....... ]:
KL-900ABasic Communication Trainer
MODULE EXPERIMENT MANUAL
K&H MFG CO., LTD.5F, No. 8, Sec. 4 Tzu-Chiang Rd., San Chung City 241, Taipei Hsien, Taiwan R.O.C.TEL 886-2-2286-0700 FAX 886-2-2287-3066E-Mail [email protected] WEB http://www.kandh.com.tw
KL-900A'c Communication TrainerBa
KL-900A CONTENTS
CONTENTSUnit 1 FR Oscillators
1.1 Objectives 1 -1
1,2 Discussion Of Fundamentals ................................ 1 -1
1.3 Equipments Required 1 -7
1.4 Experiments And Records
1 -7
Experiment 1-1 Colpitts Oscillator
Experiment 1-2 Hartley Oscillator
1.5 Questions
Unit 2 Second-order Filters
2.1 Objectives 2-1
2.2 Discussion Of Fundamentals ....... ......... ...... . ..... ........ 2-12.3 Equipments Required 2-9
2.4 Experiments And Records 2-9
Experiment 2-1 Second-order Low-pass Filter
Experiment 2-2 Second-order High-pass Filter
2.5 Questions............ ............ ....... ........ .......... ........ ........ 2-16
Unit 3 AM Modulators
3.1 Objectives 3-1
3.2 Discussion Of Fundamentals 3-1
3.3 Equipments Required 3-63.4 Experiments And Records 3-6
Experiment 3-1 Amplitude Modulator3.5 Questions .... 3-13
Unit 4 AM Demodulators
4.1 Objectives. 4-14.2 Discussions Of Fundamentals 4-1
4.3 Equipments Required 4-6
KL900A-E080327-EB
KL-900A CONTENTS
8.3 Equipments Required ............. . ... ...... .............
8.4 Experiments And Records...... ...... ............ ............... ......
........ 8-10
8-10Experiment 8-1 LM565 PLL Characteristic MeasurementsExperiment 8-2 LM565 V-F Characteristic MeasurementsExperiment 8-3 PLL Frequency DemodulatorExperiment 8-4 FM to AM Frequency Demodulator
8.5 Questions 8-22
Unit 9 A/D Converter
9.1 Objectives .......... . ............ .............. ............ ...... 9-1
9.2 Discussions Of Fundamentals
9.3 Equipments Required 9-10
9.4 Experiments And Records 9-10
Experiment 9-1 ADC0804 ConverterExperiment 9-2 ADC0809 Converter
9.5 Questions 9-15
Unit 10 D/A Converter
10.1 Objectives.... ..... ........................ ..... . ..... ..... • • .......... •• • • 10-110.2 Discussions Of Fundamentals 10-110.3 Equipments Required 10-910.4 Experiments And Records • • • • • • • • • • • •• • • • ...... -• ..... 10-9
Experiment 10-1 DAC0800 Unipolar Voltage OutputExperiment 10-2 DAC0800 Bipolar Voltage Output
10.5 Questions. ........ ............... ...... ............ .......... ..... 10-13
Unit 11 PWM Modulators
11.1 Objectives......... .............. ...... ......... ...... 11-1
11.2 Discussions Of Fundamentals . ......... ......... 11-1
11.3 Equipments Required......................... ...... ........ ....... ........... 11-8
11.4 Experiments And Records ...... 11-8Experiment 11-1..Pulse Width Modulator Using uA741
Experiment 11-2 Pulse Width Modulator Using LM55511.5 Questions ................................. .... •
Unit 12 PWM Demodulators
12.1 Objectives._.... 12-1
.... . ..... 12-1 12.2 Discussions Of Fundamentals
III KL900A-E080327-EB
KL-900A CONTENTS
16.4 Experiments And R
Experiment 16-1
Experiment 16-2
Experiment 16-3
Experiment 16-4
ecords .. . .....
