20-07-2013, 02:45 PM
DPSK MODULATOR AND DEMODULATOR
AIM:
To generate a DPSK for a given binary (digital) signal and Observe. Demodulate the same DPSK to get
back the original Digital signal, using carrier-receiving circuit & Demodulator.
THEORY:
In BPSK communication system, the demodulation is made by comparing the instant phase of the BPSK signal to an
absolute reference phase locally generated in the receiver. The modulation is called in this case BPSK absolute. The
greatest difficulty of these systems lies in the need to keep the phase of the regenerated carrier always constant. This
problem is solved with the PSK differential modulation, as the information is not contained in the absolute phase of the
modulated carrier but in the phase difference between two next modulation intervals.
In the block diagram (Fig.) 1 and 2 Shows the DPSK modulation and demodulation system.The coding is obtained by
comparing the output of an EXOR, delayed of a bit interval, with the current data bits (for detailed explanation see
experiment no.2). As total result of operation, the DPSK signal across the output of the modulator contains 180 deg.
phase variation at each data bit 1. The demodulation is made by a normal BPSK demodulator, followed by a decision
device supplying a bit 1 each time there is a variation of the logic level across its input.
PROCEDURE:
1. Refer to Block Diagram & Carry out the following connections and switch settings.
2. Connect power supply in proper polarity to the kit ADCL-01 and switch it on.
3. Select Data pattern of simulated data using switch SW1.
4. Connect SDATA generated to DATA IN of NRZ-L CODER.
5. Connect the NRZ-L DATA output to the DATA IN of the DIFFERENTIAL ENCODER.
6. Connect the clock generated SCLOCK to CLK IN of the DIFFERENTIAL ENCODER
7. Connect differentially encoded data to control input C1 of CARRIER MODULATOR