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EXPERIMENT
Objective: Study of Differential Phase Shift Keying.
Equipments Needed:
1. ST2156 and ST2157 Trainers.
2. 2 mm Banana cable 3. Oscilloscope Caddo 802 or equivalent
Circuit diagram: Refer the figure 5.1 for the connection diagram for Experiment 5
Procedure:
1. Connect the power supplies of ST2156 and ST2157 but do not turn on the power supplies until
connections are made for this experiment.
2. Make the connections as shown in the figure 5.1.
3. Switch 'ON' the power.
4. On ST2156, connect oscilloscope CH1 to Clock In and CH2 to Data In and observe the
waveforms.
5. On ST2156, connect oscilloscope CH1 to NRZ (L) and CH2 to Output of Modulator Circuit
(l) on ST2156 and observe the waveforms.
6. Adjust the Gain potentiometer of the Modulator Circuit (l) onST2156 to adjust the amplitude of
PSK waveform at output of Modulator Circuit (l) on ST2156.
7. Now on ST2157 connect oscilloscope CH1 to Input of PSK demodulator and connect CH2 one
by one to output of double squaring circuit, output of PLL, output of Divide by four ( 2) observe
the wave forms.
8. On ST2157 connect oscilloscope CH1 to output of Phase adjust and CH2 to
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output of PSK demodulator and observe the waveforms. Now vary the phase adjust potentiometer
and observe its effects on the demodulated signal waveform
9. Now connect oscilloscope CH1 to PSK output of PSK demodulator on ST2157 and connect
CH2 Output of Low Pass Filter on ST2157 and observe the waveforms.
10. Connect oscilloscope CH1 to Output of Low Pass Filter on ST2157 then connect CH2 to
Output of Comparator on ST2157 and observe the waveforms, now vary the reference voltage
potentiometer of first comparator to generate desired data pattern.
11. On ST2156, connect oscilloscope CH1 to NRZ (L) and CH2 to Output of comparator on
ST2157 and observe the waveforms.
12. Connect oscilloscope CH1 to Data In then connect CH2 output to Bit decoder and observe the
waveforms. If both data does not matches then try to match it by varying the phase adjust
potentiometer on QPSK Demodulator.
13. Now try to match the LED sequence by once pressing the reset switch on ST2156.
Observations:
1. The output at Data In is repeating sequence of bits generated by Data Source.
2. The Output of Modulator Circuit (l) is Phase Shift Keying modulated signal.
3. The output of Double squaring circuit is sinusoidal signal (carrier signal) but frequency is four
times higher than that of carrier used for modulation.
4. The output of Phase Lock Loop (PLL) is clock signal of same frequency as that of the output of
double squaring circuit and output of Divide by two ( 2) is clock signal of frequency two times
less than the output of PLL signal.
5. The output of PSK demodulator is a signal having group of positive half cycles and group of
negative half cycles of the carrier signal.
6. A low pass filter removes high frequency component from demodulated PSK signal and it makes
the signal smooth.
7. The variation in reference voltage potentiometer affect the Data, to recover Data correctly
potentiometer adjustment is necessary.
8. The Phase Adjust potentiometer on ST2157 matches the phase of regenerated
clock and carrier with input clock and carrier signal.
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Conclusion:
1. The Differential Phase Shift Keying modulation is correct for different Data pattern.
2. The differential encoding and decoding process has an advantage that data will not find inverted
after demodulation.