CVSD Modulator
CVSD Demodulator
Lowpass Filter
CVSD System at Various Clock Rates
16 .5 Questions... ..... .......... ...... .................. ....... ............ ............ 16-17
Unit 17 Manchester CVSD
17.1 Objectives...... ...... ............. ........ . ..... ..... ............ ......... 17-1
17.2 Discussions Of Fundamentals
17.3 Equipments Required
17.4 Experiments And Records..
Experiment 17-1
Experiment 17-2
Experiment 17-3
17.5 Questions..........
Manchester Encoder
Manchester Decoder
Manchester CVSD System
17-14
17-1
17-8
17-8
Unit 18 ASK System
18.1 Objectives
18.2 Discussions Of Fundamentals
18-1
18-1
18.3 Equipments Required 18-11
18.4 Experiments And Records......................... . ..... ...... ........ 18-11
Experiment 18-1 ASK Modulator
Experiment 18-2
Experiment 18-3
Experiment 18-4
18.5 Questions
Noncoherent ASK Demodulator
Manchester CVSD System
Coherent ASK Demodulator
18-22
Unit 19 PSK/QPSK System
19.1 Objectives
19-119.2 Discussions Of Fundamentals ....... ....... ......... ..... 19-1
19.3 Equipments Required......... ...... .......... ......... ............... ..... 19-12
19.4 Experiments And Records ..... ..... ...... ........ 19-12
Experiment 19-1 Measurement and Adjustment
Experiment 19-2 PSK/QPSK Modulator
Experiment 19-3 PSK/QPSK Demodulator19.5 Questions . ... .......... .... .................. ..... ......... ...... 19-31
V KL900A-E080327-EB
RF OSCILLATORS
1.1 Objectives 1 -1
1.2 Discussion Of Fundamentals 1 -1
1.3 Equipments Required 1 -7
1.4 Experiments And Records ...... 1-7
Experiment 1-1 Colpitts Oscillator
Experiment 1-2 Hartley Oscillator
1.5 Questions. 1-11
Unit 1 RF Oscillators
1.1 OBJECTIVES
Understanding the operation and characteristics of radio-frequency (RF)
oscillators.
Designing and implementing oscillators.
1.2 DISCUSSION OF FUNDAMENTALS
An oscillator is simply a signal generator converting its dc supply voltage into a
continuously repeating ac output signal without any input signal. Oscillators play
very important roles in communication systems. An oscillator generates the
carrier or local oscillation signal used in any communication system.
Fig.1-1 shows the basic block diagram of oscillator. It includes an amplifier and
a feedback network constructed by the resonator. When dc power is first
applied to the circuit, noise will appear in the circuit and is amplified by the
amplifier and then fed to the input through the feedback network that is a
resonant circuit with filter function. The feedback network permits the signal
frequency equaling the resonant frequency to pass and rejects other
frequencies. The feedback signal will be amplified and fed back again. If the
feedback signal is in phase with the signal at input and voltage gain is enough,
the oscillator will be operation.
For proper operation, an oscillator must meet Barkhausen criterion.
Barkhausen criterion is the relationship between the amplifier's gain A and the
oscillator's feedback factor 3(s) and should be equal to 1. That is
A fl(s) 1 ( 1 -1)
where
A : amplifier's gain
16' (s) : oscillator's feedback factor
Unit 1 RF Oscillators
If the frequency is not very high, the internal capacitances of transistor can be
neglected and the oscillating frequency of Colpitts oscillator can be calculated
by the formula
1 ( Hz )
27r1 + C2
L( Cl C2 )
(1-2)
Output
Fig.1-2 AC equivalent of Colpitts oscillator
In Colpitts oscillator circuit, the feedback factor 13 is C,/C2 and the voltage gain A
is g R. By Eq. (1-1)
A 13(S) = 1
we obtain
,= 1
C2
or
C2gm R = —
ci
For starting oscillation, the loop gain should be at least 1 so that the oscillation
condition can be expressed by
1 -3
Unit 1 RF Oscillators
If operating frequency is not very high, the spray capacitance of transistor canbe neglected and the oscillating frequency is determined by the componentvalues of parallel-resonant circuit and can be calculated by the formula
1( Hz )
277-AL, + L2 )C (1-4)
Output
Fig..1-4 AC equivalent of Hartley oscillator
In Hartley oscillator circuit, the feedback factor # is L2 /Li and the voltage gain A
is g„,R . By Eq. (1-1)
A NS) = 1
we obtain
gr„R---2- =1
or
L,g„,R =
L2
1-5
Unit 1 RF Oscillators
1.3 EQUIPMENT REQUIRED
1 - Module KL-920012 - Module KL-930013 - Oscilloscope4 - LCR Meter
1.4 EXPERIMENTS AND RECORDS
Experiment 1-1 Colpitts Oscillator
.
