Objectives

1. Generation of BPSK modulated signal and demodulation of the same after passing through the channel.

2. Compare the demodulation schemes using:

     (a) Squaring loop.  

     (b) Costa’s loop.

3. Observe the spectrum of BPSK Signal and effect of variation of channel bandwidth

4. Determine the error rates.

  Introduction

IIn phase shift keying (PSK), the phase of a carrier is changed according to the modulating waveform which is a digital signal. InBPSK, the transmitted signal is a sinusoid of fixed amplitude. It has one fixed phase when the data is at one level and when thedata is at the other level, phase is different by 180 degree. A Binary Phase Shift Keying(BPSK) signal can be defined as

 where b(t) = +1 or -1,  fc  is the carrier frequency, and T is the bit duration. The signal has a power , so that , where A represents the peak value of sinusoidal carrier.

Thus the above equation can be written as

       =

         =

         =   , where  E=PT is the energy contained in the bit duration.      

                          Figure1.    shows  the BPSK signal for bit sequence 1001, 

where and 

 The received signal has the form  =  , where is the phase shift introduced by the channel. The signal b(t) is recovered in the demodulator. If synchronous demodulation is used, the waveform  is required at the demodulator. Carrier recovery scheme in the demodulator is shown in Fig 2.

Figure2:   BPSK modulator and demodulator

The recovered carrier is multiplied with the received signal to generate

Assuming that integral number of carrier cycles is present in bit duration   voltage and the bit synchronizer in Fig 2 knows the end of a bit interval and beginning of the next, the output  voltage  at the output of the integrate and dump circuit is:

If the channel is noisy, some of the demodulated bits will be in error.