MULTICHANNEL MODULATION SYSTEM 



29 



in Fig. 14 in terms of compressed signal amplitude vs. uncompressed signal 

 amplitude, both in decibels. 



Slicers. The sheer circuit shown in Fig. 15 resembles a conventional 

 single-trip multivibrator in configuration, but functions somewhat differ- 

 ently because of the choice of parameters. In particular, capacitor C is 

 made large enough so that the potential drop across it does not vary appreci- 

 ably during normal operation, and plate resistor Ri is given a small value 

 such that the gain around the feedback path of the circuit is approximately 

 unity when both triodes are in their active regions. Germanium varistors 

 VRi and VR2 maintain desired bias conditions regardless of the number of 

 pulses or spaces in the input signal. Unlike the single-trip multivibrator, 

 which when tripped remains so until the charge on C has had time to relax, 



SLICING 

 POTENTIAL 



i 



+ 150V 



"TJ-L 



-150V 



Fic. 15. Slicer circuit. 



this circuit trips whenever the input signal falls through a narrow potential 

 range near the value E, and trips back again when the input rises through 

 the same potential range. A square wave of constant amplitude repre- 

 senting an accurate thin slice of the input is thus made available across 

 resistor R2. The circuit is capable of high speed, slicing pulses as narrow as 

 0.1 microsecond. 



Time gating may readily be included among the functions of this circuit, 

 through the addition of a triode having its plate and cathode connected 

 directly to the plate and cathode, respectively, of tube Fi. The grid of this 

 added triode is held normally at ground potential, and is pulsed in the nega- 

 tive direction by approximately rectangular gating pulses, having an ampli- 

 tude of about 2E. The total current passing through the common cathode 

 resistor i?^ is essentially constant, and if either or both of the paralleled 

 tubes are conducting at a given instant this current is carried by one or 

 shared by both of them. Hence the tripping action, involving transfer of 



