Ad.com Lab Manual

Advanced Communication Lab

INDEX

Sl No. Experiment Page Nos.

1 Amplitude Shift Keying 2 – 5

2 Binary Phase Shift Keying 6 – 9

3 Frequency Shift Keying 10 – 13

4 Differential Phase Shift Keying 14 – 16

5 Time Division Multiplexing 17 – 20

6 Pulse Code Modulation 21 – 23

7 Measurement of VSWR using Gunn Diode 24 – 26

8Measurement of bending loss, propagation loss, numerical aperture, connector loss in an optical fiber

27 – 29

9Measurement of Beamwidth and radiation pattern in Yagi, Dipole and Microstrip Patch antennas

30 – 44

10 Measurement of Coupling and Isolation in Directional Coupler 45 – 48

11Measurement of resonating frequency using Ring Resonator. Measurement of Power Division and Isolation in Power Divider.

49 – 54

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1. Amplitude Shift Keying Modulation and Demodulation

Aim: To design an ASK circuit to transmit digital data by a suitable carrier and demodulate the signal.

Components required: Transistor SL100, resistors – 1k,10k, opamp μA741, diode OA79,10k pot, capacitor 0.1μF,connecting wires, CRO probes Signal generators.,.

Theory: ASK is a digital modulation technique. The amplitude of the carrier varies in accordance with the message signal. When the message is ON, the carrier is transmitted. When the message is OFF, no carrier is transmitted. Therefore the output is keyed between two amplitude levels ON and OFF and hence the name ASK. Earliest forms were employed in wireless telegraphy.

Design:During the ON period of transistor [during logic 1]

Vc = 1.5V [c(t) = 3Vpp]

VCEsat ≤ 0.3V, say 0.2V

Apply KVL to the collector-emitter (CE) loop,

Vc = VCEsat + VE

VE = 1.3V.

Assume ICsat = 1.3mA

Therefore RE = VE / ICsat = 1KΩ

Also from KVL of input loop,

Vm = IBsatRB + VBEsat + VE

Vm = 2.5V [m(t) pp is 5V]

VBEsat = 0.7V, VE = 1.3V

IBsatRB = 0.5V

IBsat ≥ ICsat/hfe ≥ 1.3mA/50 = 26μA

Say IBsat = 50μA

Therefore RB = (Vm- VBEsat -VE )/IBsat =10KΩTherefore RB = 10KΩ

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ASK MODULATION

Circuit diagram:

ASK DEMODULATION

Theory: The demodulator includes two stages: i) The envelope detector which detects the envelope of the ASK signal and this is then fed to the ii) Comparator which compares the envelope with a fixed dc reference and generates a square wave varying between ±Vsat whose frequency and phase is the same as the message signal.

Design:

Stage 1: Envelope detector:

Tc < TRc < Tm

Tm = 1/fm = 1/100 = 10ms

Tc = 1/fc = 1/3KHz = 0.33ms

Let TRc = 1ms

Let C = 0.1μF, hence R = 10KΩ

Stage 2 : Comparator:

Choose an opamp in open loop configuration and connect the output of the envelope detector to NONINV input pin 3.The reference input is given to the INV input pin (2).The reference voltage should be chosen to be less than the ON time envelope of the ASK signal to get back the message signal.

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Circuit diagram:

Procedure:

1. Connections are made as shown in the circuit diagram.2. Give a message signal (square wave) of 100Hz frequency and amplitude 5Vpp and a

carrier signal (sine wave) of 3Khz frequency and amplitude 3Vpp.3. Observe the amplitude modulated output.4. Rig up the circuit of demodulator as shown in the diagram.5. Give a dc reference voltage which is less than the voltage across the 10K resistor.6. Observe and measure the frequency and amplitude of the demodulated output.7. Plot the following signals on a graph: Message, carrier, ASK output and the demodulated

signal

Expected Waveforms:

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i/p

t

0

Vm


Results: 1.The amplitude of modulated output is __________________ 2. The amplitude and frequency of the demodulated output is _____________

Inference: The modulation and demodulation of a digital signal using the ASK technique was designed, observed and understood.

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c(t)

0t

Vc

ASK

0t

Vc


2. BINARY PHASE SHIFT KEYING Modulation and Demodulation

Aim: To design a circuit to transmit digital data by a suitable carrier using BPSK technique and to demodulate the signal.

Components required: Transistor SL100, SK100, resistors – 1k(2), 10k(9), opamp μA741, diode OA79, 10k pot, capacitor 0.1μF,DC power supply, connecting wires, CRO probes,. Signal generator.

Theory: PSK is a digital modulation technique used to communicate digital information over a band pass channel. Here the phase of the carrier is shifted by 180 degrees whenever the digital information signal changes . The amplitude of the carrier remains constant. When the message is ON, the carrier is transmitted in phase. When the message is OFF, carrier is transmitted 180 degrees out of phase. Therefore the output is keyed between two phases for a binary input signal .Hence the name BPSK.The circuit includes two ASK stages and a differential amplifier (subtractor) to combine both the ASK outputs and produce the BPSK signal

Design:

Q1 and Q2 act as switches. When Q1 is ON Q2 is OFF [+’ve half cycle of m(t)] and When Q2 is ON, Q1 is OFF [-‘ve half cycle of m(t)]

During the ON period of transistor [during logic 1]

Vc = 1.5V [c(t) = 3Vpp]



Vc = VCEsat + IcsatRc


Therefore RC = (1.5-0.2) / ICsat = 1KΩ


Vm = IBsatRB + VBEsat


VBEsat = 0.7V

IBsatRB = 1.8V

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Say IBsat = 180μA

Therefore RB = (Vm- VBEsat )/IBsat =10KΩ

CIRCUIT DIAGRAM:BPSK Modulation

BPSK DEMODULATION

Theory: The demodulator uses the principle of coherent detection. It includes three stages: i) An Inverting adder circuit which adds back the carrier to the BPSK signal so as to generate an ASK signal ii) The envelope detector which detects the envelope of the ASK signal and this output is then fed to the iii) Comparator which compares the envelope with a fixed dc reference and generates a square wave varying between ±Vsat whose frequency and phase is the same as the message signal.

Design:Stage 1: Inverting Adder

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Vo = Rf/R[V1+V2]Since only addition is required, Rf/R = 1.

Therefore if R=10KΩ, Rf = 10KΩ

Stage 2 : Envelope detector:

Tc < TRc < Tm

Tm = 1/fm = 1/100 = 10ms

Tc = 1/fc = 1/3KHz = 0.33ms

Let TRc = 1ms


Stage 3: Comparator:


BPSK Demodulation

Procedure: Connections are made as shown in the circuit diagram.

1. Give a message signal (square wave) of 100Hz frequency and amplitude 5Vpp and a carrier signal (sine wave) of 2 KHz frequency and amplitude 3Vpp.

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2. Observe the phase modulated output.3. Rig up the circuit of demodulator as shown in the diagram.4. Give a dc reference voltage which is less than the voltage across the 10K resistor.5. Observe and measure the frequency and amplitude of the demodulated output.6. Plot the following signals on a graph: Message, carrier, BPSK output and the

demodulated signal

Expected Waveform:

Results: 1.The amplitude of modulated output is __________________ 2. The amplitude and frequency of the demodulated output is _____________

Inference: The modulation and demodulation of a digital signal using the BPSK technique was designed, observed and understood.

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i/p

t

0

Vm

t

PSK

0

Vc

c(t)

0t

Vc


3. FREQUENCY SHIFT KEYING : MODULATION AND DEMODULATION

Aim: To design a circuit to transmit digital data by a suitable carrier using BPSK technique and to demodulate the signal.

Components required: Transistor SL100, SK100, resistors – 1k(2), 10k(9), opamp μA741, diode OA79, 10k pot, capacitor 0.1μF,DC power supply, connecting wires, CRO probes , Signal generator.