▪
Locate Colpitts Oscillator circuit on Module KL-93001.Insert connect plugs
in J1 and J3 to set C3 = 0.001 C4 = 0.01511F and L 1 = 27 [tFl.
2. Set the vertical input of oscilloscope to AC position and connect to output
terminals (0/P). Observe and record the waveform and frequency in Table1-1. If the circuit operates improperly, recheck the dc bias of transistor.
3. Remove the connect plugs from J1 and J3. Using the LCR meter, measure
the values of C3, 04 and L 1 and record the results in Table 1-1, and thencalculate the output frequency.
4. Insert connect plugs in J2 and J4 to change C3 to C5(100 pF), C4 to
C6(1000 pF), and L 1 to L2(2.7 pH). Repeat steps 2 and 3.
Unit 1
RF Oscillators
Table 1 - 1
03 04 L1 Output Waveform
Nominal
Value0.001
liF
0.015
pf
27
p.H1
Calculated f,=
Measured fo=
Measured
Value
Nominal
Value100
pF
1000
pF
2.7
ii.H
Calculated fo=
Measured f,=
Measured
Value
1 -9
Unit 1 RF Oscillators
1.5 QUESTIONS
In experiments 1-1 and 1-2, do the calculated and measured values of outputsignal agree? Explain.
What is the function of each capacitor or inductor in Colpitts oscillator circuitshown in Fig. 1-3?
Determine the values of C3 , L1 and L2 of Hartley oscillator shown in Fig.1-5 forthe oscillating frequency of 5MHz.
When the operating frequency is in radio-frequency range, why we must payattention to the layout of circuit and the length of wire?
SECOND-ORDER FILTERS
2.1 Objectives . .
2.2 Discussion Of Fundamentals. 2-12.3 Equipments Required. 2-9
2.4 Experiments And Records 2-9
Experiment 2-1 Second-order Low-pass FilterExperiment 2-2 Second-order High-pass Filter
2.5 Questions. 2-16
Unit 2 Second-Order Filters
2.1 OBJECTIVES
Understanding the characteristics of filters.
Understanding the advantages of active filters.
3. Implementing second-order filters with integrator circuit.
2.2 DISCUSSION OF FUNDAMENTALS
Filters, which exist everywhere in communication systems, are
designed to pass a specified band of frequencies while attenuating all
signals outside this band.
Filters are usually classified according to filtering range, frequency
response in pass band, and circuit component. Classified by filtering
range, there are four types of filters: low-pass, high-pass, band-pass,
and band-reject filters. According to frequency response in pass band,
there are two types: Butterworth and Chebyshev filters. According to
circuit component, they are active and passive filters.
Passive filters are the circuits that contain only passive components
(resistors, inductors and capacitors) connected in such a way that
they will pass certain frequencies while rejecting others. Active filters,
which are the only type covered in this chapter, employ active
components (transistors or operational amplifiers) plus resistors,
inductors and capacitors. Active filters are widely used in modern
communication systems, because they have the following advantages:
1. Because the transfer function with inductive characteristic can be achieved
by particular circuit design, resistors can be used instead of inductors.
2-1
Unit 2 Second-Order Filters
C
Vin
out
1/Q
CO0
S
Fig.2-1 Miller integrator
COo
S
Output
( Vout)
Fig.2-2 Block diagram of a second-order low-pass filter
From Eq. (2-1), we can find that the Miller integrator circuit is a
first-order low-pass filter. Therefore, a second-order low-pass
filter can be easily constructed by cascading two Miller integrators
with an inverting amplifier.
The block diagram of second-order low-pass filter, shown in Fig. 2-2, is
consisted of two Miller integrators, a unity-gain inverting amplifier and adder.
Therefore, the transfer function is
2-3
R3 15 k
AA(R, 10 k
C2 1000 pF
—I I-Vin 0-1\AAriT-
12
Ci 1000 p
R1 .5 k6-11—•
14
U1:CLM348
0 Vout
U 1:A
LM348 1 :BLM348
R 15KR, 15K
Unit 2 Second-Order Filters
C2 and U.,:B is the second miller integrator and the combination of R 5 , R6 and
U i :C is a unity-gain inverting amplifier. Since this circuit design satisfies the
Butterworth criteria, the response curve in its pass band is flat and no ripple.
