DPSK Modulation and Demodulation

Wednesday, November 5th, 2008

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

DPSK and PSK modulated signals

DPSK and PSK modulated signals

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).

The lead and lag carrier signals

The lead and lag carrier signals

DPSK Modulation:

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 [10] 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 Modulator Circuit

DPSK Modulator Circuit

DPSK Demodulation:

DPSK Demodulation [12,13 & 14]is done with PLL IC 565[3 4 5]. DPSK [10] 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

DPSK Demodulator Circuit

DPSK Demodulator Circuit

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Total : 16 Comments

16 Responses to “DPSK Modulation and Demodulation”

  1. safa says:

    hi…..what signals are given under the name W and Y???
    dats a li’l confusing…
    pls clarify my doubt soon..

  2. santosh says:

    i want information about depsk can u plz tel me about in detailed????

  3. Truly wonderful desgin of this web page. It really is individual and compares for your posts. Don´t give up and make your individual issue!

  4. rosy says:

    what r flead and flag signals
    why r FET’s used??..
    can u plz give the detailed information of how the transmission takes place

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  7. jithin says:

    could you plz give block diagram representation of dpsk modulator and demodulator

  8. Empireders says:

    Great blog post. Its useful information.

  9. James says:

    Hello, I’d Like to ask about the FET used, what Kind of FET did you use? the specific. because I can only find the JFET P channel 2n5018 or 2N5019, and also the value of the Diode used. or is Q1,Q2 and Q3 just 1 IC package? I am a bit Confused. what is the Specific IC did you use. thank you, Great Blog btw :)

  10. rahul says:

    its really very use full website if its possible try to produce few more diagrams

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  12. sandhiya says:

    pls send a detailed waveforms consisting of delays

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  14. nitesh tyagi says:

    i c’t understand the mean of y,w.

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