872 



THE BELTj system TECHNICAL JOURNAL, JULY 1953 



7230 7235 7240 7245 7250 7255 7260 7265 7270 7275 7280 7285 

 FREQUENCY IN KILOCYCLES PER SECOND 



Fig. 26 — Loss characteristic of the 7266 kc crystal filter. 



the normal rectified pilot. In the presence of the interfering signal Ei 

 a diode detector with a long time constants will deliver a dc output of 

 of Ep + Ei and thus give voltage addition between the pilot and the 

 interference. With a fast time constant the detector output can follow 

 the nearly sinusodial envelope variations and the dc level is changed 

 only sUghtly. The ac component may be suppressed by the cutoff of the 

 dc amplifier, but, even if this is not the case, the thermistor being a 

 thermal device responds to the total power rather than the peak am- 

 plitude. Thus the diode time constant is made long enough to hold over 

 a few cycles of the pilot frequency but short enough to follow the im- 

 portant interference difference-frequencies. 

 The dc amplifier consists of three triode sections essentially in parallel 



a .^-sr 



Ep + E-, 



. i 



TIME 



Fig. 27 — Diode detector output in presence of interference Ei. Ep is normal 

 pilot signal. Curve a obtains with long time constant, b with short time constant. 



