UNIT 2: ANGLE MODULATIONPART A

1. Differentiate phase modulation and frequency modulation.

Ans: In phase modulation the phase shift of the carrier signal is varied in proportion with the amplitude of the modulating signal. In Frequency modulation, the frequency of the carrier is changed in proportion with the amplitude of the modulating signal.2. What do you understand by narrowband FM?Ans: When the modulation index is less than 1, the angle modulated systems are called low index. The bandwidth requirementof low index systems is approximately twice of the modulating signal frequency. Therefore low index systems are called narrowband FM3. Why Armstrong method of FM is superior to reactance modulator?

Ans: Reactance modulator is direct FM, where as Armstrong method is indirect FM. Armstrong method generates FM from PM. Hence crystal oscillators can be used in Armstrong method. Therefore frequency stability is better than reactance modulator.4. Differentiate between narrow band FM and wideband FM

Ans: In narrow band FM, the frequency deviation is very small. Hence the frequency spectrum consists of two major sidebands likeAM. Other sidebands are negligible and hence they can be neglected. Therefore the bandwidth of narrowband FM is limited only to twice of the highest modulating frequency. If the deviation in carrier frequency is large enough so that other sidebands cannot be neglected, then it is called wideband FM. Thebandwidth of wideband FM is calculated as per Carson’s rule.5. What is direct FM?Ans: In this type of angle modulation, the frequency of the carrier is varied directly by the modulating signal. This means;

an instantaneous frequency deviation is directly proportional to amplitude of the modulating signal6. What is indirect FM?Ans: In this type of angle modulation, FM is obtained by phase

modulation of the carrier. This means, an instantaneous phase of

the carrier directly proportional to amplitude of the modulating

signal

7. Define instantaneous frequency deviation.

Ans: The instantaneous frequency deviation is the instantaneous

change in the frequency of the carrier and is defined as the

first derivative of the instantaneous phase deviation.

8. Define frequency deviation.

Ans: Frequency deviation is the change in frequency that occurs

in the carrier when it is acted on by a modulating signal

frequency. The peak frequency deviation is simply the product of

the deviation sensitivity and the peak modulating signal voltage

and is expressed mathematically as Df=K1 Em Hz

9. State Carson rule.

Carson rule states that the bandwidth required to transmit an

angle modulated wave is twice the sum of the peak frequency

deviation and the highest modulating signal frequency.

Mathematically carson’s rule is

B=2(δ +fm) Hz.

10. Define Deviation ratio.

Ans : Deviation ratio is the ratio of maximum frequency deviation

to the maximum modulating signal frequency. Mathematically, the

deviation ratio is

Deviation ratio = maximum frequency deviation / fm (max)

11. Define percentage of modulation for angle modulation

Ans: The percentage of modulation is given as the ratio of actual

frequency deviation to maximum allowable frequency deviation.

Percent modulation = (∆f (actual)/∆f (max))*10012. What do you mean by Tuned Frequency Discriminator?Ans: Tuned Frequency Discriminators convert FM to AM and then demodulates the envelope using conventional Peak Detectors.13. What are the merits and demerits of angle modulation?

AnsMerits

Noise reduction

Improved System Fidelity

More Efficient Use of Power.

Demerits Requires wider bandwidth

The transmitter and receiver circuits are

very complex

14. What is the bandwidth required for an FM wave in which the

modulating frequency signal is 2 KHz and the maximum frequency

deviation is 12 KHz?

Ans : 2( 2 + 12 ) K =28 K Hz.

15. For FM modulator with deviation sensitivity k1=5 KHz, Em(t)=

2cos(2Π2000t) determine ∆f and m.

Ans: ∆f = k1Em=5KHz*2=10KHz.

m=∆f/fm=10KHz/2000 =5.

16. Compare FM and PM.

Frequency Modulation Phase Modulation

The frequency of carrier

signal is varied in

accordance with the

instantaneous value of the

modulating signal.

