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THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



oscillator. The useful output of the magnettor is the second harmonic of 

 the carrier frequency. The amplitude of the second harmonic signal is 

 proportional to the magnitude of the dc input current and the phase of 

 the second harmonic signal is determined by the polarity of the dc input 

 current. The output voltage of the magnettor is applied to an active 

 filter which is tuned to the second harmonic frequency. The signal is 

 then amplified in a tuned amplifier and applied to a diode gating circuit. 

 Depending on the polarity of the dc input current, the gating circuit 

 passes either the positive or negative half cycle of the second harmonic 

 signal. In order to accomplish this, a square wave at a repetition rate 

 equal to that of the second harmonic signal is derived from the carrier 

 oscillator and actuates the gating circuit. 



A circuit diagram of the AZS circuit is shown in Figs. 17(a) and 17(b). 

 The various sections of the circuit are identified with the blocks shown 

 in Fig. 16. The active filter is adjusted for a Q of about 300, and the gain 

 of the active filter and tuned amplifier is approximately 1000. The AZS 

 circuit provides ±1.0 volt of dc output voltage for ±0.05 microamperes 

 of dc input current. The maximum sensitivity of the circuit is limited 

 to ±0.005 microamperes because of residual second harmonic generation 

 in the magnettor with zero input current. 



When the transistor integrator is used together with the magnettor 

 AZS circuit, the slope of the voltage ramp is maintained constant to 

 within one part in 8,000 over a temperature range of 20°C to 40°C. 



5.0. The Voltage Comparator 



The voltage comparator is one of the most important circuits used in 

 analog to digital converters. The comparator indicates the exact time 

 that an input waveform passes through a predetermined reference level. 

 It has been common practice to use a vacuum tube blocking oscillator 

 as a voltage comparator. ^^ Due to variations in the contact potential, 

 heater voltage, and transconductance of the vacuum tube, the maximum 



DC 

 INPUT 



DC 



OUTPUT 



Fig. 16 — Block diagram of AZS circuit. 