Fig.2-3 Second-order low-pass filter circuit
Q/K
1/Q
Vin S
K Vout
Fig.2 -4 Block diagram of a second-order high-pass filter
2-5
R5 s2
Vow (S)R2
Vm(S) S2 + R3C 5R
I S +1
R4 C2
R 2S
R2( 2-9 )
S2 +VR
4R5 X 1
R3 C I R4 R5 R4 R5C2
then
Unit 2 Second-Order Filters
Comparing these two figures, the U l :A performs the functional combination of
the first adder and Miller integrator. The U i :B performs the functional
combination of the second adder and the unity-gain inverting amplifier. If
C1=C2=C
R7=R6=R5
the transfer function will
R5 s2
be
R5 ( 1 R2 \
Voia (S) R2 CR2 R, RiR42-8 )(
V^ (S) S 2 +1
S 1R3C R4R,C2
and if
R 1 R4=R2R3
Comparing Eqs. (2-7) to (2-9), we yield
2-7
Unit 2 Second-Order Filters
2.3 EQUIPMENT REQUIRED
1 - Module KL-92001
2 - Module KL-93001
3 - Oscilloscope
2.4 EXPERIMENTS AND RECORDS
Experiment 2-1 Second-Order Low-Pass Filter
1. Locate the Second Order LPF circuit on Module KL-93001. Insert
connect plugs in J1 and J2 to set C 1 = 02 = 0.0011AF.
2. Connect a 100mVp-p, 10Hz sine wave to the input (I/P). Using the
oscilloscope, observe the output signal and record the output
amplitude in Table 2-1.
3 Observe and record the output amplitudes in Table 2-1 for input
frequencies of 100Hz, 1KHz, 2KHz, 5KHz, 8KHz, 10KHz, 20KHz,
50KHz and 100KHz.
4. Calculate each voltage gain for each input frequency and record
the results in Table 2-1.
Using the results of Table 2-1, sketch Bode plot of voltage gain in
Fig. 2-6.
6. Remove the connect plugs from J1 and J2 and then insert them in
J3 and J4 to set C3=C4=0.01[1.F.
2-9
Unit 2 Second-Order Filters
Experiment 2-2 Second-Order High-Pass Filter
1 Locate Second Order HPF circuit on Module KL-93001. Insert
connect plugs in J1 and J2 to set C 1 = C2 = 0.004711F.
2. Connect a 100mVp-p, 10Hz sine wave to input (I/P). Using the
oscilloscope, observe the output signal and record the output
amplitude in Table 2-3.
3. Observe and record the output amplitude in Table 2-3 for input
frequencies of 100Hz, 1KHz, 2KHz, 5KHz, 8KHz, 10KHz, 20KHz,
50 KHz and 100 KHz.
4. Calculate each voltage gain for each input frequency and record
the results in Table 2-3.
5. Using the results of Table 2-3, sketch Bode plot of voltage gain in
Fig. 2-8.
E16. Remove the connect plugs from J1 and J2 and then insert them in
J3 and J4 to set C3=C4=0.0151AF.
7. Observe and record the output amplitude in Table 2-4 for input
frequencies of 10Hz, 100Hz, 200Hz, 500Hz, 800Hz, 1KHz, 2KHz,
5KHz, 10KHz and 100KHz.
8. Calculate each voltage gain for each input frequency and record
the results in Table 2-4.
9. Using the results of Table 2-4, sketch Bode plot of voltage gain in
Fig. 2-9.
2-11
Unit 2 Second-Order Filters
Table 2-2
(C1 = 02 = 0 01[1.F)
InputFrequency
(Hz)
10 100 200 500 800 1k 2k 5k 10k 100k
OutputAmplitude
(mV)
Voltage Gain
(dB)
VoltageGain
(dB)
Frequency ( Hz )
Fig.2-7
2-13
Unit 2 Second-Order Filters
Table 2-4
(C = 02 = 0 01 51.19
InputFrequency
(Hz)
10 100 200 500 800 1k 2k 5k 10k 100k
OutputAmplitude
(mV)
Voltage Gain
(d B)
VoltageGain
(dB)
Frequency ( Hz )
Fig.2-9
2-15
AM MODULATORS
3.1 Objectives.................. ....................... 3-1
3.2 Discussion Of Fundamentals 3-1
3.3 Equipments Required 3-6
3.4 Experiments And Records 3-6
Experiment 3-1 Amplitude Modulator3.5 Questions... ..... ....... ...... ............... ...... ........ 3-13
Unit 3 AM Modulators
3.1 OBJECTIVES
Understanding the principle of amplitude modulation (AM).Understanding the waveform and frequency spectrum of AM signal andcalculating the percent of modulation.