The phase of carrier

signal is varied in

accordance with the

instantaneous value of the

modulating signal.The maximum frequency

deviation depends upon the

amplitude of modulating

voltage and modulating

signal frequency.

The maximum phase

deviation depends upon the

amplitude of modulating

voltage.

Modulation index is

increased as modulating

signal frequency is

reduced and vice versa.

Modulation index remains

same with change in

modulating signal

frequency.

17. Compare AM and FM.

Amplitude Modulation Frequency ModulationThe amplitude of high

frequency carrier signal is

varied in accordance with

the instantaneous value of

the modulating signal.

The frequency of carrier

signal is varied in

accordance with the

instantaneous value of the

modulatingPoor fidelity Better FidelityOnly 2 sidebands Infinite number of

sidebandsTransmitter and receiver

circuits are simple.

Transmitter and receiver

circuits are complex.18. What are the types of FM demodulators?

Slope Detector

Foster Seeley Discriminator

Ratio Detector

PLL FM Demodulator

PART - B

1. With necessary block diagram explain Direct method and

indirect method of FM transmission (Crosby method and Armstrong

Method) (16)

FM TRANSMITTERS

Crosby Direct MethodAs previously stated, if a crystal oscillator is used to

provide the carrier signal, the frequency cannot be varied too much (this is a characteristic of crystal oscillators). Thus, crystal oscillators cannot be used in broadcast FM, but other oscillators can suffer from frequency drift. An automatic frequency control (AFC) circuit is used in conjunction with a non-crystal oscillator to ensure that the frequency drift is minimal.Figure 4 shows a Crosby direct FM transmitter which contains an AFC loop. Thefrequency modulator shown can be a VCO since the oscillator frequency as much lowerthan the actual transmission frequency. In this example, the oscillator centre frequency is5.1MHz which is multiplied by 18 before transmission to give ft =91.8MHz.

Calculation of Modulation IndexWhen the frequency is multiplied, so are the frequency and

phase deviations. However,the modulating input frequency is obviously unchanged, so the modulation index ismultiplied by 18. The maximum frequency deviation at the output is 75kHzmaximum allowed deviation at the modulator output is

Since the maximum input frequency is fm = 15kHz for broadcast FM, The modulation index at the antenna then is = 0.2778 x 18 =

5.The AFC loop aims to increase the stability of the output withoutusing a crystaloscillator in the modulator

.The modulated carrier signal is mixed with a crystal reference signal in a non-linear device. The band-pass filter provides the difference in frequency between the master oscillator and the crystal oscillator and this signal is fed into the frequency discriminator.The frequency discriminator produces a voltage proportional to the difference between the input frequency and

its resonant frequency. Its resonant frequency is 2MHz, which will allow it to detect low frequency variations in the carrier.

The output voltage of the frequency discriminator is added to the modulating input tocorrect for frequency deviations at the output. The low-pass filter ensures that the frequency discriminator does not correspond to the frequency deviation in the FM signal (thereby preventing the modulating input from being completely cancelled).

Indirect Armstrong method.Indirect transmitters have no need for an AFC circuit because thefrequency of the crystalis not directly varied. This means that indirect transmitters provide a very stable output,since the crystal frequency does not vary with operating conditions.

Figure shows the block diagram for an Armstrong indirect FM transmitter. This worksby using a suppressed carrier amplitude modulator and adding a phase shifted carrier to

this signal. The effect of this is shown in figure 6, where the pink signal is the output andthe blue signal the AM input. The output experiences both phase and amplitudemodulation. The amplitude modulation can be reduced by using a carrier much largerthan the peak signal amplitude, as shown in figure 7. However, this reduces the amountof phase variation.

The disadvantage of this method is the limited phase shift it canprovide. The rest offigure 5 shows the frequency shifting to the FM broadcast band bymeans of frequencymultiplication (by a factor of 72), frequency shifting and frequency multiplication again.

This also multiplies the amount of phase shift at the antenna, allowing the required phaseshift to be produced by a small phase variation at the modulator output.

