modulating signal

7/31/2019 Modulating Signal

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DOUBLE SIDEBAND AM GENERATOR

This experiment investigates the generation of Double Sideband Amplitude

Modulated (AM) waveform using the Transmitter Kit. By removing the

carrier from such an AM waveforms, the generation of Double Sidebands

Suppressed Carrier (DSBSC) AM is also investigated. To avoid unnecessary

loading of monitored signals oscilloscope probes should be used

throughout this experiment.

1. Ensure that the following initial conditions exist on the board.

a). AUDIO INPUT SELECT switch in INT position.

b). MODE switch in DSB position.

c). OUTPUT amplifiers gain preset in fully

clockwise position. d). SPEAKER switch OFF

position.

2. Turn ON power of the Transmitter Kit.

3. Turn the AUDIO OSCILLATOR blocks AMPLITUDE preset to its fully clockwise

(MAX) position and examine the blocks output (TP14) on an oscilloscope.

(FIGURE 1)

TP14Modulating Signal (Frequency preset fully rotated counter-clockwise)

Horizontal scale: 1mS/div 3.3divisions

Vertical Scale: 1v/div 2 divisions


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(FIGURE 2)

Modulating signal (frequency preset fully rotated clockwise)

Horizontal scale: 50S/div 5.9 divisions

Vertical Scale: 1v/div 2 divisions

This is the audio frequency sine wave which will be as our

modulating signal. Note that the sine wave frequency can be

adjusted from about 300Hz (Figure 1) to approximately 3.4Hz

(FIGURE 2) by adjusting the AUDIO OSCILLOSCOPEs FREQUENCY

preset.

Note also that the amplitude of this modulating signal can be

reduced to zero, by turning the AUDIO OSCILLATOR AMPLITUDE


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preset to its fully counter clockwise (MIN) position. Return the

AMPLITUDE preset to its MAX position.

4.Turn the BALANCE preset, the BALANCE MODULATOR & BANDPASS FILTER

CIRCUIT 1 block at TP1 and TP9 note that:

a). The signal at TP1 is the audio

frequency sinewave from the AUDIO OSCILLATOR block. This is themodulating input to our double sideband modulator.

A b). Test point 9 carriers a sinewave of frequency of 1MHz and 75mV

peak to peak bbbbbbz zapproximately. This the carrier input to our double

sideband modulator.

Carrier Frequency (below)

Horizontal scale: 0.5 S/div 2 divisions

Vertical Scale: 50 m V/div 2.2 divisions

5. Next, examine the output of the BALANCE MODULATOR & BANDPASS FILTER

CIRCUIT 1 block at (TP13) together with the modulating signal at TP1. Trigger

the oscilloscope on the TP1 signal.


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Channel 1- fm(above) Channel 2- (below)

For channel 1 For channel 2

Horizontal scale: 2mS/div Horizontal scale: / div

Vertical scale:1v/div Vertical scale: 0.2 v/ div

The output from the BALANCED MODULATOR & BANDPASS

FILTER CIRCUITS 1 block at (TP3) is a double sideband. AM

waveform which has been performed by amplitude

modulating the 1Mhz carrier sinewave with the audio

frequency sinewave from the audio oscillator.

The frequency spectrum of this AM waveform is shown

below, where fm is the frequency of the audio modulating

signal.

6. To determine the depth of modulation, measure the maximum amplitude

(Vmax) and the minimum amplitude (Vmin) of AM waveform at TP3 and use the

following formula:

Vmax= 2.2 div x 0.2 v/div = 0.44 v

VMIN=1 divX 0.2 V/div=0.2 v

M = [(Vmax Vmin) / (Vmax+ Vmin)] x 100

= [(0.44v 0.2v) / (0.44v + 0.2v) ] x 100

= 37.5 % (Percent Modulation)


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7. Now vary the amplitude and the frequency of the audio frequency sinewave,

adjusting the AMPLITUDE and FREQUENCY present in the AUDIO OSCILLATOR

block. Note the effect that varying each preset has on the amplitude modulated

waveform. The amplitude and frequency amplitudes to the two sidebands can

be reduced to zero by reducing the amplitude of the modulating audio signal to

zero. Do this by turning the AMPLITUDE preset to its MIN position, and note the

signal at TP3 becomes an unmodulated sinewave of frequency 1MHz, indicating

that only the carrier components now remains. Return the AMPLITUDE preset to

its maximum position.

Now turn the BALANCE preset in the BALANCE MODULATOR & BANDPASS FILTER

CIRCUIT 1 block, until the signal at TP3 is as shown.

By adjusting the preset until the peaks of the waveform (A, B, C and so on) have

the same amplitude we are removing the carrier component altogether. We say

that the carrier has been balanced out (or suppressed) to leave only the two

sidebands.

Note once the carrier has been balanced out the amplitude of TP3 waveform

should be zero at minimum points X, Y, Z etc. If this is not the case, it is becauseof the two sidebands is being amplified more than the other. To remove this

problem, the BANDPASS filter in the BALANCED MODULATOR & BANDPASS FILTER

CIRCUIT 1 block must be adjusted so that it passes both sidebands equally. This is

achieved by carefully trimming transformer T1, until the waveform amplitude is as

close to zero as possible at the minimum points.

Vertical scale: 0.2 v/div

Horizontal Scale: .5ms/div


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(The above figure illustrates an AM waveform with 100% modulation.)

8. Changed the amplitude frequency of the modulating audio signal(by adjusting

the AUDIO OSCILLATOR blocks AMPLITUDE and FREQUENCY presets) and the

two sidebands can be reduced to zero by reducing the amplitude of the

modulating audio signal to zero. Do this by turning the AMPLITUDE preset to its

MIN position, and more no frequency components present. Return the

AMPLITUDE preset to its MIN position.

9. Examine the output from the OUTPUT AMPLIFIER block (TP13) together with the

audio modulating signal (at TP1) triggering the scope with the audio modulating

signal. Note that the DSBSC waveform appears slightly at TP13. As we will see

later it is the OUTPUT AMPLIFIERs output signal which will be transmitted to the

receiver.

The amplified signal

Vertical Scale: 0.2 v/div

Horizontal scale: .5ms/div


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