Theory: FSK is a digital modulation technique used to communicate digital information over a band pass channel. Here the frequency of the carrier is changed whenever the digital information signal changes. The amplitude of the carrier remains constant. The simplest FSK is binary FSK (BFSK).BFSK literally implies using a couple of discrete frequencies to transmit binary (0’s & 1’s) information. With this scheme, when the message is ON, one frequency of carrier is transmitted-called the mark frequency. When the message is OFF, another frequency of carrier is transmitted –called as space frequency. Therefore the output is keyed between two frequencies for a binary input signal .Hence the name BFSK.The circuit includes two ASK stages with two different carrier frequencies and a differential amplifier (subtractor) to combine both the ASK outputs and produce the BFSK signal

Design:

Q1 and Q2 act as switches. When Q1 is ON Q2 is OFF [+’ve half cycle of m(t)] and When Q2 is ON, Q1 is OFF [-‘ve half cycle of m(t)]

During the ON period of transistor [during logic 1]

Vc = 1.5V [c(t) = 3Vpp]



Vc = VCEsat + IcsatRc


Therefore RC = (1.5-0.2) / ICsat = 1KΩ


Vm = IBsatRB + VBEsat


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VBEsat = 0.7V

IBsatRB = 1.8V


Say IBsat = 180μA

Therefore RB = (Vm- VBEsat )/IBsat =10KΩ

CIRCUIT DIAGRAM:BFSK Modulation

BFSK DEMODULATION

Theory: The FSK demodulator includes three stages:i) A Low Pass Filter circuit which attenuates the high frequency component and allows only the lower frequency to pass on to the next stage hence resembling an ASK signal .ii) The second stage is an envelope detector which detects the envelope of the ASK like signal and this output is then fed to the iii) Comparator which compares the envelope with a fixed dc reference and generates a square wave varying between ±Vsat whose frequency and phase is the same as the message signal.

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Design:

Stage 1: Low Pass Filter

fc = 1/2ПRC

Choose fc > 1KHz, the lower carrier frequency

Let fc = 1.5KHz

Therefore, if C=0.1μF , R = 1/2ПfcC = _______

Generally a 10Kohm potentiometer can be chosen for R.

Stage 2 : Envelope detector:

Tc < TRc < Tm

Tm = 1/fm = 1/100 = 10ms

Tc = 1/fc = 1/3KHz = 0.33ms

Let TRc = 1ms


Stage 3: Comparator:


FSK Demodulation

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Procedure:

1. Connections are made as shown in the circuit diagram.2. Give a message signal (square wave) of 100Hz frequency and amplitude 5Vpp and two

carrier signals (sine waves) of frequencies 1 KHz and 3KHz and amplitudes 3Vpp.3. Observe the frequency modulated output.4. Rig up the circuit of demodulator as shown in the diagram.5. Give a dc reference voltage which is less than the voltage across the 10K resistor.6. Observe and measure the frequency and amplitude of the demodulated output.7. Plot the following signals on a graph: Message, carrier, BFSK output and the

demodulated signal

Results: 1. The amplitude of modulated output is __________________ 2. The mark and space frequencies are ___________ & _______________ 3. The amplitude and frequency of the demodulated output is _____________

Inference: The modulation and demodulation of a digital signal using the BFSK technique was designed, observed and understood.

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4. DPSK GENERATION AND DETECTION

Aim: To modulate and demodulate DPSK

Components: Data formatting and carrier modulation Trainer kit, carrier demodulation and data reformatting receiver trainer kti, 8 bit variable data generator.

Theory: We may view Differential Phase Shift Keying as a non coherent version of PSK. It eliminates the need for coherent reference signal at the receiver by combining two basic operations at the transmitter namely (1) Differential encoding of input binary wave (2) Phase shift keying. In effect to send symbol ‘0’, the current signal waveform does not undergo any phase change and to send symbol ‘1’, there is a phase change of 1800. The receiver is equipped with a storage capacity, so that it can measure the relative phase difference between the waveforms received during the successive bit intervals.

Procedure: 1. Connection is done as shown in the block diagram.2. Switch on the power.3. Give any 8 bit data from data generator and carrier of 960 kHz frequency internally

generated in the kit.4. The NRZ (M) data is obtained from the 8 bit data input. (This is similar to carrying out

differential encoding).5. This NRZ (M) data is given to Unipolar to Bipolar converter to obtain the polar version

of the data.6. The carrier and the polar version of the data are given to the PSK balanced modulator on

the kit.7. The DPSK output is obtained as shown in the figure.8. Connect the modulated output to the PSK demodulator on the demodulation trainer kit.9. This output is connected to a comparator which compares the incoming signal with a

reference value which is adjustable.10. The output of the comparator is then fed to a decoder along with the clock which is the

same that is used in the transmitter.11. The demodulated output is observed at the output of the decoder.12. Plot the input 8 bit data, NRZ (M) data, DPSK and the demodulated waveforms.

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Circuit diagram:

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Expected Waveform:

Result: The given digital bit sequence is ____________________The corresponding NRZ-M signal is _________________The demodulated data is ______________________

Inference: The digital data was transmitted using DPSK technique The different stages involved were understood and the waveforms at different stages were

observed The different stages in demodulation were understood and the demodulated signal was

obtained.

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t

01 0 01 1 1 1i/p

0

t

NRZ(M)

0

11 1 10 1 0 1

t0

DPSK


5. TIME DIVISION MULTIPLEXING

Aim: i) To demonstrate the working of TDM for PAM signals ii) To demultiplex the TDM signal and get back the PAM signals iii) To obtain the original signal by low pass filtering.

Components required: IC 4051(2), resistors, transistors – SL100 & SK100, Signal generator, VRPS

Theory: An important feature of pulse amplitude modulation is a conservation of time. That is, for a given message signal, transmission of the associated PAM wave engages the communication channel for only a fraction of the sampling interval on a periodic basis. Hence, some of the time interval between adjacent pulses of PAM wave is cleared for use by other independent message signals on a time shared basis. By doing so, we obtain a time division multiplex system, which enables the joint utilization of a common channel by a plurality of independent message signals without mutual interference. Each input message is first restricted by a low pass filter. The filter outputs are then applied to commutator following which the multiplexed signal is then applied to PAM to transform the multiplexed signal into a form suitable for transmission over the communication channel.

Suppose N message signals to be multiplexed are having similar properties, Ts denotes the sampling period so determined for each message signal, Tx denotes the time spacing between adjacent samples in the time multiplexed signal, then T = Ts/N.

Procedure:1. Make connections as in the circuit diagram.2. Give two message signals of frequency 100Hz to PAM circuits.3. Connect the PAM outputs to multiplexer ICs4. Give a control signal of higher frequency .5. Connect the multiplexed output to demux IC & observe reconstructed PAM outputs.6. Plot the waveforms of message, carrier, PAM signals, Multiplexed signal and the

demultiplexed signal.

TDM – Circuit diagrams

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Expected Waveforms:

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w:

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Results and Inference: The TDM operation was observed for the different signals and the waveforms were observed as shown.

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6. PULSE CODE MODULATION AND DEMODULATION

Aim: To generate and detect Pulse Code Modulated wave using codec chip

Components required: IC 44233, IC 7493 (2), resistors, VRPS, signal generators

Theory: Pulse Code Modulation systems are complex in that the message signal is subjected to a large number of operations. The essential operations in the transmitter of a PCM system are sampling, quantizing and encoding. The sampling, quantizing and encoding operations are usually performed in the same circuit, called as analog to digital converter. Regeneration of impaired signals occur at intermediate points along the transmission path. At the receiver, the essential operation consists of one last stage of regeneration followed by decoding, then demodulation of the train of quantized samples. The operation of decoding and reconstruction are usually performed in the same circuit called a digital to analog converter. When time division multiplexing is used, it becomes necessary to synchronize the receiver to the transmitter for overall system to operate satisfactorily.

Procedure: 1. Connections are made as per circuit diagram.2. DC power supplies are switched ON and the specified voltages are applied.3. A TTL clock of 2 MHz are applied to the counter IC 7493 at pin no. 14 and observe the

output on CRO at pin no. 11. It should be a 125 kHz signal.4. Check the output at pin no. 11 of 2nd IC 7493 that will be approximately 8 kHz.5. Apply a sinusoidal message frequency of 1V, 1 kHz at pin no. 1 of IC 44233.6. Observe the PCM output at pin no. 8 of IC 44233. You may have to change the time

range of oscilloscope to convenient range to observe the frame time (50 us) and 8 bit word length.

7. Observe the demodulated output at pin no. 5 of IC 44233 and compare it with original analog message signal.

8. Observe the changes at the PCM output and demod output by changing the frequency and amplitude of message signal.

Circuit Diagram:

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Result and Inference: The Pulse Code Modulated wave was observed and the message was detected successfully as shown in the figure.

