7-15] 



THE SECOND DETECTOR (ENVELOPE DETECTOR) 



383 



Video Amp. 



Fig. 7-12 Typical Second Detector Circuit 



times IF frequency is greater than 50. As a result of this operation it is 

 necessary that Rq (Co + Ci + C2) be considered as a low-pass filter estab- 

 lishing the video bandwidth.] 



The efficiency of the diode detector is the ratio of the d-c voltage (Edc) 

 to the peak carrier voltage applied to the circuit. The efficiency depends on 

 the ratio of the diode resistance plus source resistance of the IF network as 

 seen by the diode to the load resistor Rq. Efficiency, however, also depends 

 on the ratio of the load resistance to the reactance of Co + Ci at the IF 

 frequency. In practice Co is usually on the order of 10 to 20 iJifif. Smaller 

 values of Co result in less voltage impressed on the diode because of the 

 division of voltage between Cq and C^. Ro is then selected on the basis of 

 video bandwidth requirements. A typical example is a requirement that the 

 video bandwidth be 10 Mc with a network impedance as seen by the 

 detector of approximately 500 ohms and a capacitance Ci of 10 finf. The 

 value of Ro is then fixed by Ci and the smallest value of Co that can be 

 employed. Assuming Co to be 10 nnf, Ro is required to be 796 ohms. An Ro 



