Communications Engineering I (Lab.)
Islamic University of Gaza
Faculty of Engineering
Electrical Department
Prepared by:
Eng. Mohammed K. Abu Foul
Eng. Omar A. Qarmout
Experiment # (5)
DSB-SC and SSB Modulator
Experiment theory:
Figure 5-1 shows the wave forms of the amplitude modulation (AM).
cos 2
and carrier signal be cos 2
, then the
Let the audio signal be
amplitude modulation can be expressed as
1
Where
=
:
:
:
:
:
:
cos 2
cos 2
cos 2
cos 2
5
1
/
DC signal amplitude.
Audio signal amplitude.
Carrier signal amplitude.
Audio signal frequency.
Carrier signal frequency.
Modulation index or depth of modulation.
Figure 5-1 Signal waveform of amplitude modulation
We can rewrite equation (5-1) as
1
cos 2
2
cos 2
cos 2
5
2
The first term represents the double sideband signals; the second term
represents the carrier signal. From equation (5-3), we can sketch the frequency
spectrum of the amplitude modulation as shown in figure 5-3(a). Since the AM signal
is hidden the double sidebands and the carrier signal does not contain any infotmation,
therefore the power is consumed in the carrier during transmission of amplitude
modulation signal. The double sidebands suppressed carrier (DSB-SC) modulation
means the term
cos 2
equals to zero, therefore, it can suppress the carrier
signal and only left the double sidebands. DSB-SC modulation can be used to obtain
the SSB modulation. Two DSB-SC modulators are utilized and the phase difference
between the two audio signals and the carrier signal is adjusted to be 90 degree, i.e.
(DSB-SC)Q and (DSB-SC)I , as shown in equations (5-3) and (5-4)
DSB
SC
I
DSB
SC
Q
1
2
cos 2
cos 2
5
3
1
2
cos 2
cos 2
5
4
Equation (5-5) and (5-6) shows that both (DSB-SC)I and (DSB-SC)Q signals are
connected to the adder, then USSB or LSSB signal can be obtained at the output port.
XLSSB
DSB
SC
I
DSB
SC
Q
cos 2
5
5
XUSSB
DSB
SC
I
DSB
SC
Q
cos 2
5
6
Figure 5-3(a) is the frequency spectrum of AM signal. We can see that the frequency
spectrum consists of three kinds of signals, which is
,
and
. The
output voltage of is higher than the other two signals, therefore, the carrier does not
contain any information, and the power is consumed in the carrier during transmission
of amplitude modulation signal. Figure 5-3(b) is the frequency spectrum of DSB-SC
signal. We can see that the frequency spectrum consists of two kinds of signals, which
are
and
. These two kinds of signals consists of the transmission
signal, therefore, by using this type of modulation, the power will not consume in the
carrier. Besides, as a result of the audio signal is hidden in the double sidebands, so,
the stronger the double sidebands signal, the better efficiency of transmission. From
equation (5-3), we notice that the larger the modulation index, the better the
transmission efficiency. Generally, the modulation index is smaller or equal to 1. If
the modulation index is greater than 1, we call this situation as over modulation. For
DSB-SC modulation, the modulation index is adjusted to be as large as we can get in
order to suppress the carrier from the signal. Figure 5-3(c) and 5-3(d) are the
frequency are the frequency spectrum of SSB signal. We can see that the frequency
spectrum consists of either
signal or
signal. Therefore, during
transmission, the power consumption of SSB modulation is less than DSB-SC
modulation. From above-mentioned discussion, we know that the sequence of power
consumption of the three different types of modulation is:
Figure 5-2 Block diagram of DSB-SC modulation
(a) Frequency spectrum of AM
(b) Frequency spectrum of DSB-SC.
(c) Frequency spectrum of LSSB
(d) Frequency spectrum of HSSB
Figure 5-3 Different frequency spectrums of AM modulation.
1- Implementation of DSB-SC Modulator
DSB-SC modulation is a kind of AM modulation; therefore, we can utilize the
structure of AM modulator to implement the DSB-SC modulator. Figure 5-2 is the
block diagram of DSB-SC modulator. We utilize balanced modulator MC1496 to
design the DSB-SC modulated signal. Figure 5-4 is the circuit diagram of AM
modulator. We can see that the carrier signal and the audio signal belong to signal
ended input. The carrier signal is inputted from pin 10 and the audio signal is inputted
determine the gain of the whole circuit and
determine the
from pin 1. Therefore
magnitude of bias current. If we adjust the variable resistor VR1 or change the input
amplitude of audio signal, then we can control the percentage modulation of
amplitude modulation, which means we can adjust the output become the DSB-SC
modulation. By adjusting variable resistor VR2, we can control the magnitude of the
output amplitude, which is also the gain.
Figure 5.4 Circuit diagram of DSB-SC modulation by utilizing MC1496.
