addition to that effected by the pulse design noted above individual pulses in a doublet can be shaped in such a way as to concentrate their energy in certain frequency bands this method is well known in the field of radar this method can be used if required to accommodate various spectrum restrictions we recover the encoded information from a doublet by computing the correlation at a lag given by d this operation is performed by an electronic circuit called a pulse pair correlation a block diagram of the pulse pair correlation is depicted in fig2 a pulse pair correlation consists of a delay 21 a signal multiplier 22 and a finite time integrator 23 the signal is split into two paths one of which is delayed by delay 21 the two versions of the received signal are multiplied in multiplier 22 and the product is integrated over a specified time tc by integrator 23 the integration time is determined by the number of doublets making up the transmitted bit the delay is such that the leading pulse of the delayed circuit path is registered in time with the trailing pulse of the undelayed circuit path this non zero mean product is integrated over a symbol interval tc to produce an output signal note that in practice the output of the finite time integrator is only required at some relatively small number of sampling instants therefore the continuously integrating finite impulse response fir integrator.