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7. DETERMINATION OF VSWR AND OPERATING FREQUENCY

Aim: Conduct an experiment to measure the operating frequency and VSWR using Gunn diode as a source.

Block Diagram:

Theory: A Gunn diode, also known as a transferred electron device (TED), is a form of diode used in high-frequency electronics. It is somewhat unusual in that it consists only of N-doped semiconductor material, whereas most diodes consist of both P and N-doped regions. In the Gunn diode, three regions exist: two of them are heavily N-doped on each terminal, with a thin layer of lightly doped material in between. When a voltage is applied to the device, the electrical gradient will be largest across the thin middle layer. Conduction will take place as in any conductive material with current being proportional to the applied voltage. Eventually, at higher field values, the conductive properties of the middle layer will be altered, increasing its resistivity and reducing the gradient across it, preventing further conduction and current actually starts to fall down. In practice, this means a Gunn diode has a region of negative differential resistance. The negative differential resistance, combined with the timing properties of the intermediate layer, allows construction of an RF relaxation oscillator simply by applying a suitable direct current through the device. In effect, the negative differential resistance created by the diode will negate the real and positive resistance of an actual load and thus create a "zero" resistance circuit which will sustain oscillations indefinitely. The VI curve of a Gunn diode is shown in the fig below.

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Gunn Power Supply

Gunn Oscillator

Ferrite Isolator

PIN Modulator

Attenuator Frequency Meter

Slotted Section with Carriage

Crystal Detector

SWR Power Meter / CRO

Matched Load

http://en.wikipedia.org/wiki/Direct_current

http://en.wikipedia.org/wiki/Relaxation_oscillator

http://en.wikipedia.org/wiki/Radio_frequency

http://en.wikipedia.org/wiki/Negative_resistance

http://en.wikipedia.org/wiki/Negative_resistance

http://en.wikipedia.org/wiki/Semiconductor

http://en.wikipedia.org/wiki/Doping_(semiconductors)

http://en.wikipedia.org/wiki/Electronics

http://en.wikipedia.org/wiki/Diode


Procedure:

1. Set up the equipment as shown in the above setup taking due care for biasing PIN and Gunn diode.

2. Set up the Gunn oscillator micrometer tuning screw at suitable frequency. Adjust the attenuator and PIN modulator to get the maximum output on the CRO.

3. To measure the operating frequency, the frequency meter is tuned to get the dip on the CRO and the frequency is read directly from the wave meter making the Gunn diode to operate in the negative resistance region.

4. To find Vg move the carriage on the slotted line to get the maximum output and note the reading on the scale on slotted line and vernier scale. Say d1 in cms. Move the carriage to right or to the left to get the next maximum output position. Note the reading on the slotted line scale say d2 cms. The difference between the two maximum output positions is the group wavelength

λg / 2 = d1 ~ d2 = ______________ cmsλg = 2 x a = ______________ cmsa = Width of the waveguide

5. To find VSWR: Move the carrier to the maximum output and set the VSWR to 1 on the VSWR meter by adjusting the gain. Move the carrier to minimum output position. The reading of the VSWR on the VSWR meter gives the VSWR.

6. Find the maximum output voltage Vmax and the minimum output voltage Vmin.

VSWR = Vmax / Vmin

7. To find the cutoff wavelength.

1 / ( λo )2 = 1 / ( λc )2

+ 1 / ( λg )2

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Waveform:

Result: Operating Frequency fo = ________ Hz (Practically)

________ Hz (Theoritically)

VSWR = ______ Vλg = _______ cmAttenuation = ______ dB

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d1d2λg / 2

o/p power or voltage

Distance in cms


8. MEASUREMENT OF LOSSES IN OPTICAL FIBER

Aim: The objective of this experiment is to measuring loss in the fiber.1. Propagation Loss.2. Bending Loss.3. Connector Loss.

Components required:FCL-01 & FCL-02.1 & 3 Meter Fiber cable.0.5 meter connectorized fibers.Patch chordsPower supply (Use only the provided)20 MHz Dual Channel Oscilloscope.

Theory: Optical fibers are available in different variety of materials. The materials are usually selected by taking into account their absorption characteristics for different wavelengths of light. In case of optical fiber, since the signal is transmitted in the form of light, which is completely different in nature as that of electronics, one has to consider the interaction of matter with the radiation to study the losses in fiber. Losses are introduced in fiber due to various. As light propagates from one end of fiber to another end, part of it is absorbed in the material exhibiting absorption loss. Also part of the light is reflected back or in some other direction from the impurity particles present in the material contributing to the loss of the signal at the other end of the fiber. In general terms it is known as propagation loss. Plastic fibers have higher loss of the order of 180dB/Km. whenever the condition for angle of incidence of the incident light is violated the losses are introduced due to refraction of light. This occurs when fiber is subjected to bending. Lower the radius of curvature more is the loss. Another loss are due to the coupling of fiber at LED & photo detector ends. When light travels down optical fibers, some of the light is absorbed by the glass or plastic.This means the light coming out of the end of the fiber is not as strong as the light going into the fiber. When designing a fiber communications system, you need to know the size of thisLoss to calculate the maximum distance the signal will travel. In this experiment you will try one way of measuring the loss in the fiber.

Procedure:

A. MEASUREMENT OF PROPAGATION LOSS

1. Make connections as shown in the figure. Connect the power supply cables with proper polarity to FCL-01 & FCL-02 Kits. While connecting this, ensure that the power supply is OFF.

2. Keep the jumpers JP1, JP2,JP3 & JP4 on FCL-01 as shown in fig.3. Keep the jumpers JP1 & JP2 on FCL-02.4. Keep switch S2 in V1 position on FCL-01.5. Switch on the power supply.

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6. Slightly unscrew the cap of LED SFH756V (660 nm). Do not remove the cap from the connector. Once the cap is loosens, insert the 1 meter fiber into the cap, Now tighten the cap by screwing it back.

7. Now rotate the Optical power control pot P3 in FCL-01 in anticlockwise direction. This ensures minimum current flow through LED.

8. Slightly unscrew the cap of photo Diode SFH250V. Do not remove the cap from the connector. Once the cap is loosened, insert other end of fiber into the cap. Now tighten the cap by screwing it back.

9. Keep switch SW1 to SIGNAL STRENGTH position in FCL-02.10. Connect the output of the photo diode detector post OUT to post IN of signal strength

indicator block.11. Observe the signal strength LED’s adjust the TRANSMITTER LEVEL using intensity

control pot P3 until you get the reading of all LED’s glow.12. We will measure the light output using the SIGNAL STRENGTH section of the kit. The loss

will be larger for a longer piece of fiber, so you will measure the loss of the long piece of fiber. In order to measure the loss in the fiber you first need a reference of how much light goes into the piece of fiber from the LIGHT TRANSMITTER. You will use the short piece of fiber to measure this reference.

13. Now remove the 1 meter fiber & insert 3 meter fiber.14. What reading do you get? Loss in optical fiber systems is usually measured in dBs. Loss of

the fiber itself is measured in dBs per meter. Subtract the length of the short fiber from the length of long fiber to get the difference in the fiber lengths (3m-1m). The extra length of two meters is what created the extra loss you measured. Then take the signal strength reading you obtained for the loss of the long fiber & convert it to dB using the equation 1. Finally divide dB reading by the length to get the loss in dB per meter

Power=10 log(p2/p1)dB =10 log (8/6)dB =1025dB

P2: Reference reading by 1 meter FIBER.P1: Reading obtained after replacing 3 meter fiber.For example, your signal strength reading is 6, & then your loss in dB would be 1.25 dB.Taking 1.25/2 gives 0.625 dB per meter.The reason for converting to dB per meter is that now in order to find the loss of any length of fiber you just have to multiply the dB per meter by the length of the fiber. For e.g. if you have a 10 meter long piece of fiber the loss will be 0.625 dB per meter * 10 meters=6.25 dB.

B. MEASUREMENT OF BENDING LOSS

1. Keep the connections with 1 meter fiber as per the above procedure.2. Adjust the transmitter power so that the SIGNAL STRENGTH reading is 8. Now take the

portion of the fiber & loop it to match the bends as shown in the fig. As you match each bends, write down the reading from SIGNAL STRENGTH indicator. What happens as bends the fibers ? Don’t bend the fiber too tightly or it may not come back to shape.