3. Designing an amplitude modulator using MC1496.Measuring and adjusting an amplitude modulator circuit
3.2 DISCUSSION OF FUNDAMENTALS
Modulation is the process of impressing a low-frequency intelligence signal ontoa high-frequency carrier signal. Amplitude Modulation (AM) is a process that ahigh-frequency carrier signal is modulated by a low-frequency modulatingsignal (usually an audio). In amplitude modulation the carrier amplitude varieswith the modulating amplitude, as shown in Fig. 3-1. If the audio signal isA mcos(2 nfmt) and the carrier signal is A sos(2 nf,t), the amplitude-modulated signalcan be expressed by
X 4m(t) = [ Amcos(2#,M] A, cos(2nf, t)
= p +mcos(27if.t)}A,cos(271f,t)
= Apc A, [1+ mcos(27ifmt)]cos(27-tfet)( 3-1 )
whereApc = dc levelA m = audio amplitudeAc = carrier amplitude
fm = audio frequency
= carrier frequency
m = modulation Index or depth of modulation = A m /A DC
3-1
Aix. A c
0.5mA 1„ 0.5mA DC A
Unit 3 AM Modulators
fc fm fc f fm f(Hz)
Fig.3-2 Spectrum of AM signal
The m in Eq.(3-1), called modulation index or depth of modulation, is animportant parameter. When m is a percentage, it is usually called percentagemodulation. It is defined as
Modulating Amplitude x 100% =
Am x 100%
m= DC Level A Dc
( 3-3 )
It is difficult to measure the A DC in a practical circuit so that the modulation indexis generally calculated by
E.— E„, x100%Ems +
where Emax=i4c+A„, and Emin =A c-A,, , as indicated in Fig. 3-1.
( 3-4 )
As mentioned above, audio signal is contained in the side bands so that thegreater the sideband signals the better the transmitting efficiency. FromEq.(3-2), we can also find that the greater the modulation index, the greater thesideband signals and the better the transmitting efficiency. In practice, themodulation index is usually less or equal to 1; if m > 1, it is called overmodulation.
X( 1 )
( V )
3-3
4)Modulatinginput
o Gain° adjust
Bias adjust(5)
(12)O_
0+ Output(6)
(10)Carrier oinput +0
(8)
R3500(14)
-V 0
C2. 0Audio _ ,
input O--II I
C0.1uF
Carrierinput o— II
R451
luF
14 5RI R2 R R610K 10K 51 51
R9
6.8K
VR 150K-5V
R3IK
R71K
RII
3.9K
0+12VC3
0.1uFR8 1K
EAM—39K
2 38
Coutput
100.1 uF
MC1496
1 124
Unit 3 AM Modulators
Fig.3-3 MC1496 internal circuit
Fig. 3-4 shows an AM modulator circuit whose carrier and audio signals are
single-ended inputs, carrier to pin 10 and audio to pin 1. The gain of entire
circuit is determined by the R8 value. The R9 determines the amount of bias
current. Adjusting the amount of VR1 or the audio amplitude can change the
percentage modulation.
Fig.3-4 Amplitude modulator using MC1496
3-5
Unit 3 AM Modulators
7. Repeat steps 4 and 5.
8. Connect a 150mVp-p, 1 kHz sine wave to the input (I/P2), and a 100
mVp-p, 100kHz sine wave to the carrier input (I/P1).
9. Using the oscilloscope, observe the AM signal at output terminal
(0/P) and record the result in Table 3-3.
10. Using the spectrum analyzer, observe and record output spectrum
in Table 3-3.
11. Using the results above and Eq. (3-4), calculate the percentage
modulation of output signal and record the results in Table 3-3.
1E12. Repeat steps 9 to 11 for carrier amplitudes of 200mVp-p and
300mVp-p.
13. Connect a 150mVp-p, 3kHz sine wave to the audio input (I/P2),
and a 250mVp-p, 100kHz sine wave to the carrier input (I/P1).
14. Using the oscilloscope, observe the modulated signal at output
terminal (0/P) and record the result in Table 3-4.
1 5 . Using the spectrum analyzer, observe and record the output signal
spectrum in Table 3-4.
16. Using the results above and Eq. (3-4), calculate and record the
percentage modulation of output signal in Table 3-4.
