734 BELL SYSTEM TECHNICAL JOURNAL 



protection against complete "blocking" of the IF amplifier under these 

 conditions involves the use of extremely sniall-valued filter or by-pass ele- 

 ments which are associated with the grid and cathode circuits to present 

 very short time constants and, accordingly, assure recovery of the amplifier 

 in fractions of a microsecond after overloading by a large pulse signal. The 

 gain control method is also chosen to minimize possible overloading by reduc- 

 ing the gain of the amplifier at a point as far forward in the radar receiver 

 chain as possible. 



Intermediate Midband Frequency 



The choice of the intermediate frequency for a radar receiver is essentially 

 a compromise between the need for reduction of unwanted external inter- 

 ference and noise, and the desire to realize maximum performance in terms 

 of gain and noise figure. 



The tendency to employ a high IF midband value arises from a considera- 

 tion of the character of the certain local beating oscillator noise sidebands. 

 As mentioned previously, the noise sideband output decreases as the fre- 

 quency interval from the oscillator frequency is increased and it is apparent 

 that a high value for the IF will be advantageous. In the case of balanced 

 converters where the oscillator noise sidebands are reduced by the circuit bal- 

 ance, this factor assumes less importance. A moderately high IF is also 

 advantageous in the elimination of the IF signal from the final detected video 

 output which must be accomplished by the use of low-pass filters. The 

 automatic frequency-control problem is somewhat simplified by the use of 

 a high IF. The wider separation of the desired tuning point and the image 

 response and the reduction of interfering TR pulse energy is a positive help 

 in the performance of the automatic frequency-control circuits and will be 

 discussed in detail in a later section. 



There are also a number of factors which indicate that a low value of 

 midband IF may be desirable. The noise figure of the input stage of the 

 IF amplifier is generally better at a low frequency, though this improvement 

 tends to be quite small for the band widths employed in the military radar 

 system. A more important advantage of a low IF value is found in the 

 improvement of the absolute frequency stability of the IF transmission char- 

 acteristic under the influence of variations of tube and circuit capacitance. 



Intermediate midband frequencies of 30 mc and 60 mc have been employed 

 in the majority of military radar systems developed in the United States 

 during World War II. In general, 30 mc has been employed quite exten- 

 sively in naval and ground radar equipments for fire control where radar 

 pulse widths of the order of one microsecond are used. For airborne radar 

 ecjuipments with the emphasis on compactness, weight, and the trend toward 

 higher microwave transmission frequencies, 60 mc has proven to be an 

 extremely popular intermediate frequency value. 