3. If you are designing fiber optic communications system, you would need to know the relationship between the size of the bend & the light loss from the bend. In order to describe

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this relationship, you can measure the loss for a number of different bends & plot them on a graph.

C. MEASUREMENT OF CONNECTOR LOSS

1. Keep the connections with 1 meter fiber as per the above procedure.2. Adjust the transmitter power so that the SIGNAL STRENGTH reading is 8.

Remove the 1 meter fiber & insert 0.5 meter connector loss is then 8.0dB-7dB=1dB. This is actual connector loss.

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Microwave and Antennas LabIntroduction to the Instruments

1.1 Instruments-Main featuresThe instruments required for all the experiments are common; one is a microwave signal source and the other is VSWR meter which is to be used in conjunction with a coaxial detector.Microwave Signal SourceThe microwave signal source has been an operating frequency range from 2.2 to 3GHz. It is a compact source with a minimum power output of 10mW over this frequency range. It has a built-in modulation facility, provision for varying the frequency and RF power output, and a digital display for frequency readout on the front panel. The source has a built-in 1KHzAM preset mode so that standard VSWR meters can be used as test instruments for making measurements.Coaxial DetectorThe coaxial detector incorporates a nonlinear non-reciprocal device (Schottky barrier diode). The nonlinearity of the diode is used to demodulate the 1 KHz amplitude modulated microwave signal. The desired demodulated output at 1 KHz is filtered out in the detector. The amplitude of the corresponding current in the diode is proportional to the RF power of the input signal; i.e the square of the RF voltage. This square-law range is the desired operating range of the detector and hence the detector is referred to as a square-law detector.VSWR MeterThe measuring instrument for all the experiment is a VSWR meter, the input to which is the detected output from the coaxial detector. The VSWR meter is basically a high gain low noise audio amplifier tuned to a mean frequency of 1 KHz. On the front panel is a display meter that is square-law calibrated to read the SWR directly and relative power levels in dB. It has a RANGE SWITCH covering 0 to 60dB in steps of 10dB and a GAIN CONTROL knob that provides continuous variation over about 10dB. In the current lab experiments, there is no requirement to read SWR directly; you only need to read the relative power levels on the dB scale.

1.2 Measurement Principle and General InstructionsPrincipal of Measurement

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In all the experiments, you will be using the microwave signal with 1 KHz amplitude modulation. Hence the input to the MIC component (or transmit antenna in antenna experiment) is the microwave carrier modulated with 1 KHz square wave. The output signal from the components (or receive antenna) is fed to the coaxial detector. The detector demodulates this modulated microwave signal and produces an output which is the 1 KHz modulation envelope. The output of the detector is fed to the VSWR meter which is square-law calibrated to read (relative) power levels in dB with the maximum (i.e., VSWR=1 or 0dB).Detectors offer square-law response over a restricted range of input powers. In order to enable correct measurements over a larger range of input power levels, the calibration curve that is provided with the coaxial detector is to be used.

Note:All VSWR meter readings are to be recorded as minus dB.No correction is required for reading in the range from -70dB to -60dB because the graph is linear (the detector response is square law). Deviation from square law increases with an increase in the input power level above approximately -60dB. For readings above -60dB locate the point corresponding to the VSWR meter reading (minus value) on the x- axis and then the correct value on y-axis.

Experiment-1Measurement of Directivity and Gain of Antennas

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[Printed Yagi, Printed dipole and Micro strip patch antenna]2.1 THEORY

2.1.1 Introduction to planar Antennas

The antennas provided for the experiment are in planar form. The printed dipole is a planar version of the conventional dipole and the yogi antenna is a planar form of the conventional Yagi that is commonly made of conducting rods or tubes. The micro strip patch antenna is a basic antenna belonging to a class of planner antennas based on microstrip techniques. The antenna that is provided has a rectangular shaped conducting patch fed by a microstripThe function of an antenna is to transform guided electromagnetic energy in a transmission line into free space radiated energy and vice versa. In the first case, the antenna functions as a transmitting antenna and in the second case it functions as a receiving antenna. Antenna forms an essential part of any system required to either transmit or receive electromagnetic energy. The basic parameters of an antenna remain same whether it is used for transmission or reception.

Advantages of planar antennas Planar antennas in the form of printed antennas offer several advantages over the conventional antennas mentioned above. They are lightweight, low profile antennas and can be made conformal with the use of flexible substrates. These features make them well suited for aerospace application such as for aircraft, missile and satellites and also for land mobile system. Microstrip patch antennas in particular are thin and flat, and hence are ideal for mounting in the interior of a vehicle, a cellular mobile phone system and portable manpack radars.

2.1.2 Directivity and gain – definitions and formulas

Directivity and gain are two important parameters of any antenna. Before defining these parameters, we need to understand certain other characteristics of the antenna; namely, radiation pattern, far field region, E- and H- plane half-power beam widths, and radiation intensity.

Radiation pattern: practical antennas do not radiate uniformly in all directions. Every antenna has a radiation pattern. It is a graphical representation of the distribution of radiated energy as a function of angle about the antenna in the three-dimensional space. The radiation pattern is generally measured in the far field region

Far field region: The far field region is defined as that region of space where the angular field distribution of the antenna is essentially independent of the distance from the antenna. If the maximum overall dimension of the antenna is D, then the far field region is commonly taken to exist at the distance grater than 2D2 /λ0 from the antenna where λ0 is the free space wavelength.

The strength of radiation is usually measured in terms of field strength relative to some reference level, and this reference level is usually the peak of main beam. Radiation pattern plots however, can be shown in terms of field strength or power density or decibels (dB). Thus a complete radiation pattern gives relative power radiated (or field strength) at all angles of θ and ø in spherical coordinate system (Fig. 2.1) and requires a 3-dimentional presentation. However, in

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practice, it is common to present cross section of the radiation pattern in two principal planes of interest. For linearly polarized antennas, these planes are the E- and H- planes.

E-plane: the E-plane is the plane passing through the antenna in the direction of the beam maximum and parallel to the far –field E-vector.

H-plane: the H-plane is the plane passing through the antenna in the direction of the beam maximum and parallel to the far –field H-vector.

Beam width: the radiation pattern of a typical antenna consists of a main beam and a few minor lobs usually represent radiation in the undesired direction. The beam width is a measure of sharpness of the main radiated beam. The 3dB beam width is the angular width of pattern between the half-power points; i.e. -3dB points with respect to the maximum field strength. In the electric field intensity pattern, it is the angular width between points that are 1/√2 times the maximum intensity(Fig. 2.2).

Radiation intensity: Radiation intensity in a given direction is defined as the power radiated by the antenna per unit solid angle. It is obtained by multiplying the power density in the far field region by the square of the radial distance from the antenna. If U denotes the radiation intensity in W/unit solid angle (steradian or square degree) and Srad denotes the power density in W/m2, then we can write

U= R2 Srad (2.1)

Where R is the distance from the antenna to the far field point of observation. It may be noted that in the far zone, the power density Srad depends on the radial distance from the antenna, but the radiation intensity U is independent of the distance

Directivity:

The directivity D of an antenna is defined as the ratio of maximum radiation intensity (Umax) to the average radiation intensity (Uav).

D = U(θ, ø)max / Uav = S(θ, ø)max / Sav (2.2)

Where S is the radiated power density (or the pointing vector); Sav is the average value over a sphere and Smax is the maximum value.

Sav =1/4Π ∫02 Π ∫0

2 Π S(θ, ø) dΩ (2.3)

Therefore the directivity D can be written asD = 4П/ ΩA (2.4)

In practice, the directivity is calculated from the measured E-plane and H-plane radiation patterns. There fore, we need to take in to account the power lost in the minor lobes. An approximate formula that is commonly used for directivity in practice is

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D ≈ 32400/∆θE0 ∆θH

0 (2.5)

The directivity expression given in (2.2), (2.4) and (2.5) are dimension less. In decibels, the directivity given by

D(dBi)=10log10(D) (2.6)

For example, directivity D=100 is 20dBi; that is 20dB above the isotropic radiator.

Gain

The gain G of an antenna is defined as the product of its directivity and radiation efficiency.

G = ηrad D (2.7)Where ηrad is the radiation efficiency of the antenna and is defined as ηrad = Prad/Paccepted= Prad/Prad+Pdis (2.8)

Where Prad is the power radiated by the antenna, and Paccepted is the power accepted by the antenna at its input terminals. Paccepted is equal to the sum of power radiated and power dissipated (Pdis) in the antenna. The power dissipated includes the conductor loss as well as the dielectric loss in the antenna.