E 17. Repeat steps 14 to 16 for the audio frequencies of 2kHz and 1kHz.
3-7
Unit 3 AM Modulators
Table 3-2
(Vc=250mVp-p, fc=100kHz, fm=1 kHz)
AudioAmplitude
Output Waveform Output Signal SpectrumPercentageModulation
250 mVp-p
Emax=
Em,n
200 mVp-p
E = min
150 mVp-p
E„ax=
Emit,
Unit 3 AM Modulators
Table 3-4
(V,=250mVp-p, Vm = 150mVp-p, fa =100 kHz)
AudioFrequency Output Waveform Output Signal Spectrum
PercentageModulation
3 kHz
Ems
Emin
2 kHz
F=-nun(
Emirs
1 kHz
Emax-Emit,
Unit 3 AM Modulators
3.5 QUESTIONS
In Fig. 3-4, if we change the value of R8 from 1 kS2 to 2 IcC2, what is the
variation of the AM output signal?
In Fig. 3-4, if we change the value of R9 from 6.8 k0 to 10 1(0, what is
the variation in the dc bias current of the MC1496?
Determine the ratio of Em„ to Emm if m=50%.
What is the function of the VR1?
3-13
AM DEMODULATORS
4.1 Objectives...... ...... ..... ......... ...... ...............
4.2 Discussion Of Fundamentals 4-1
4.3 Equipments Required.................. ........ ...... ........ 4-6
4.4 Experiments And Records . ....... . 4-6
Experiment 4-1
Experiment 4-2
4.5 Questions
Diode Detector
Product Detector
4-13
Unit 4 AM Demodulators
4.1 OBJECTIVES
Understanding the principle of amplitude demodulation.
Implementing an amplitude demodulator with diode.
3. Implementing an amplitude demodulator with a product detector.
4.2 DISCUSSION OF FUNDAMENTALS
A demodulation process is just the opposition of a modulation process. As
noticed in Chapter 3, an AM signal is a modulated signal that is high-frequency
carrier amplitude varied with low-frequency audio amplitude for transmission.
To recover the audio signal in receiver, it is necessary to extract the audio
signal from an AM signal. The process of extracting a modulating signal from a
modulated signal is called demodulation or detection. It is shown in Fig. 4-1. In
general, detectors can be categorized into two types: synchronous and
asynchronous detectors. We will discuss these two types of AM detectors in the
rest of this chapter.
AmplitudeDemodulator
AM Signal Audio Signal
Fig.4-1 Illustration of an amplitude demodulation
Diode Detector
Since an AM modulated signal is the signal that the carrier amplitude varies with
the modulating amplitude, a demodulator is used to extract the original
modulating signal from the AM signal.
4-1
Unit 4 AM Demodulators
Product Detector
Demodulation for AM signal can be also accomplished with the balancedmodulator discussed before. Such demodulator is called synchronous detectoror product detector. Fig. 4-4 provides the internal circuit of MC1496 balanced
modulator. See the discussion in Chapter 3 for details. If xAM(t) represents the
AM signal and x,(t) is the carrier, and are expressed by
X Aivi (i) = V De [1+ mcos(27if,„01[ cos(27-tf, t)] ( 4-1 )
x, = V, cos(27tfct) ( 4-2 )
If these two signals are connected to the inputs of balance demodulator, thenthe output of balance demodulator will be
x ou,(1) = kx ,(t) x xAm(t)
k170( V, 2 [1 +mcos(27zfm t)]cos2 (271f, t)
kV Dc V, 2 + kV 1V2 mcos(27zycnt)2 2
kV .V 2 r+
2 [1+ mcos(2gf„,t)]cos[2(27zict)] (4-3)
where k is the gain of balanced modulator. The first term on the right side ofEq.(4-3) represents dc level, the second term is the modulating signal, and thethird term is the second-order harmonic signal. To recover the modulating signal,the intelligence must be extracted from the AM signal xout(t).
Unit 4 AM Demodulators
Cl
R2 1 k R,1 2k
• 0Cu +12V0 luk
Ri1 C4 - R5 270
0.1u 0.1uR2k
—WV—s— 8 2 31k
Carrierinput
VR,100k
C2
10
0 0.1u
0 II 1UI
MC 1496
AMinput
C3O. lu
R9 1 k C lap 2.2uVR200k 14 12
A A,0
Demodulatedoutput
5Ic, — C9
0.1u T 2.2uT i'°461T
1000T
Fig. 4-5 Product detector circuit
Unit 4 AM Demodulators
8. Adjust the VR1 of AM modulator to get maximum amplitude of AM
signal output.
