7-17] PULSE STRETCHING 387 



is not usually coherent with the IF signal. The higher frequency compo- 

 nents of the gating signal are the signal components which cannot be 

 adequately filtered. Transients caused by the modulating signal will 

 generally produce outputs from the IF filter when rectangular gate pulses 

 are employed. Noise modulation of the desired signal results from these 

 transients. 



In typical designs the noise modulation caused by the transients is at 

 least 40 db below the signal. In addition, the seriousness depends on the 

 signal processing following the gating. Appropriate sampling of the gated 

 signal prior to integration reduces the noise to a negligible value. In video 

 gating circuits the modulating signal is coherent with the detected signal. 

 Thus the noise mentioned does not occur. 



7-17 PULSE STRETCHING 



In tracking radars it is required that the modulation signal associated 

 with a pulse-amplitude-modulated signal be recovered. The modulated 

 pulse signal is 



/i(/) = [1 + m cos (a;„/ + 0)] ^fljue^^ (7-40) 



for periodic pulses of shape /(/) 



where r„ = ^///W exp (^^^^V/. (7-41) 



If the pulses are passed through a low-pass filter having a cutoff frequency 

 below the first harmonic of the pulse, the modulation is recovered and will 

 have an amplitude m{tjT) cos [w^/ + <^]. Since t jT typically may be on the 

 order of xoVo this is a very inefficient process. Pulse-stretching circuits are 

 therefore used to lengthen a series of pulses without changing the relative 

 pulse amplitudes in order to obtain more gain in the process of recovering 

 the amplitude-modulating signal. For most efficient demodulation the 

 pulse is lengthened for a full period. In either case — whether a pulse is 

 simply filtered or is lengthened and then filtered, time selection of the pulse 

 is required prior to the lengthening to prevent cross modulation by un- 

 desired pulses. 



A pulse lengthener converts the modulation function 1 + m cos (co^^ + <^) 

 into a new function F{f). Two types of lengtheners are used. In one, F{t)^ 

 is set at a fixed reference level prior to a signal pulse input; in the other the 

 output is changed from the value measured to the new value. Typical 

 circuits of these lengtheners are shown in Fig. 7-15. The lengthened pulse 

 on which the desired signal is modulated is an exponential pulse. The 

 decrement is small and approaches zero in many practical cases. 



