REFLEX OSCILLATORS 551 



signal frequency would require centimeter range amplifiers haying good 

 signal to noise properties. No such amplifiers existed for the centimeter 

 range, and it was necessary to beat the signal frequency to an intermediate 

 frequency for amplification before rectification. For a number of reasons, 

 such intermediate frequency amplifiers operate in the range of a few tens 

 of megacycles, so that the beating oscillator must generate very nearly the 

 same frequency as the transmitter oscillator. 



In radar receivers operating at frequencies up to several hundred mega- 

 cycles, conversion is frequently achieved with vacuum tubes. For higher 

 frequencies crystal converters have usually been employed. With few ex- 

 ceptions, the oscillators to be described were used with these crystal con- 

 verters which require a small oscillator drive of the order of one miUiwatt. 

 In general it is desirable to introduce attenuation between the oscillator and 

 the crystal to minimize effects due to variation of the load. Approximately 

 13 db is allowed for such padding so that a beating oscillator need supply 

 about 20 milliwatts. Power in excess of this is useful in many applications 

 but not absolutely necessary. Since the power output requirements are 

 low, efficiency is not of prime importance and is usually, and frequently 

 necessarily, sacrificed in the interest of more important characteristics. 



The beating oscillator of a radar receiver operating in the centimeter 

 range must fulfill a number of requirements which arise from the particular 

 nature of the radar components and their manner of operation. The inter- 

 mediate frequency amplifier must have a minimum pass band sufficient to 

 amplify enough of the transmitter sideband frequencies so that the modu- 

 lating pulse is reproduced satisfactorily. It is not desirable to provide much 

 margin in band width above this minimum since the total noise increases 

 with increasing band width. It is therefore necessary for best opera- 

 tion that the frequency of the beating oscillator should closely follow fre- 

 quency variations of the transmitter, so that a constant difference frequency 

 equal to the intermediate frequency is maintained. 



This becomes more difficult at higher frequencies, inasmuch as all fre- 

 quency instabilities, such as thermal drifts, frequency pulling, etc. occur as 

 percentage variations. Some of the frequency variations occur at rapid 

 rates. An example of this is the frequency variation which is caused by 

 changes in the standing wave presented to the transmitter. Such varia- 

 tions may arise, for instance, from imperfections in rotating joints in the 

 output line between the transmitter to the scanning antenna. 



For correction of slow frequency drifts a manual adjustment of the fre- 

 quency is frequently possible, but instances arise, notably in aircraft installa- 

 tions, in which it is not possible for an operator to monitor the frequency 

 constantly. Rapid frequency changes, moreover, occur at rates in excess 

 of the reaction speed of a normal man. Hence for obvious tactical reasons 



