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Phase Shift Keying (PSK) Modulation & Demodulation Process

Phase Shift Keying (PSK) Modulation & Demodulation Process: In PSK (Phase Shift Keying) technique, the phase of the carrier frequency is varied in accordance with the data stream. Since it involves the phase change of the carrier sine wave between 0° and 180°, it is also known as Phase Reversal Keying (PRK).

 PSK MODULATION PROCESS:

Functionally, the PSK modulator is very similar to the ASK modulator. Both uses balanced modulator to multiply the carrier with the modulating signal. But in contrast to ASK technique, the digital signal applied to the modulation input for PSK generation is bipolar i.e. have equal positive and negative voltage levels. When the modulating input is positive the output of modulator is a sine wave in phase with the carrier input. Whereas for the negative voltage levels, the output of modulator is a sine wave which is shifted out of phase by 180° from the carrier input. This happens because the carrier input is now multiplied by the negative constant level.

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Phase Shift Keying Modulation & Demodulation

 PSK DEMODULATION PROCESS:

The incoming PSK signal with 0° & 180° phase changes is first fed to the signal square, which multiplies the input signal by itself. The output of this block is a signal of twice the frequency with the frequency of the output doubled, the 0° & 180° phase changes are reflected as 0° & 360° phase changes. Since phase change of 360° is same as 0° phase change, it can be said that the signal squarer simply removes the phase transitions from the original PSK waveform. The PLL block locks to the frequency of the signal square output & produces a clean square wave output of same frequency. To derive the square wave of same frequency as the incoming PSK signal, the PLL output is divided by two in frequency domain by the ‘Divide by 2’ circuit.

Phase Adjust Circuit allows the phase of the digital signal to be adjusted with respect to the input PSK signal. Also its output controls the closing of an analog switch. When the output is high the switch closes, the original PSK signal is passed through the detector. When the Phase Adjust Block’s output is low, the switch opens and the detector’s output falls to 0 Volts. The average level information of the demodulator output which contains the digital data information is extracted by the following low pass filter. The low pass filter output is too rounded to be used for digital processing. Therefore it is ‘Squared Up’ by a voltage comparator. The output of the voltage comparator is the original data stream.

 PSK MODULATION PROCEDURE:

 1. The input data stream is generated.

2. Data Stream is converted from Unipolar to Bipolar Data.

3. The carrier sine wave is generated.

4. They are mixed in Modulator.

5. The output PSK waveform is generated and transmitted.

 PSK DEMODULATION PROCEDURE:

 1. The incoming PSK signal with 0° & 180° phase changes is first fed to the signal square, which multiplies the input signal by itself.

 2. The PLL block locks to the frequency of the signal square output & produces a clean square wave output of same frequency.

 3. The PLL output is divided by two in frequency domain by the ‘Divide by 2’ circuit.

4. Phase of the digital signal to be adjusted with respect to the input PSK signal using the Phase Adjust Circuit. Its O/P controls closing of the analog switch.

 5. When the output is high the switch closes, the original PSK signal is passed through the detector. When the Phase Adjust Block’s output is low, the switch opens and the detector’s output falls to 0 Volts.

 6. Low Pass Filter extracts the average level information of the demodulator output which contains the digital data information.

7. The low pass filter output is ‘Squared Up’ by a voltage comparator.

CONCLUSION:

PSK is a complex but effective way for transmission of digital information signal.


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1 Comment

  1. Juliana says:

    Very helpful thanks. But I do not understand the phase adjust circuit, can you explain more?

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