EXPERIMENT 3
DOUBLE SIDE BAND SUPPRESSED CARRIER MODULATION AND
DEMODULATION
OBJECTIVES
1- To study the modulation of double side band suppressed carrier signal.
2- Demodulation of the double side band suppressed carrier signal using synchronous
detection.
MODULES REQUIRED
Oty
Description
1
AUDIO OSCILATOR
1
QUADRATURE UTILITIES
1
TUNEABLE LPF
1
WIDEBAND TRUE RMS METER
THEORY
The transmission of the Carrier Wave (as in AM) represents a waste of power. To overcome
this, we may suppress the carrier component from the modulated wave. Thus, we obtain a
modulated wave that is proportional to the product of the carrier wave and the baseband signal.
φ DSB − SC (t ) = m(t ) A cos ω c t
(3.1)
The signal given by Eq. (3.1) is known as DSB-SC AM signal. If m(t ) = Am cos ω m t , the
corresponding DSB-SC wave is given by:
φ DSB − SC (t ) = 0.5 AAm cos( ω c + ω m ) t + 0.5 AAm cos (ω c − ω m )t
(3.2)
Thus the modulated wave is seen to have only two components: the upper-side frequency at
(ω c + ω m ) and the lower-side frequency at ( ω c − ω m ) .
Coherent Demodulation of DSB-SC
The baseband signal m(t) can be uniquely recovered from DSB-SC wave φ DSB − SC (t ) by
coherent detection. The block diagram for this method will be the same as that shown in
Fig.2.3, but without the coupling capacitor. This process can be explained mathematically as
follows:
φ DSB − SC (t ) cos ω c t = Am (t ) cos ω c t cos ω c t
= 0.5 Am (t ) [1 + cos 2ω c t ]
(3.3)
The output of the low pass filter will be 0.5 Am (t ) .
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PROCEDURE AND QUESTION
DSB-SC Modulation and Demodulation
1- Connect the circuit shown in Fig. 3.1. Set the audio oscillator to have a cosine wave of 3
kHz and adjust the control of the buffer amplifier to get a peak to peak value for
modulating signal equal to 4 V.
2- Save and print the signal at the output of the first multiplier, the signal at the input of LPF
and the signal at the output of LPF.
3- Save and print the spectrum of the signal at the output of the first multiplier, the signal at
the input of LPF and the signal at the output of LPF.
Q.1 For the signal at the output of the first multiplier:
a)
What type of signal you have at the output of the first multiplier?
b)
Using its spectrum find the frequency components that present in it also find its
bandwidth.
c)
Find the theoretical frequency components and bandwidth for this signal and compare
them with the practical one.
Q.2 What frequency components will be present at the input of the LPF? By using theoretical
method justify your answer.
4
Measure the frequency of the signal at the output of LPF.
Q.3 What type of signal will be present at the output of the LPF? Why?
5-
Modify the connection of your layout such that the square law detection is used to recover
the information signal.
Note: Set the LPF switch to WIDE position.
6-
Save and print the waveforms at the input and output of LPF.
7-
Save and print the spectrum of the signals at the input and output of LPF
Q.4 Find the frequency components for the signal at the input of LPF. Theoretically justify your
answer.
8-
Measure the frequency of the signal at the output of LPF.
Q.5 According to step 8, is the demodulated (recovered) signal the same as the modulating signal?
Why? Justify your answer theoretically.
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Fig. 3.1 DSB-SC modulation and demodulation connection diagram
FURTHER QUESTIONS:
Q.6 Assume
the
information
signal
m(t ) = 0.4 cos(60π t )
and
the
carrier
c(t ) = 2 cos(2π 1000 × 10 3 t ) .Write the equation for the DSB-SC signal in time domain and
frequency domain. Sketch its spectrum.
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