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THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1952 



is provided between the first and second stages. With the rec gain 

 control at maximum the third stage is driven to full output when the 

 input to the receiving filter is greater than about —50 dbm*. Where it 

 is not necessary to detect the presence or absence of carrier for super- 

 vision as described below, the control is generally set for maximum gain. 



The limiter output is passed through a frequency discriminator con- 

 sisting of two anti-resonant circuits in series, tuned so that one has a 

 parallel resonance at the low frequency edge and the other at the high 

 frequency edge of the channel band. The voltages appearing across the 

 anti-resonant circuits are rectified separately by germanium varistor 

 diodes and the resultant d-c output voltages are added algebraically, 

 filtered and applied to the control grids of the output tubes. 



Since at normal receiving levels the limiter removes all magnitude 

 variations, the output from the discriminator detector circuit is depend- 

 ent in magnitude and sign only on the signaling frequency. A negative 

 voltage from the detector causes cut-off of the amplifier tubes and a 

 positive voltage causes plate current to flow. A switch between the 

 discriminator and the detector provides means for reversing the out- 

 put connections of the discriminator so that a positive voltage from 

 the detector can be obtained with either the higher or the lower sig- 

 naling frequency. Fig. 4 shows the dc voltage output versus frequency 

 characteristic obtained with a typical discriminator. 



D 

 O 



0-40 



-240 -200 -160 -120 -80 -40 40 80 120 160 200 240 



FREQUENCY IN CYCLES PER SECOND FROM MID -BAND 



Fig. -1 — Frequency characteristic of typical discriminator. 



'dbm" is an abbreviation for "decibels with respect to one milliwatt." 