2.1.2 Printed dipole

The simplest type of antenna and one of the most commonly used is the centre fed half wave (λ0/2) dipole, where λ0 is the free space wavelength. For a thin wire dipole (diameter less than about λ0 /100), the current distribution is approximately sinusoidal. Fig 2.3a shows the approximate current distribution on a thin centre fed λ0/2 dipole.

In the far zone, the only nonzero field components are Eθ and Hø And they are related by the expression Hø =Eθ /η0 where η0 is the intrinsic impedance of free space. The expression for Eθ is given by Hø

Figure 2.3(b) shows the variation of Eθ in the E- and H- planes. The 3dB beam width is about 900 in the E-plane. In the H-plane, the pattern in omni-directional. The directivity of the l0/2 dipole is D=1.643 or 2.16 dBi

Dipoles are generally constructed from conducting cylindrical rods. The length l of the dipole for its first resonance is in the range 0.47l 0 to 0.480 depending on the diameter of the wire. The fatter the dipole, shorter is its resonant length.

The centre fed wire dipole as shown in fig. 2.3a has a balanced input in the form of a two wire line. The printed dipole has 50W coaxial connector at the input end which is an unbalanced

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input. We therefore need an unbalanced to balanced line converter (balun) plus an impedance transformer to execute the printed dipole. A micro strip feed which incorporate both these features is printed on the reverse side of the same substrate. The substrate containing the entire printed pattern is mounted on a ground plane with a suitable bracket.

The microstrip feedline has a quarter- wave transformer and a sub line exciting the resonant slot, which in turn excites the dipole. The dimensions are chosen so as to achieve an input impedance of 50W at the connector point. The conductor plate on which the printed dipole is mounted serves as reflector. The reflector allows radiation only in the forward direction and thus enhances the directivity of dipole. Thus, the pattern of the dipole is different from that of the simple dipole in free space. Another advantage of the printed dipole is its enhanced return loss bandwidth over the dipole in free space. This is accomplished in view of its decrease in the length to width ratio.

Radiation pattern : The field patterns of the printed dipole in the E – and H –planes are shown in fig.2.4b. The 3dB beamwidth is approximately 780 in the E –plane, and 1900 in the H –plane.

Directivity : The theoretical directivity of the printed dipole is 4.6dBi which is higher than that of a wire dipole. The bandwidth corresponding 10dB return loss for the printed dipole is ~29%.

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2.1.4 Microstrip Patch Antenna

A microstrip patch antenna basically consists of a conducting patch on a grounded dielectric substrate. That radiation and impedance characteristics of the patch antenna depend on the shape and size of the patch and the feeding arrangement. The patch may be of various shapes; rectangular, square, circular, triangular etc, and it may be excited at one or more feed points from the edge of the patch or through the ground plane. Of the various shapes, the most popularly used one is the rectangular shape. In the following we shall consider the theory of the rectangular patch.

Radiation Mechanism of Rectangular Patch Antenna

In order to understand the radiation mechanism in a rectangular microstrip patch, consider a linearly polarized radiating patch fed by a microstrip as shown in Fig.2.5a. the substrate is electrically thin (typically about 0.02 λ0, where λ0 is the free space wavelength) such that the electric field between the patch and the ground plane is essentially x-directed and independent of the x- coordinate. At resonance, the length L of the patch is approximately half wavelength (lg/2) in the microstrip medium .The input impedance of the patch is mainly governed by the patch width W and this width is generally chosen to be between 0.5 to 2 times the length L, but much larger than the strip width of the microstrip line feeding it. The width W should however be kept less than 2L in order to avoid higher order modes.

Since the path length is approximately λg/2, the electric field lines (Fig.2.5a)at each of the two edges of width W are out of phase. The fringing fields at these two edges can be resolved into normal and tangential components with respect to the ground plane. The normal components are oppositely directed because the patch is nearly λg/2 long, and therefore the far fields produced by them cancel in the broadside direction (normal to the surface of the patch). On the other hand, the far fields due to the tangential components that are parallel to the ground plane (see fig. 2.5) add in phase to give a maximum in the broadside direction. The patch antenna can therefore be treated as two radiating slots separated by a distance L. It may be noted that the fringing fields at the other two edges, each of length L do not contribute to radiation. This is apparent from the reversal of electric field lines at edge as shown in Fig.2.5c.

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Figure 2.6b shows the theoretical E- and H- plane far field patterns. The 3dB beamwidth is around 750 in the E-plane and 820 in the H-plane.radiation. This is apparent from the reversal of electric field lines at edge as shown in Fig.2.5c.

Directivity

Since the radiation of the patch is one sided, it has a larger directivity than a simple dipole.

Directivity of the order of 6 to 7dBi is easily achievable with the patch antenna. But the bandwidth of the patch antenna is inherently narrow (~2.5%).

2.1.5 Printed Yagi Antenna

Principle of OperationThe Yagi antenna consists of one excited dipole, one reflector and several parasitic directors figure 2.7 shows the basic geometry of a 5-element Yagi antenna. Parasitic elements are shorted dipoles that are not directly excited, but carry induced current due to proximity coupling. By adjusting the lengths and spacing’s of the parasitic elements with respect to the main excited dipole we can control the amplitude and phase of the induced currents in the parasitic element.

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If the length of the parasitic element is longer than the resonant length λ0/2, the induced current lags the voltage s in an inductor, and if the length is less than λ0/2 the current leads the voltage as in a capacitor. The reflector is a parasitic element with a lagging current and the directors are parasitic element with leading current. Yagi antenna is essentially an array supporting a travelling wave, and its performance is determined by the current distribution in each element and the phase velocity of the traveling wave.The Yagi antenna produces a highly directive unidirectional radiation pattern. The field intensity is maximum along the line of the array and towards the directors. The directivity is a function of the number of director or the length of the array. Normally only one reflector is used since increasing the number of reflector does not improve the directivity significantly. With five element (one reflector, one dipole and three directors) we can get a directivity of about 10dBi in the forward direction (in the direction of the directors). The positions and length of the parasitic elements strongly affect the input impedance of the Yagi antenna, and the feed network has to be appropriately designed taking into consideration these effects.As shown in fig. 2.7, the excited dipole and the directors are of width w. the lengths and spacing of the various elements as marked in the figure from design parameters. The Yagi antenna radiates an endfire beam with nearly the same beamwidth in both the E-and H-planes.

Printed 5-Element Yagi ArrayFig. 2.8a shows the geometry of a 5-element printed Yagi antenna. It uses a printed dipole with the addition of printed parasitic elements o a dielectric substrate. The printed dipole configuration is the same as that described in section 2.1.3 All the five elements are located on one side of te substrate and the microstrip feed is on the reserve side of the substrate.Radiation Pattern: Figure 2.8b shows typical field patterns of a Yagi array. The half-power beamwidth is 520 in the E-plane and 640 in the H-plane.Directivity: The antenna offers a directivity of about 10dBi. The bandwidth corresponding to 10dB return loss is approximately 10%.

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2.2 ExperimentThree types of planner antennas are provided: printed Yagi antenna, printed Yagi antenna, printed dipole and microstrip rectangular patch antenna. All are designed t operate within the S-band with a centre frequency around 2.4GHz. the experiment involves.

1. Measurement of E-and H-plane pattern and calculation of directivity

2. Measurement of gain –Absolute gain method and Comparison method

2.2.1 Measurement of Directivity

Objective: (a) To measure the E-and H-plane radiation patterns of an antenna (b) To determine the half-power beamwidths in the principal planes and calculate

the directivity of the antenna

Equipment/Components required:Microwave Signal Source(2.2-3GHz)VSWR meterCoaxial DetectorN(m) to SMA(F) adapterAttenuator pad(3dB)BNC/SMA connector fitted cablesAntenna standsPlanar antennas - printed Yagi antenna, printed dipole and microstrip rectangular patch antenna

Procedure 1. Assemble the set up as shown in fig. 2.9. Mount the two (identical) Yagi antenna on the

two stands.

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Do not switch ‘ON’ the signal source or the VSWR meter until you read the instructions given at Sl. Nos. 2 and 3 below.[ In general, when the two antenna are not identical, the antenna whose pattern is to be measured must be mounted on the stand which has provision for rotation as a function of angle. You may consider this as the receiving antenna. The other antenna on the fixed stand will then be the transmitting antenna.]

