802 BELL SYSTEM TECHNICAL JOURNAL 



severe military field conditions. In the case of the usual radio-communi- 

 cation system, some knowledge of character and extent of the information 

 which is being transmitted is available to the receiving location which may 

 serve to evaluate the receiver operating performance, but in the case of the 

 military radar system such reference data is not always available. The 

 usual military operating conditions for radar systems are extremely severe 

 which, in general, tends to degrade the performance. Mistuning of the 

 radar receiver and the attendant reduction of the performance of the system 

 must be immediately evident to the operator even under conditions where no 

 radar signal returns are present. 



During the first years of the past war, this tuning problem was recognized 

 and the initial attempts at solution involved the inclusion of receiver tuning 

 indicators to serve as an indication of adjustment. As the radar systems 

 became more complex and with the trend toward the use of higher trans- 

 mission frequencies the necessity for completely automatic continuous 

 tuning adjustment of the receiver became increasingly evident and the 

 present types of automatic frequency control devices were developed. It 

 has been determined that in the specific case of airborne radar bombing 

 equipment operating at 10,000 mc that the automatic frequency control of 

 the receiver tuning is an absolute necessity, since the radar operator cannot 

 under the normal military operating conditions maintain the system per- 

 formance in this regard to a small fraction of the optimum. 



Functions and Requirements 



The basic reference for a radar automatic frequency control system must 

 be the transmitter frequency since it is required that the receiver be properly 

 tuned under the condition where no radar return signals are available. 

 Either the frequency of the transmitter magnetron or the local beating 

 oscillator frequency may be adjusted from an electrical error signal whose 

 characteristics are related to the tuning point. Magnetrons whose fre- 

 quency was conveniently controllable by remote electrical means were not 

 then available so that the later method has been universally apj)lied in mili- 

 tary radar systems developed during the past war period. 



It is ])ertinent to review the nature and extent of the frequency instability 

 of a radar .system to derive the requirements to be imposed upon an AFC de- 

 vice. The sources of frequency instability are associated with the trans- 

 mitter as well as the receiver elements of a radar system. The magnetron 

 frequency is determined in part by the physical dimensions of its oscillatory 

 cavity structure, and as would be expected, ambient temperature and 

 pressure conditions exert a decided influence. For example, a typical 

 thermal coefficient of frequency for a magnetron may be as high as 200 kilo- 

 cycles per degree Centigrade which ()\-er the range of ambient tcnii)eralures 





