316 GENERIC TYPES OF RADAR SYSTEMS AND TECHNIQUES 



From Equation 6-22, it is possible to evaluate the extent to which the 

 angular noise sidebands of a large signal will interfere with the detection of 

 another signal. However, it is difficult to apply Equation 6-22 in a general 

 manner since the effective modulation index is a periodic function of the 

 two variables /m and t where the maximum values of/m are at «/2r (where 

 « = 1, 3, 5, ... ) and the zeros are at/m = n /t (where « = 0, 1, 2, 3, ...). 

 Of more usefulness is a consideration of the two extremes of the relationship 

 as divided by -a cutoff frequency /^ at which the returned deviation, by 

 definition, equals the transmitted deviation. Below/c, where the time delay 

 is short compared to a modulation cycle, the angular indices of the detected 

 and transmitted signals are related simply by 



Mfr = M/,(27rr/,„). (6-25) 



Far above /c, where the time delay is long compared with a modulation 

 cycle, the two indices are, on the average, equal. 



For small indices of modulation such as are descriptive of useful trans- 

 mitting tubes it is more convenient to discuss potential interference in terms 

 of sideband power ratios relative to the carrier. Therefore, the ratio of the 

 power in a single sideband Psb, relative to the carrier power Pc for a detected 

 signal, is determined (for small indices only) by: 



n 



£f4^-i- »-j« 



Typically, for a 1200-cps power supply ripple component producing 240 cps 

 of frequency deviation, a carrier to single sideband power ratio of 100 would 

 exist. 



Random noise modulation may be considered to be composed of distrib- 

 uted components having a mean angular excursion per cycle of A/. T+te 

 composite mean deviation AF associated with a band of frequencies B cps 

 wide is then 



AF = Af^fB. (6-27) 



A not uncommon composite noise deviation in a 100-kc band B for practical 

 CW radars is 100 cps rms (AF), indicating about a 1 /3 cps rms/cps density 

 (A/)_. 



Fig. 6-18 indicates typical signal power levels which might be present in 

 a hypothetical FM /CW radar. 



FM/CW Airborne Radar Systems Applications. FM/CW doppler 

 systems are most commonly employed for applications requiring high 

 clutter rejection and a relatively low range information rate. AI radars, 

 missile seekers, and altimeters are good examples of such applications. 