2. Procedure for switching ‘ON’ the Microwave Signal Source

(a) Before switching ‘ON’ the signal source, rotate the RF power level knob on the front panel anti-clockwise to minimum position (lowest power output). Connect a 3dB attenuator pad at the RF output port as shown in the diagram.

The RF power should not be switched ‘ON’ without a load (attenuator pad or antenna) connected to avoid damage to the RF circuits inside the source.(b)Switch on the signal source in the following sequence: First Power Switch to ‘ON’ position and then RF Power Switch to ’ON’ position.Set modulation switch to AM and modulation frequency to the 1 KHz preset position (click at extreme left).Before making any change in the setup, i.e., changing cable connections, device or attenuator, ensure that there is at least a 3dB attenuator pad at the RF output port of the source. Alternatively, you can switch OFF the RF power while making any changes.

3. Procedure for switching ‘ON’ the VSWR meter

The VSWR meter is to be used in conjunction with the coaxial detector. Keep the Range Switch in the 40dB position and the Variable Gain Knob close to maximum.The choice of range initially, is to avoid the meter needle from kicking in case the input power is high.Switch ‘ON’ the VSWR meter. Then change the Range setting to 50 dB, 60dB till the meter needle is within the reading range. You can vary the source RF power to get reading in one of these ranges.

4. How to record VSWR meter readings

Take all VSWR meter readings on the dB scale and record them as ‘minus’ dB. Positive dB numbers that you read refer to the VSWR meter gain, but for the input signal, it is negative dB.For example, if the Range Switch is in 40dB position and the needle on the meter points to 6dB on the dB scale, then note down the readings as –(40+6) = -46dB.

5. Keep the receiving antenna in the far zone of the transmitting antenna. That is, the distance R between the two antennas must satisfy the relation R>2D2/ λ0, where D is the maximum size of the antennas(S), and λ0 is the free space wavelength. Calculate this value for the given antennas and make sure that the distance between the antennas is greater than this R.

6. For E-plane Pattern: Align the two Yagi antennas along their main beam peaks (boresight direction) and for horizontal polarization. Set the pointer on the receiving antenna stand to read 00.

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7. Set the frequency of the source near 2.4GHz and vary frequency around this value to get maximum reading on the VSWR meter. [When the frequency of the source is to set to the centre frequency of the antennas, the VSWR meter will show maximum reading.]

8. With the antennas properly aligned and the pointer on the rotating stand set at ,adjust the power output of the source to indicate high power in dB on the VSWR meter (say, -46dB). This is the reference value at the peak of the beam.

9. Next, rotate the antenna clockwise in steps of 50 at a time till 900 (or till the meter reading falls to -70dB). Record the angles in column 1 and VSWR meter readings as ‘minus’ dB in column 2 of Table 2.1.

10. Return to 00 position. The VSWR meter needle should return to the reference level (-46dB).In case of any minor deviation (which can occur due to power fluctuation), adjust the gain on the VSWR meter slightly to read the same reference value. Repeat measurements by rotating the antenna anticlockwise in steps of 50 till -900 (or till the meter reading falls to -70dB). Record the angle and VSWR meter readings at every step in columns 5 and 6, respectively.

This completes the measurement in the E-plane.11. For H-plane Pattern: Now turn both the antennas by 900 and mount them for vertical

polarization. Align the antennas for maximum reading on the VSWR meter.

Follow the same procedure as given above in steps 8 to 10 and tabulate the readings in Table2.2 in the respective columns (as in Table 2.1).This completes the measurement in the H-plane.

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Relative power level Relative Power levelAngle

(degrees)VSWR meter

reading (dB)

Corrected value (dB)

Normalized value (dB)

Angle (degrees)

VSWR meter reading (dB)

Corrected value (dB)

Normalized value (dB)

0

5

10

:

:

:

0

:

:

:

0

-5

-10

:

:

:

0

:

:

:

1. Refer to the calibration Graph that is provided with the detector and VSWR meter. Locate the VSWR meter readings of columns 2 and 6 on the x-axis of the graph. Read the corrected values on the y-axis and record them in columns 3 and 7, respectively.

[See Fig.1.2 for an example. In Fig.1.2, if x = -48dB, the corrected value y is -49.5dB]

2. Normalize all the readings by taking the reference value as 0dB. [For example, if the corrected reference value is y = -49.5dB, then add 49.5dB to all the readings of column 3 and 7 and enter the normalized values in the respective adjacent columns. Plot the E- and H- plane patterns on a polar plot showing normalized values in dB versus the angle.

3. For both the patterns, locate the -3dB points on either side of the peak (0dB) and note the angle between them. This gives the -3dB beam widths ∆θ0

E ∆θ0H , in the E- and H-planes,

respectively.

4. The pattern directivity D can be calculated using the approximate formula given in (2.5b).

D ≈ 32400/∆θE0 ∆θH

0

or D(dBi)=10log10(32400/∆θE

0 ∆θH0)

Measurement of Gain

Objective(A)To measure the absolute gain of an antenna using two identical antennas(B) To measure the gain of a given antenna using a reference antenna with known gain. (comparison method) (a)Measurement of Absolute gain an antenna using Two identical antennas

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The absolute gain of an antennae can be measured if we have two identical antennas. For making this measurement, tow identical printed Yagi antennas are provided.

This experiment involves measurement of RF power in put to the transmit antenna and the power received at the receive antenna.

Procedure

1.Measure RF power input to the transmit antenna. First we sent the transmit power level. Connect all the three attenuator pads (3dB+6dB+10dB) at the source out put and then connect the detector and VSWR meter.

2. Switch ‘ON’ the RF power with source in AM 1KHz modulation and frequency 2.4 GHz. Set the VSWR range switch to 40dB range and variable gain knob to maximum.

Increase the RF power so that the VSWR meter shows reading in the 40db range. This is the reference power level. If the needle is at 46dB, then note this reference reading as Pref (dB)=-46dB. Do not vary RF power setting on the source throughout the gain measurement.

3. Now, switch ‘OFF’ the RF power output without disturbing the power level setting of the source. Disconnect the detector and VSWR meter from the source.

5. Connect the equipment as in the experimental arrangement shown in Fig.2.9. Mount the two identical Yagi antennas on the two antenna stands. The distance between the two antennas must satisfy the far zone criterion.6. Align the two antennas for the same polarization (say vertical). Start with a minimum

distance R that satisfies the far zone criterion.7. Switch ‘ON’ the RF power. If the VSWR meter does not show any reading, increase the

transmit power by removing one or two of the attenuator pads. The VSWR meter gives the received power level Prec (dB) at distance R.

8. Record R (cm),Pref (dB) (minus), value of attenuator pad(s) removed as A(dB) (plus) and the received power level Prec (dB) (minus) in columns 1, 2, ,4 and 6, respectively, of Table 2.3.

9. Increase the distance R by 10cm at a time and record the VSWR meter readings. Do not change the RF power level setting at the source. You may remove the attenuator pads to increase the power to the transmit antenna. Record Prec (dB) and A(dB) for four different values of R in Table 2.3.

10. The experiment can be repeated at other frequencies to obtain gain versus frequency plot.

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Worksheet

1. Refer to the Calibration Graph that is provided to you. Locate the reading of column 2 on the x-axis of the graph. Read the corrected values on the y-axis and record them in column 3 as Pref(dB). Similarly get the corrected values for Prec(column 6) from the User Graph and record them as Pr(dB) I column 7.

2. The power input Pr(dB) to the transmit antenna is calculated by adding the value of the attenuator pad(s) removed to the corrected reference value Pref(dB).

For example, if Pref(dB) =-49.5dB,and one 10dB pad has been removed, then Pt(dB)=(-49.5+10)dB=-39.5dBRecord Pt(dB) in column 5of Table 2.3.

3. For each value of R, calculate (Pt-Pr)dB and enter at column 8. Calculate the power the power ratio (Pt/Pr) using the following formula.

4. Plot a graph with R (cm) along the x-axis and the power ratio√ Pt/Pr along the y-axis.

5. Form the graph, find the slope which is equal to 4π/(λ0G). the derivation of the relevant formula is given in section 2.1.7. Determine λ0 (in cm) from the frequency setting of the source and then calculate the gain G. In absolute gain is 10log10G.

