376 THE RADAR RECEIVER 



approximations to this value when the 3-db bandwidth is approximatehy 

 1.2 /(pulse length).^ 



Additional filtering can be applied to the postdetection or video signal 

 when there is more than one pulse. A series of periodic pulses will have a 

 spectrum consisting of a number of harmonics. The filter which is matched 

 to such a signal will be tuned to these harmonics so as to amplify them and 

 attenuate the intervening noise. Because of the shape of the frequency 

 response of such a filter, it is sometimes called a comb filter. It is often more 

 convenient to obtain the effect of a matched filter by operating in the time 

 domain. The comb filter, which is appropriate for a series of pulses, can be 

 simply represented by adding the pulses after they are delayed by appro- 

 priate multiples of the repetition period. This operation is normally called 

 pulse integration and, for search radars, is often performed by the phosphor 

 of a B or PPI scope display. When the more elaborate technique of time 

 domain filtering is utilized, it is sometimes referred to as signal correlation. 

 A more detailed discussion of matched filters is given in Paragraph 5-10. 



In selecting a bandwidth characteristic for the receiver, three considera- 

 tions must be made over and above signal to thermal noise: 



1. Adjacent channel (frequency) attenuation and discrimination 

 against clutter 



2. Compatibility of transient response with required resolution 



3. Large signal operation 



The usual response characteristics that might be encountered were 

 indicated in Paragraph 7-7. The transient response of these networks 

 governs the resolution and large-signal behavior. The rectified envelope of 

 this response corresponds to the video signal. 



A typical transient response would 

 appear as shown in Fig. 7-9. At the 

 receiver output, a loss in sensitivity 

 may occur for the time /2 shown in 

 Fig. 7-9 if the signal becomes suffi- 

 ciently large that amplifier stages 

 are driven into saturation. In pulse 



T7 1 n ^ ■ ] r\ ^ ^^ • .. doppler radar receivers this is a more 



i*iG. 7-9 Typical Output Transients as ^^ 



They Appear on the Rectified Envelope serious problem than in conventional 



of the IF Response to an IF Pulse Input, radar receivers. 



When all of the networks have identical transfer impedance of the form 



^ £ 



(s — Si){s — Si*) 



6See J. I. Lawson and G. E. Uhlenbeck, Threshold Signals, Vol. 24, Sec. 8-6 (Radiation 

 Laboratory Series), McGraw-Hill Book Co., Inc., 1950. 



