384 



THE RADAR RECEIVER 



of 750 ohms would be used. The efficiency of the detector would be 0.21, 

 assuming a diode resistance of 200 ohms^ and a 60-Mc IF frequency. A gain 

 loss of 13.6 db is thus exhibited by the detector. This is a typical loss; the 

 loss usually ranges between 6 and 15 db, depending on the video bandwidth 

 and IF frequency involved. 



An important design consideration is the loading on the IF network 

 produced by the detector. An approximation of this loading is given by 



R = ^ (7-37) 



where R is the IF network loading and rj is the efficiency of rectification. 

 Efficiency of rectification depends on the diode resistance Rd plus the IF 

 signal source resistance. Since the value of Ra depends on the voltage 

 applied to the diode, the detector is nonlinear at low levels. A typical 

 second-detector characteristic is shown in Fig. 7-13. Reproduction of the 

 modulation on a PAM (pulse amplitude modulated) signal depends there- 



1.00 



0.10 



0.01 



0.001 



1.0 10 



INPUT VOLTS (rms) 



100 



Fig. 7-13 Transfer Characteristic of a Typical Wide-Band Envelope Detector. 



'Determination of efficiency and input impedance is relatively complicated. Methods for 

 determining these quantities may be found in K. S. Sturley, Radio Receiver Design, Vol. 1. 