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Experiment-II Determination of Coupling and Isolation Characteristics of Microstrip Directional Couplers

3.2.1 Measurement of Coupling and IsolationObjective: (a) To measure the coupling characteristics of a microstrip directional couple (b) To measure the isolation characteristics of a microstrip directional coupler Note:For both the couplers, the impedance of input/output lines is 50Ω.Choose any one coupler for your experiment. The procedure given below applies to both.Identify any one port as input port (port 1). With respect to the input port, identify the coupled port (port 3) and the isolated port (port 4). Measurement of coupling involves measuring the transmission response between the input port (port 1) and the coupled port (port 3). Similarly, measurement of isolation of the coupler involves measuring the transmission response between the input port and the isolated port (port 4). While making the measurement between any two ports, the remaining two ports will have to be terminated in 50Ωmatched loads.Procedure1. Assemble the set up as shown fig.3.6. Do not switch on the microwave signal source

or the VSWR meter until you read the instruction given at Sl. Nos.2 and 3 below.


Before switching on the signal source, rotate the RF power level knob on the front panel anti-clockwise to minimum position (lowest power output). Remember to connect a 6dB (or 10dB) attenuator pad at the RF output port of the source as shown in the diagram.Switch on the signal source in the following sequence:First Power Switch to ‘ON’ position,

Set modulation switch to AM and modulation frequency to the 1 KHz preset (click at extreme left).

3.Procedure for switching ‘ON’ the VSWR meterKeep the Range Switch in the 40dB range position and variable gain knob close to maximum. [Choice of 40dB range initially is to avoid the meter needle from kicking in case the input power is high].Switch ‘ON’ the VSWR meter.

4.To Measure the coupling First measure reference power level by connection the cable end at P to Q directly (Refer fig. 3.6). Set the frequency of the source to 2.3 GHz. Increase the RF power output of the source till the VSWR meter shows a reading in the 50dB range (say 55dB). Record the frequency (in GHz) in column 1 and the VSWR meter readings as P1i dB (minus value) in column 2 of Table 3.1. Increase the frequency of the VSWR meter. Column 2 now gives the reference input power at different frequencies.

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Next insert the coupler (branchline or parallel coupled ) between P and Q with input port (say port1) connected to P and the coupled port (port 3) to Q. Terminate ports 2 and 4 of the coupler in 50Ω matched loads. Record the readings of the VSWR meter at the above frequencies as P3s dB (minus value) in column 3of Table3.1.

6.To measure the Isolation

The value of isolation is generally much greater than coupling. Therefore, choose a higher reference values so that with the device connected, the meter needle does not go below 70dB.a. Connect the cable end at P to Q directly (Refer fig. 3.6). Set the frequency of the source to 2.3 GHz. Increase the RF power output of the source till the VSWR meter shows reading in the 40dB range (say 48dB). Record the frequency (in GHz) in column 1 and the VSWR meter readings as P2idB(minus value) in column 2 of Table 3.2. Increase the frequency of the source in steps of 0.1 GHz up to 2.8GHz and note the corresponding of the VSWR meter in column 2. Of Table 3.2. Increase the frequency of the source in steps of 0.1GHz up to 2.8GHzand note the corresponding readings of the VSWR meter. Column 2 now gives the reference input power at different frequencies.

b.Connect the isolated port (port 4) to Q. Terminate ports 2 and 3 in matched loads. Record the reading of the VSWR meter at the same frequencies as P4s d (minus value) in column 3 of the same Table.

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1. Determination of coupling in decibels:

Using the Calibration Graph, get the corrected values of P1i(column 2)of table 3.1 and record them as P1i (dB) in column 4 of the same Table.Simplify, get the corrected values of P3s (column 3) and record them as P3s(dB)in column 5 of Table 3.1.Coupling C (dB) =P1i(dB)-P3s(dB). Enter this value at the column 6 of Table 3.1.

2. Determination of Isolation in decides:

Using the Calibration graph (User graph), get the corrected values of P1i(column 2)of Table 3.2 and record them as P1i(dB) in column 4 of the same Table.Similarly, get the corrected values of P4s(dB) in column 5 of Table 3.1.

Isol. (dB) = P1i –P4s(dB). Enter this value at column 6 of Table 3.2.3. Plot C(dB) and Isol.(dB) as a function of frequency.

Note that C(dB)=|S31|(dB) and Isol.(dB)=|S41|(dB)

Branchline Directional Coupler4. Plot of coupling versus frequency: In the ideal case coupling is 3dB at the centre

frequency. In the actual device, because of the losses n the connectors and in the microstrip line, the measured coupling may be slighter higher. Away from the centre frequency, observe the variation in the coupling as a function of frequency. Explain the variation.

5. Plot of isolation vs. frequency:

6. Determine the % bandwidth of the branchline coupler corresponding to (a) coupling variation betweet 3 and 4.5dB and (b) 1dB isolation. Compare threes two bandwidths. Which of the two parameters of the coupler limits the bandwidth?

Parallel Coupled Directional Coupler

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7. Plot of coupling versus frequency: The coupling value of this coupler is provided on the component. Observe the variation in coupling as function of frequency. Explain the variation.

8. Plot of isolation vs. frequency

9. % BW

Page no 44.Voltage signals appearing at ports 2 and 3 differ in phase by 900. Further, the voltage at the coupled port 3 attains a maximum value when the electrical length θ=π/2 or the physical length L=λg0/4 where λg0 is the guide wavelength in microstrip.

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Experiment III – Measurement of Resonance Characteristics of Microstrip Ring Resonator and Determination of Dielectric Constant of the Substrate

4.1 Theory of Ring ResonatorMicrostrip ring resonators are commonly used in the design of MIC components such as filters, oscillators, and mixers. Microstrip ring resonator is formed by bending the strip conductor of a microstrip in the form of a ring. shows the configuration along with the input and output feed lines, The gap between the feed line and the ring determines the coupling. Smaller the gap, tighter is the coupling.Resonance is established when the mean circumference of the ring is equal to integral multiples of guide wavelength in microstrip.Where, R is the mean radius of the ring and n is the mode number. The other symbols are,λg = guide wavelength in the microstripεef = effective (relative)dielectric constant of the microstripν0 = free space velocityfr = resonant frequency of the ringthe expression for εef is given in (1.3) and is repeated below:where h is the height of the dielectric substrate, and w is the width of the strip conductor in the ring. We note that the lowest order resonance occurs when the mean circumference is one wavelength (n=1) in the microstrip.Ring resonator provides a simple experimental means of characterizing a microstrip substrate. For a given microstrip ring, the dimensions are R, w and the substrate height h are known. The resonant frequency of the ring is determined experimentally by measuring its transmission response and noting the frequency at which the output shows a peak. The value of εef is first calculated using (4.1). In order to calculate εr from the knowledge of εef andw/h, we can recast(4.2) in the form4.2 Experiment4.2.1 Measurement of Resonant FrequencyObjective

(a) To measure the resonance characteristics of a microstrip ring resonator

(b) To calculate the (relative) dielectric constant εr of the substrate

Parameters given: Strip conductor width (in the ring) w=1.84 mm Height of the substrate h=0.76 mm Mean radius of the ring R=12 mmProcedure


Before switching on the signal source, rotate the RF power level knob on the front panel anti-clockwise to minimum position (low power output). Connect a 3dB attenuator pad at the RF output port of the source as shown in the diagram.Switch on the signal source in the following sequence:First on Power to ‘ON’ position, then RF power switch to ‘ON’ position.Set modulation switch to AM and modulation frequency to the KHz preset (click at extreme left).

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2 Procedure for switching ‘ON’ the VSWR meter The VSWR meter is to be used in conjunction with the coaxial detector. Keep the range Switch in the 40dB range and the variable gain Knob to Maximum or close to maximum.Switch ‘ON’ the VSWR meter.4 Set the frequency of the source t 2.2Ghz. connect P to Q directly.Increase the power output of the source till the VSWR meter shows a reaing of about 45dB.5 Next insert the ring resonator between P and Q.You may notice that the power output suddenly drops. The VSWR meter may not even show any indication. That is because the ring resonator offers large attenuation away from resonance.Vary the frequency of the source slowly from 2.3GHz to 2.8Ghz and observe the frequency at which the VSWR meter reading shows a sharp peak. If no peak is observed, increase the power output of the source and vary the frequency again. Note the frequency at which the VSWR meter shows a peak. This is the (first order) resonant frequency fr of the resonator.

