744 BELL SYSTEM TECHNICAL JOURNAL 



objective resulting in an over-all gain requirement of the order of 110 db. 

 If the radar system employs RF amplification, the entire IF amplification 

 may be provided in one unit. However, in radar systems operating above 

 1000 mc it has proved advantageous to provide the total IF gain required in 

 two separate amplifier sections. The IF preamplifier assembly is commonly 

 designed to be mounted adjacent to the crystal converter located in the 

 transmitter portion of the radar system and usually consists of two stages 

 of IF amplification. The main IF amplifier is usually located at some 

 distance from the preamplifier, commonly associated with the indicator 

 components of the radar receiver. The main IF amplifier assembly includes 

 the second detector circuit and occasionally one stage of video amplification 

 is included. 



The IF preamplifier location as described above is quire desirable, elimi- 

 nating the need for a long transmission line connecting the IF output circuit 

 of the crystal converter to the input stage of the IF amplifier. As has been 

 discussed previously, the impedance transformation employed in the IF 

 input stage is chosen to realize optimum signal-to-noise performance. The 

 output impedance of the crystal converter is normally of the order of 

 400 ohms. To assure negligible impedance reflection losses in this circuit, 

 any connecting cable employed would have to be designed to present a char- 

 acteristic impedance of this order of magnitude which is inconvenient. The 

 practical solution as employed in past military radar systems is obtained by 

 locating the IF preamplifier in close proximity to the converter. The 

 absence of long leads at this IF input stage is also advantageous in reducing 

 the interference pickup into this low signal level point. After moderate 

 amplification the output of the IF preamplifier is usually fed over a 75-ohm 

 coaxial transmission line to the main IF amplifier. 



Figure 28 illustrates the converter and IF preamplifier assembly as em- 

 ployed on the AN/APQ-13 and AN/APQ-7 airborne radar bombing equip- 

 ments operating at 10,000 mc. The local oscillator and silicon crystal con- 

 verter are arranged in a manner similar to a basic type previously described 

 in this paper. The IF out})ut of the crystal converter is introduced directly 

 into the preamplifier assembly without exposure. This preamplifier is 

 arranged to offer two stages of amplification employing the 717A pentode 

 and using a double-tuned input, interstage, and output network. Figure 2^ 

 indicates the circuit arrangement. The gain of this IF preamplifier is 30 db 

 and an IF band width of 6 mc is provided. The outi)ut transformer network 

 is arranged to operate into a 75-ohm coaxial transmission line. It should be 

 observed that provision is here included to disable the preamplifier by a|)pli- 

 cation of a positive pulse to the cathode circuit of the second amplifier tube. 

 This feature reduces the gain of the IF ])reamplilier during the short interval 

 coincident with the outgoing radar pulse, which assures that the TR tube 