9. Set the vertical input of scope to DC coupling and observe the output
waveforms of the amplifier and the diode detector, and record the
results in Table 4-2.
D 1 O. Change the audio frequencies for 2kHz and 1kHz, and repeat step 9.
Unit 4 AM Demodulators
Table 4-1
(Vc=250mVp-p, Vm=150mVp-p, fc=200kHz)
AudioFrequency
Input Waveform Detector Output Waveform
3 kHz
2 kHz
1 kHz
4-9
Unit 4 AM Demodulators
Table 4-3
(V,=250mVp-p, Vm = 150mVp-p, fc=500kHz, m=50%)
AudioFrequency
Input Waveform Detector Output Waveform
3 kHz
2 kHz
1 kHz
4-11
Unit 4 AM Demodulators
4.5 QUESTIONS
1. In the diode detector circuit of Fig. 4-3, if the operational amplifier pA741
is neglected, what is the output signal?
2. In the product detector circuit of Fig. 4-5, if the carrier signal and the AM
ignal are asynchronous, what is the output signal?
What is the function of R9, C 7 or Cg in Fig. 4-5?
What is the function of VR, or VR 2 in Fig. 4-5?
5. What is the function of R5 or R6 in Fig. 4-5?
DSB-SC AND SSB MODULATORS
5.1 Objectives 5-1
5.2 Discussion Of Fundamentals ......... ..... ...... 5-1
5.3 Equipments Required............ ....... ............ ....... . 5-6
5.4 Experiments And Records 5-6
Experiment 5-1 DSB-SC Modulator
Experiment 5-2 SSB Modulator5.5 Questions. 5-21
Unit 5 DSB-SC and SSB Modulators
5.1 OBJECTIVES
Learning how to generate double-sideband suppressed carrier and
single-sideband modulated signals.
Learning how to test and adjust double-sideband suppressed carrier and
single-sideband balanced modulators.
5.2 DISCUSSION OF FUNDAMENTALS
The principle of circuit operations of this chapter is similar to that of Chapter 3
mentioned before. The circuit of Fig. 5-1 is a double-sideband
suppressed-carrier (DSB-SC) modulator. The balance circuit consisted by the
VR i is used to control the LM1496 operating in balance state. By adjusting the
VR, properly, this will ensure that the modulator operates in balance state. In
short, the major difference between DSB-SC and AM modulated signals is the
DSB-SC modulated signal containing no carrier. To achieve the requirement of
suppressing carrier, we should first connect the audio input to ground, and then
observe the LM1496 output to ensure no carrier presented by carefully
adjusting the VR i . If this is made and then reconnects the audio signal, the
DSB-SC modulated signal containing the upper- and lower-sideband signals will
be presented at LM1496 output.
Unit 5 DSB-SC and SSB Modulators
Since the amplitude-modulated signal contains these two sideband signals, it is
sometimes called as double-sideband AM. In double sideband suppressed
carrier modulation, the carrier signal is removed or suppressed by the balanced
modulator, and the modulated signal containing no carrier as shown in Fig. 5-2c.
Notice that these two sidebands contain the same audio signal when the
modulated signal is transmitted, while receivers may recover the audio signal
from each sideband signals by demodulation technique. This means that only
one of two sidebands is need in transmitting process. Thus an amplitude
modulation called single-sideband (SSB) is shown in Fig. 5-2d.
Suppose the audio input signal (pins 1 and 4) of LM1496 is Amcos2nfmt and the
carrier input signal (pins 8 and 10) is A ccos24t, then its output signal at pin 6
should be
V0 (t) = k(A„, cos27Cf„,t)(A, cos27Lfet)
kA. A, [cos271-(fn, + fc )t +cos27-1-(fm — L)t] ( 5-1 )
where k is the modulator gain, and ( fc-f-fm ) and ( fc-fm ) are the upper and lower
sideband modulated signals, respectively.
In Fig. 5-1, the source follower consisted of Q 1 and Q2 acts as a buffer due to
the characteristics of high input impedance and low output impedance. The
coupling capacitors C1, C2, C4, C5 and C8 are used for blocking dc signal while
coupling ac signal. The R 11 is for adjusting the gain of the balanced modulator
and the R12 is for bias current adjustment. Resistors R 1 , R2 , R13 and R14 provide
dc bias for operating requirement. Resistors R5 and R 10 are for AGC control.