2. With neat circuit diagram explain the operation Ratio detector

and Foster Seeley Discriminator.(16)

Ratio detectors When circuits employing discrete components were more widely

sued, the Ratio and Foster-Seeley detectors were widely used. Of these the ratio detector was the most popular as it offers a better level of amplitude modulation rejection of amplitude modulation. This enables it to provide a greater level of noise immunity as most noise is amplitude noise, and it also enables the circuit to operate satisfactorily with lower levels of limiting in the preceding IF stages of the receiver.

The operation of the ratio detector centres around a frequency sensitive phase shift network with a transformer and the diodes that are effectively in series with one another. When a steady carrier is applied to the circuit the diodes act to produce a steady voltage across the resistors R1 and R2, and the capacitor C3 charges up as a result. The transformer enables the circuit to detect changes in the frequency of the incoming signal. It has three windings. The primary and secondary act in the normal way to produce a signal at the output. The third winding is un-tuned and the coupling between the primary and the third winding is very tight, and this means that the phasing between signals in these two windings is the same.

The primary and secondary windings are tuned and lightly coupled. This means that there is a phase difference of 90 degrees between the signals in these windings at the centre frequency. If the signal moves away from the centre frequency thephase difference will change. In turn the phase difference between the secondary and third windings also varies.When this occurs the voltage will subtract from one side of the secondary and add to the other causing an imbalance across the resistors R1

and R2. As a result this causes a current to flow in the third winding and the modulation to appear at the output. The capacitors C1 and C2 filter any remaining RF signal which may appear across the resistors. The capacitor C4 and R3 also act as filters ensuring no RF reaches audio section of the receiver. When this occurs the voltage will subtract from one side of the secondary and add to the other causing an imbalance across the resistors R1 and R2. As a result this causes a current to flow inthe third winding and the modulation to appear at the output. The capacitors C1 and C2 filter any remaining RF signal which mayappear across the resistors. The capacitor C4 and R3 also act as filters ensuring no RF reaches the audio section of the receiver.

The Foster Seeley detector has many

similarities to the ratio detector. The circuit topology looks very similar, having a transformer and a pair of diodes, but there is no third winding and instead a choke is used.

The Foster-Seeley detector

Like the ratio detector, the Foster-Seeley circuit operates using a phase difference between signals. To obtain the differentphased signals a connection is made to the primary side of the transformer using a capacitor, and this is taken to the centre tap of the transformer. This gives a signal that is 90 degrees out of phase.

When an un-modulated carrier is applied at the centre frequency, both diodes conduct, to produce equal and opposite voltages across their respective load resistors. These voltages cancel each one another out at the output so that no voltage is present. As the carrier moves off to one side of the centre frequency the balance condition is destroyed, and one diode conducts more than the other. This results in the voltage across one of the resistors being larger than the other, and a resultingvoltage at the output corresponding to the modulation on the incoming signal.

The choke is required in the circuit to ensure that no RF signals appear at the output. The capacitors C1 and C2 provide a similar filtering function. Both the ratio and Foster-Seeley detectors are expensive to manufacture. Wound components like

coils are not easy to produce to the required specification and therefore they are comparatively costly. Accordingly these circuits are rarely used in modern equipment.

3. How FM is generated with the help of Varactor diode both

Direct method and indirect method? Explain with the help of neat

diagram. (16)

4. i) Describe about PLL with block diagram (8 marks)

The way in which a phase locked loop, PLL FM demodulatorworks is relatively straightforward. It requires no changes tothe basic phase locked loop, itself, utilising the basicoperation of the loop to provide the required output.

When used as an FM demodulator, the basic phase locked loopcan be used without any changes. With no modulation applied andthe carrier in the centre position of the pass-band the voltageon the tune line to the VCO is set to the mid position. Howeverif the carrier deviates in frequency, the loop will try to keepthe loop in lock. For this to happen the VCO frequency mustfollow the incoming signal, and in turn for this to occur thetune line voltage must vary. Monitoring the tune line shows thatthe variations in voltage correspond to the modulation applied tothe signal. By amplifying the variations in voltage on the tuneline it is possible to generate the demodulated signal.