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4.2.2 Write-up1. For the ring resonator provided in the experiment, R=12mm, w=1.84mm and h=0.76mm. substitute the value of the measured resonant frequency in (4.1)and value of εef of the microstrip.Next, substitute the value of εef in (4.3) and calculate the relative dielectric constant εr of the substrate.

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Experiment III-B : Measurement of Power Division and Isolation Characteristics of Microstrip 3dB Power Divider

5.1 Theory of Power Divider Scattering Matrix of 3 dB Power DividerFigure shows below the line diagram of Y-junction as a power divider. Let port 1 be the input that is matched to the source that is, input reflection coefficient S11=0

As an equal-split power divider, power incident at port gets divided equally between the two output port 2 and 3. Equal power division implies |S21|=|S31|=1/√2. The phase factor of S21and S31canbe made equal to zero (multiple s of 3600) by choosing the reference planes of ports 2 and 3 symmetrical with respect to port 1. Further, the device is reciprocal.S22 = -S23 (5.3)and|S22|=|S23|=|S33|=1\2 (5.4)Matched Power DividerFigure 5.3 shows a matched power divider , popularly known also as the WIkinson power divider. It uses an isolation resistor R of value 2Z0 Between ports 2 and 3 . with this resistor, the device completely matched at all the three ports, and ports 2 and 3 are isolation from each other at the centre frequency (f0).S22= S33=0 (5.5)

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5.2.1 Measurement of Power Division and IsolationProcedure1 Assemble the set as shown in fig. 5.5 Do not switch on the microwave signal source or the VSWR meter until you the instruction given at Nos. 2 and 3 below.2 Procedure for the switching ‘ON’ the Microwave signal SourceBefore switching on the signal source, rotate the RF power level knob on the front panel anti-clockwise to minimum position. Remember to connect a 6dB (or 10dB) attenuator pad at the RF output port of the source as shown in the diagram.Switch on the signal source in the following sequence:First Power Switch to ‘ON’ position, the RF Power Switch to ‘ON’ position.Set modulation switch to AM and modulation frequency to the 1 KHz preset (click at extreme left).4 Procedure for switching ‘ON’ the VSWR meterKeep the Range Switch in the 40dB range position and the Variable Gain Knob close to Maximum. Switch ‘ON’ the VSWR meter5 To measure the power division propertyInsert the power divider between P and Q with input port (port 1) connected to P coupled port 2 to Q. Terminate port 3 in a matched load.Set the frequency of the source to 2.3GHz. record the reading of the VSWR meter as P2s dB in column 3 of table 5.1. next, interchange connections at port 2 and port 3. That is connect port 3 to Q. Terminate ports 2 in matched load. Record the reading of the VSWR meter as P3s dB in column 4 of Table 5.1.6 To measure the isolation propertyRemove the power divider from the set-up. Measure the reference power level again at the same frequencies by following the procedure given at above. Since the values of isolation are much higher, you can keep the reference level slighter higher.

(a) Set the frequency of the source to 2.3 GHz. Increase the RF power output of the source till the VSWR meter shows reading in the 40dB range (say 48dB). Record the frequency (in GHz) in column 1 and the VSWR meter readings as P2idB in column 2 of Table 5.2. Increase the frequency of the source in steps of 0.1 GHz up to 2.8GHz and note the corresponding of the VSWR meter in column 2.

Insert the power divider between P and Q with port 2 as the input connected to P and port 3 to Q. Terminate port in a matched load. Record the VSWR meter at the same frequencies as P3s dB in column 3 Table 5.2.

1. Determination of Power Division

Using the Calibration Graph, get the corrected values of p1i (column 2)of Table 5.1 and record them as P1i (dB) column 5 of the same Table.Similarly, get the corrected values of p2sand record them as P2s (dB) and P3s (dB) in columns 6 and 7, respectively of table 5.1.Power division (loss) from port 1 to port 2 = P1i(dB)-P2s (dB)= - 20log10|S21|. Denote this loss as S21(dB) and enter at column 8 of the Table 5.1Power division (loss) from port 1 to port 3 = P1i (dB) = - 20log10|S21|. Denote this loss as S31 (dB) and enter at column 9 of the Table5.1

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2. Determination of Isolation

Using the Calibration Graph, get the corrected values of P2i (column 2) and P3s (column 3) of the same Table.Isolation (dB) = P2i (dB) – P3s (dB) = 20log10|S32|.Denote this as S32(dB) and enter at column 6 of the Table 5.2

3. Plot power division S21 (dB) and S31 (dB) as a function of frequency.

Ideally the values of both these should be 3dB at the centre frequency. In the actual device, because of the losses in the connectors and in the microstrip line, the measured loss will be slightly higher. Compare the variation in the loss characteristic with the ideal response given in Fig.5.4. From the plot determine the centre frequency.4.Plot isolation S32 (dB) as a function. Compare with the ideal response and explain the difference.5.Calculate the magnitudes of the scattering parameters from the measured loss at the centre frequency. Compare with the theoretical values.

VIVA QUESTIONS

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1. Define the term Signals.2. Define the term Systems.3. Define the term Signal Space.4. Define Bandpass signals and system.5. Define Baseband signal &system.6. Give the basic difference between Analog and Digital communication.7. State Sampling Theorem.8. What is impulse sampling, Natural sampling9. Define Pulse code Modulation.10. In what way PCM is different from other Modulation?11. What is the meaning of Quantization?12. Define uniform Quantization 13. Define nonuniform Quantization.14. What is meant by Quantizing Noise?15. How Delta modulation is different from PCM?16. Which two noises are occurred in Delta modulation?17. What is meant by coherent?18. What is meant by non coherent?19. Write the advantages and disadvantages of a digital communication system.20. Explain the difference between baseband transmission and bandpass transmission.21. Give the concept of Basis Function in Digital communication.22. What do you mean by Orthogonal Basis function?23. What do you mean by Orthonormal Basis Function?24. Explain the term optimum in digital communication.25. Explain the term correlators.26. What is the application of Gram –Schmidth orthogonalization procedure?27. What is the meaning of signal space representation?28. State optimum receiver Algorithm.29. Spectral density of PSK, FSK & ASK can be improved using which factor?30. Non – coherent scheme do not require a ----- local oscillator?31. Band width required for ASK and PSK is same or not?32. FSK signal can be demodulated using ---detector?33. For each symbol 1 & 0 in PSK phase of carrier differs by --- degrees?34. In ASK symbol 1 is represented by transmitting a --- wave?35. Phase synchronization ensures these two waves in phase in the Ask receiver transmitter?36. PSK transmission is polar or non polar?37. PSK is liner or non-linear modulation scheme?38. What is microwave?39. What is the frequency range of microwave?40. What is the frequency range of UHF?41. Give the applications of microwave.42. What is a waveguide?43. Where the waveguide is used?44. What are the types of waveguide?45. Name the microwave tubes.

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46. What is Klystron?47. How much power gain is possible with two-cavity klystron amplifier?48. What is reflex klystron?49. Give its applications.50. How can klystron amplifier be converted into an oscillator?51. What is Transit time?52. What is the problem with transit time in diode?53. What do you understand by Transit time in Triode?54. Give application of two cavity kystron?55. What do mean by Bunching cavity?56. What do you understand by catcher cavity?57. What is bunching parameter?58. Name any two properties of scattering matrix.59. What are the transmitting coefficients?60. What are the reflection coefficients?61. How do you form H-plane Tee?62. What is the advantage of Magic Tee?63. What is rat race junction?64. Define coupling factor of DC.65. Define directivity of DC?66. Define isolation of DC?67. Name any two DC?68. What are ferrites?69. What is Faraday rotation?70. Name the microwave devices that make use of Faraday rotation?71. What is Scattering matrix?72. Define VSWR?73. What is Isotropic radiator?74. Define directivity.75. How is antenna efficiency calculated?76. What does the effective length of an antenna represent?77. Define antenna bandwidth.78. Where is parasitic array used?79. Define the term polarization.80. What does lower value of Q in an antenna indicate?81. What is duct propagation?82. Define skip distance.83. What is Yagi Antenna84. What is Dipole Antenna?85. What is Patch Antenna?86. Define Fresnel region87. Define beamwidth88. What is the difference between branchline and parallel couplers?89. What is PIN modulator?90. How do waves propogate in a waveguide?

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