324 GENERIC TYPES OF RADAR SYSTEMS AND TECHNIQUES 



target and clutter signals produces two symmetrical sidebands around each 

 PRF line. The folding effect places a further limitation on the minimum 

 allowable PRF: considering the effects of adjacent spectral-line interference 

 due to folding, the minimum PRF is 



fr > — rj:!!^ (when signal "folding" occurs) (6-32) 



A ^ 



The resulting video signal then is passed through a bandpass filter to 

 remove all the zero-frequency clutter components and all of the higher- 

 frequency sidebands of both signal and clutter. The resulting output, then, 

 is simply the doppler return associated with the central line of the trans- 

 mitted spectrum. 



The process of "folding" also doubles the thermal noise which competes 

 with the doppler return associated with the central line of the transmission. 

 For this reason, some form of single-sideband detection process is usually 

 employed in pulse doppler and CW doppler systems in preference to the 

 simpler system described here. 



The doppler filter may be a single filter as shown, a bank of contiguous 

 narrow-band filters as was shown in Fig. 6-15, or a single narrow band filter 

 which sweeps over the total range between /d,min and/d,max- The comments 

 in the preceding paragraph concerning the various filter types for CW 

 systems also apply to pulsed-doppler systems. 



The simple pulsed-doppler system considered does not possess a range- 

 measurement capability. Thus its information matrix and information rate 

 are the same as shown for the simple CW system in Fig. 6-16. Actually the 

 only reason for using a simple system of the type described is to eliminate 

 the duplexing problem of a CW system and permit the use of a single 

 antenna. In most other respects, this simple system is inferior to a simple 

 CW system. Specifically, it possesses as deficiencies (1) greater complexity, 

 and (2) less efficient use of power, since only the target power associated 

 with the central spectral line is detected. 



The second point deserves further amplification. In a CW radar of 

 transmitted power P, the peak and average powers are equal because the 

 duty cycle is unity. All of this power is effective for detection of the target. 

 However, for a pu/sed radar, the peak and average powers are related by the 

 "duty cycle" — i.e., by the ratio d of "on" time to total time. Thus 



Ptd = Pare. (6-33) 



For pulsed doppler detection — as previously described — only the power 

 in the central spectral line is used for detecting the target. The ratio of this 

 power to the total power may be expressed 



Power effective for target doppler detection = {Pave)d. (6-34) 