PLL FM demodulator performanceWhen designing a PLL system for use as an FM demodulator, one ofthe key considerations is the loop filter. This must be chosen tobe sufficiently wide that it is able to follow the anticipatedvariations of the frequency modulated signal. Accordingly theloop response time should be short when compared to theanticipated shortest time scale of the variations of the signalbeing demodulated.A further design consideration is the linearity of the VCO. Thisshould be designed for the voltage to frequency curve to be aslinear as possible over the signal range that will beencountered, i.e. the centre frequency plus and minus the maximumdeviation anticipated.In general the PLL VCO linearity is not amajor problem for average systems, but some attention may berequired to ensure the linearity is sufficiently good for hi-fisystems.

4. ii) Explain the operation of the slope detector, giving the circuit diagram and response characteristics.(8 marks)FM The very simplest form of FM demodulation is known as slopedetection or demodulation. It consists of a tuned circuit that istuned to a frequency slightly offset from the carrier of thesignal.As the frequency of the signals varies up and down in frequencyaccording to its modulation, so the signal moves up and down theslope of the tuned circuit. This causes the amplitude of thesignal to vary in line with the frequency variations. In fact atthis point the signal has both frequency and amplitudevariations.It can be seen from the diagram that changes in the slope of thefilter, reflect into the linearity of the demodulation process.The linearity is very dependent not only on the filter slope asit falls away, but also the tuning of the receiver - it isnecessary to tune the receiver off frequency and to a pint wherethe filter characteristic is relatively linear.The final stage in the process is to demodulate the amplitudemodulation and this can be achieved using a simple diode circuit.

One of the most obvious disadvantages of this simple approach isthe fact that both amplitude and frequency variations in theincoming signal appear at the output. However the amplitudevariations can be removed by placing a limiter before thedetector.A variety of FM slope detector circuits may be used, but the onebelow shows one possible circuit with the applicable waveforms.The input signal is a frequency modulated signal. It is appliedto the tuned transformer (T1, C1, C2 combination) which is offsetfrom the centre carrier frequency. This converts the incomingsignal from just FM to one that has amplitude modulationsuperimposed upon the signal.

This amplitude signal is applied to a simple diode detectorcircuit, D1. Here the diode provides the rectification, while C3removes any unwanted high frequency components, and R1 provides aload.

ADVANTAGES DISADVANTAGES Simple - can be used to

provide FM demodulationwhen only an AM detectoris present.

Enables FM to bedetected without anyadditional circuitry

Not linear as the outputis dependent upon thecurve of a filter.

Not particularlyeffective as it relieson centring the signalpart of the way down thefilter curve wheresignal strengths are

ADVANTAGES DISADVANTAGESless.

Both frequency andamplitude variations areaccepted and thereforemuch higher levels ofnoise and interferenceare experienced.

5. i) Explain in detail about frequency receiver with block diagram.(10 marks)

ii) What are the advantages of FM over AM? (6) FM has following advantages over AM.

i) The amplitude of FM is constant. It is independent of depth of

modulation. Hence transmitter power remains constant in FM

whereas it varies in AM.

ii) Since amplitude of FM constant, the noise interference is

minimum in FM. Any noise superimposing amplitude can be removed

with the help of amplitude limits. Whereas it is difficult to

remove amplitude variations due to noise in AM.

iii) The depth of modulation has limitation in AM. But in FM the

depth of modulation can be increased to any value by increasing

the deviation. This does not cause any distortion in FM signal.

iv)Since guard bands are provided in FM, there is less

possibility of adjacent channel interference.

v) Since space waves are used for FM, the radius of propagation

is limited to line of sight. Hence it is possible to operate

several independent transmitters on same frequency with minimum

interference.

vi)Since FM uses UHF and VHF ranges, the noise interference is

minimum compared to AM which uses MF and HF ranges.