Capacitors C3, C6 and C7 are used to bypass undesired noise. The VIR, are for
balancing, optimum operating point, minimizing distortion and determining types
of output signal (i.e., AM or DSB-SC).
5-3
R1
Carrier CI •
Input 10u
0-71
2k
Audio c2QiK30Ainput I or,
I0
SSBoutput
o455
_
Unit 5 DSB-SC and SSB Modulators
Amplitude
Lowersideband
Uppersideband
Frequency
fF La' f fctfnil
Fig. 5-2d Spectrum of SSB signal
To generate a SSB modulated signal from DSB-SC, a low-pass or high-passfilter is commonly used to filter one sideband signal. Unfortunately, it is difficultto take out single sideband signal from DSB-SC signal with 1st- or 2nd-orderlow- or high-pass filters because these two sideband spectrums are so close toeach other. A good solution of this problem is the use of ceramic or crystal filters.For example, we use the FFD455 ceramic band-pass filter to take out the uppersideband signal in experiment circuit, as shown in Fig. 5-3.
R7 270
C30 I u
C60 lu
1 k
R„1 K
WV'R,„ 1K
Ru 270
PA/Vt-1C5 8 2 30.1u
10
R41 k
LM1496
C4 VR1
0.1U R6 14
2k 100K100K4 5
RR26.8k
Rlk
Fig. 5-3 SSB modulator circuit.
5-5
Unit 5 DSB-SC and SSB Modulators
6. Using the oscilloscope, measure and record the waveforms listed in
Table 5-1.
7. Using the spectrum analyzer, observe and record the output signal
spectrum in Table 5-1.
8, Change the audio amplitude to 600mVp-p. Measure and record the
waveforms listed in Table 5-2 using the oscilloscope.
9. Using the spectrum analyzer, observe and record the output signal
spectrum in Table 5-2.
10. Change the carrier amplitude to 600mVp-p. Measure and record the
waveforms listed in Table 5-3 using the oscilloscope.
11. Using the spectrum analyzer, observe and record the output signal
spectrum in Table 5-3.
12. Change the audio amplitude to 300 mVp-p and frequency to 2kHz,
and the carrier amplitude to 300mVp-p and frequency to 1 MHz. Using
the oscilloscope, measure and record the waveforms listed in Table
5-4.
13. Using the spectrum analyzer, observe and record the output signal
spectrum in Table 5-4.
14. Remove the connect plug from J1 and insert it in J2 to change R11
(270Q) to R15 (3300). Change the audio amplitude to 600mVp-p and
frequency to MHz, and the carrier amplitude to 600mVp-p and
frequency to 500kHz. Hold VIR, position. Using the oscilloscope,
measure and record the waveforms listed in Table 5-5.
5-7
Unit 5 DSB-SC and SSB Modulators
Experiment 5-2 SSB Modulator
1. Locate SSB Modulator circuit on Module KL-93003. Insert the connect
plug in J2 to bypass ceramic filters.
2. Check each of source follower circuits for a proper bias. Set the vertical
input of oscilloscope to AC and observe the source output signal and
the input signal. Ensure that these two signals are the same but the
output amplitude is slightly smaller than the input amplitude. If done,
insert connect plugs in J3 and J4.
3. Turn the VIR, to its mid-position.
4. Connect the audio input (I/P2) to ground and connect a 500mVp-p, 457
kHz sine wave to the carrier input (I/P1). Carefully adjust VR, to get a
minimum output or zero. Then remove the connect plug from J2 and
insert it in J1.
5. Connect 'a 300mVp-p, 2kHz sine wave to the audio input and change
the carrier amplitude to 300mVp-p.
6. Using the oscilloscope, measure and record the waveforms listed in
Table 5-7.
7. Using the spectrum analyzer, observe and record the output signal
spectrum in Table 5-7.
8. Change the audio amplitude to 600mVp-p. Measure and record the
waveforms listed in Table 5-8 using the oscilloscope.
5-9
Unit 5 DSB-SC and SSB Modulators
Table 5-1
(R 11 =27052, R 12 =6.8k0, Vc= 300mVp-p, Vm=300mVp-p, fc=500kHz, fm=1 kHz)
CarrierWaveform
AudioWaveform
OutputWaveform
OutputSpectrum
5-11