kl-900a basic communication trainer

....... ]:

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



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


17.3 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


18-1

18-1


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 1-2 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


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


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


1.3 EQUIPMENT REQUIRED

1 - Module KL-920012 - Module KL-930013 - Oscilloscope4 - LCR Meter

1.4 EXPERIMENTS AND RECORDS


.

▪

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


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


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.


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


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


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


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


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


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.



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.



9. Using the results of Table 2-4, sketch Bode plot of voltage gain in

Fig. 2-9.

2-11


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


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




Experiment 3-1 Amplitude Modulator3.5 Questions... ..... ....... ...... ............... ...... ........ 3-13


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


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


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


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


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


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,


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,


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...... ...... ..... ......... ...... ...............


4.3 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


4.1 OBJECTIVES

Understanding the principle of amplitude demodulation.

Implementing an amplitude demodulator with diode.

3. Implementing an amplitude demodulator with a product detector.


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


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).


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


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.


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


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


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


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.


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.


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

_


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


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


8, Change the audio amplitude to 600mVp-p. Measure and record the

waveforms listed in Table 5-2 using the oscilloscope.



10. Change the carrier amplitude to 600mVp-p. Measure and record the




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.



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


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.



8. Change the audio amplitude to 600mVp-p. Measure and record the


5-9


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

kl-900a basic communication trainer

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