382 



THE RADAR RECEIVER 



-500t20- 



-20 -16 -12 -8-4 4 8 12 



SIGNAL POWER (dbm) 



Fig. 7-11 Transfer Characteristic of a Microwave Mixer at Large-Signal Input 



Levels (1N23C Crystal). 



7-15 THE SECOND DETECTOR (ENVELOPE DETECTOR) 



An envelope detector is employed to produce an output voltage which 

 corresponds to the envelope of the IF signal. The envelope detector is 

 actually a mixer in which the sidebands of the signal are heterodyned 

 against the signal carrier thereby producing as one output the modulation 

 that existed on the IF signal. In the ordinary noncoherent pulse radar set, 

 a diode detector is frequently employed. A typical circuit is shown in 

 Fig. 7-12, together with the current- voltage relations that exist under 

 large-signal conditions. A pulse of IF voltage is indicated as being applied 

 to the detector. A large diode current pulse flows for a short time following 

 the application of the signal. Capacitor Co is a relatively low impedance at 

 the IF frequency compared to i?o, and RFC is a high impedance to these 

 frequency components; therefore negligible voltage appears across i?o due 

 to the IF frequency components and their harmonics which appear in the 

 diode current. The average value of the current pulse, however, does 

 produce a voltage across i?o- This voltage builds up at a rate dependent on 

 the capacitance Co + Ci + Ci and the diode resistance, and reaches an 

 average value Edc as shown in Fig. 7-12. The diode only conducts when the 

 instantaneous voltage applied to the diode exceeds Ex. As shown, conduc- 

 tion during time ab occurs and the capacitance Co + C\ is charged at a rate 

 dependent on the diode resistance and this capacitance. When the IF pulse 

 ceases, the diode is back-biased and returns to the unbiased condition with 

 a time constant /?o (Co + Ci + C2). [The effect of the inductance of the 

 RFC on this transient is usually negligible when the product of pulse length 



