Differential phase shift keying (DPSK), a common form of phase modulation conveys data by changing the phase of carrier wave. In Phase shift keying, High state contains only one cycle but DPSK contains one and half cycle. Figure illustrates PSK and DPSK Modulated signal by 10101110 pulse sequence
High state is represented by a M in modulated signal and low state is represented by a wave which appears like W in modulated signal DPSK encodes two distinct signals of same frequency with 180 degree phase difference between the two. This experiment requires two 180 degree out of phase carrier and modulating signals. Sine wave from oscillator is selected as carrier signal. DSG converts DC input voltage into pulse trains. These pulse trains are taken as modulating signals. In actual practice modulating signal is digital form of voice or data. Sine wave is selected as carrier and 180 degree phase shift is obtained using Opamp as shown in figure below. Different methods are used to demodulate DPSK. The analog scheme is the PLL (Phase Locked loop).
In DPSK, during HIGH state of the modulating signal flead signal is allowed to pass and during LOW state of the modulating signal flag signal is allowed to pass. Figure below shows DPSK  modulator circuit. The Opamp is tied in the inverting amplifier mode. The closed loop voltage gain of the Opamp is given by
RF + rDS (on) 3
AV(CL) = – ———————
RI + rDS (on) 1,2
Where: rDS (on) 3 is the drain- source resistance of Q3 FET
rDS (on) 1,2 is drain-resistance of the conducting FET(Q1 or Q2)
The drain source resistance is of the order of 100Ω which is very small compared to RF and RI.
AV(CL) = – ——
DPSK Demodulation [12,13 & 14]is done with PLL IC 565[3 4 5]. DPSK  signal is given as input at DPSK input terminal of PLL as shown in the figure below.
A capacitor C is connected between pin7 and power supply forms first order low pass filter with an internal resistance 3.6KW, The capacitor C should be large enough to eliminate variations in the demodulated output voltage in order to stabilize the VCO frequency. The cut-off frequency of Low pass filter is made equal to carrier frequency. The cutoff frequency of low pass filter is given by
fH = ———-
R = 3.6KW, fH = 18.7KHz
The value of C designed by
C = ———-
C = ——————— = 2.3nF
C selected is 3nF