2- Implementation of SSB Modulation
From equations (5-5) and (5-6), we know that the SSB modulator is the
combination of two DSB-SC modulators. Figure 5-5(a) is the block diagram of SSB
modulator, where the phase difference of each audio signal and carrier signal of the
two DSB-SC modulators is 90o (i.e. 90o phase difference between TP1 and TP2, and
90o phase difference between TP3 and TP4). In figure 5-5(a), the blocks of the
quadrature phase shift the phase shift represent the phase shifters. The circuit diagram
of phase shifter is in figure 5-5(b). By adjusting the variable resistor, we can control
the phase difference between the input and the output phase. The circuits of balance
modulator 1 and balance modulator 2 are similar to circuit in figure 5-4. Then the
output terminals of the two balanced modulators, which are TP5 and TP6, will be
added by the linear adder, and then we can obtain the modulated SSB signal. The
circuit diagram of the linear adder is shown in figure 5-5(c). We utilize oscilloscope
or spectrum are to observe TP5 and adjust VR1 of the balanced modulator 1, so that
the output is modulated DSB-SC signal. By adjusting the variable resistor VR2, we
can control the gain of the DSB-SC modulator so that the output amplitude is
maximum value without distortion. Similarity, utilize oscilloscope or spectrum are to
observe TP6 and adjust VR1 of the balanced modulator 2, so that the output is
modulated DSB-SC signal. By adjusting the variable resistor VR2, we can control the
gain of the DSB-SC modulator so that the output amplitude is maximum value
without distortion. Finally we use spectrum analyzes to observe the output signal
terminal is whether the modulated SSB signal. If the frequency spectrum is not true,
we can adjust the variable resistor of the quadrature phase shift
(a) Block diagram of SSB modulator
(b) Circuit diagram of phase shifter.
(c) Circuit diagram of linearity adder
Figure 5-5 Circuit diagram of linearity adder
Experiment Items
Experiment: DSB-SC modulator and SSB Modulator
1. To implement a DSB-SC modulator as shown in figure 5-4 or refer to figure
ACS5-1 on ETEK ACS-3000-03 module.
2. At the audio signal input port (Audio I/P), input 300mV amplitude and 1 kHz
sine wave frequency. Next at the carrier signal port (Carrier I/P), input 300mV
amplitude and 100 kHz sine wave frequency.
3. By using oscilloscope, observe on both the audio signal output ports TP1 and
TP2 at the same time. Next adjust variable resister 'QPS' so that the phase
difference between TP1 and TP2 is 90o. Then record the measured result in table
5-1. By using oscilloscope, observe on both the carrier signal output ports TP3
and TP4 at the same time. Next adjust variable resistor 'phase Adjust' so that the
phase difference between TP3 and TP4 is 90o. Then record the measured results
in table 5-1.
4. By using oscilloscope, observe the output signal waveforms of DSB-SCQ
modulator modulation output port TP5. Next adjust variable resistor VR1 (gain
adjustment) so that the output amplitude is maximum without distortion, and
also adjust variable resistor VR3 (modulation index adjustment) so that the
modulation index is large enough to suppressed the carrier from the signal.
Finally, record the measured results in table 5-1.
5. Change the oscilloscope to spectrum analyzer, observe on the output signal
waveform of TP5 and record the measure in table 5-1.
6. By using oscilloscope, observe the output signal waveforms of DSB-SCI
modulator modulation output port TP6. Next adjust variable resistor VR2 (gain
adjustment) so that the output amplitude is maximum without distortion, and
also adjust variable resistor VR4 (modulation index adjustment) so that the
modulation index is large enough to suppressed the carrier from the signal.
Finally, record the measured results in table 5-1.
7. Change the oscilloscope to spectrum analyzer, observe on the output signal
waveform of TP6 and record the measure in table 5-1.
8. By using oscilloscope, observe on the output signal waveforms of SSB
modulation output port (SSB O/P), then record the measured results in table 5-1.
9. By using spectrum analyzer, observe on the output signal waveforms of SSB
modulation output port (SSB O/P), then record the measured results in table 5-1.
10. According to the input signals table 5-2 repeat steps 3-9 and record the results.
11. According to the input signals table 5-3 repeat steps 3-9 and record the results.
Table 5-1 (Audio I/P VP=300mV, f =1 kHz; Carrier I/P VP=300mV, f =100 kHz)
Measured results of phase adjustment.
TP1 and TP2
TP3 and TP4
Measure results of modulated DSB-SC signal (TP5)
Oscilloscope
Spectrum analyzer
Measure results of modulated DSB-SC signal (TP6)
Oscilloscope
Spectrum analyzer
Measure results of modulated SSB signal (SSB O/P)
Oscilloscope
Spectrum analyzer
Table 5-2 (Audio I/P VP=300mV, f =1 kHz; Carrier I/P VP=300mV, f =300 kHz)
Measured results of phase adjustment.
TP1 and TP2
TP3 and TP4
Measure results of modulated DSB-SC signal (TP5)
Oscilloscope
Spectrum analyzer
Measure results of modulated DSB-SC signal (TP6)
Oscilloscope
Spectrum analyzer
Measure results of modulated SSB signal (SSB O/P)
Oscilloscope
Spectrum analyzer
Table 5-3 (Audio I/P VP=500mV, f =1 kHz; Carrier I/P VP=500mV, f =500 kHz)
Measured results of phase adjustment.
TP1 and TP2
TP3 and TP4
Measure results of modulated DSB-SC signal (TP5)
Oscilloscope
Spectrum analyzer
Measure results of modulated DSB-SC signal (TP6)
Oscilloscope
Spectrum analyzer
Measure results of modulated SSB signal (SSB O/P)
Oscilloscope
Spectrum analyzer