MULTICHANNEL MODULATION SYSTEM 



27 



preferable for use in the collector, where twelve samplers are multipled to a 

 common load. On the other hand the triode type affords a d-c.path (without 

 the series blocking capacitors that are required by circuit a) between source 

 and load. In cases where a holding capacitor is to be charged to a succession 

 of sample amplitudes which must be kept mutually independent to avoid 

 crosstalk, the d-c. path is very desirable for it avoids "memory" effects 

 associated with passage of the charging currents through the blocking 

 capacitors. A further useful property of the triode circuit is its abiHty to 



LOW- IMPEDANCE 

 SOURCE OF 

 SIGNAL TO BE 

 SAMPLED r 



\ 



PULSE 

 TRANSFORMER 



(a) 



LOAD 



CONTROL PULSE 



LOW-IMPEDANCE, 

 SOURCE \ 



(b) 



1\ 



I — ^MSu- 



L 





PULSE 

 ■TRANSFORMER 



LOAD 

 (R OR C) 



CONTROL PULSE 



Fig. 12. Diode and triode sampling circuits. 



sample signals of greater amplitude than that of the control pulses. With 

 the diode circuit, the signal amplitude must be kept less than half the pulse 

 height. 



Instantaneous Compressor. The simple configuration of the non-linear 

 circuit used in the compressor and in the feedback path of the expandor is 

 shown in Fig. 13. Two selected silicon rectifiers are connected in parallel, 

 but poled oppositely, with a small padding resistor {Ri or R'^ in series with 

 each. A parallel padding resistor {Rz) of large value is also provided. The 

 direct-current resistance of this combination varies from about 6000 ohms 

 at zero signal to about 190 ohms at the peak of a full-load signal. Input is 

 applied as a current through the relatively high resistor R\, and the voltage 



