238 



BELL SYSTEM TECHNICAL JOURNAL 



Heterodyne Oscillator. The major requirement to consider in the 

 design of the heterodyne oscillator was that of frequency stability. As 

 the only function of the oscillator was to furnish a current for hetero- 

 dyning the one being measured the output requirements were moderate, 

 10 milliamperes into 600 ohms being ample, but it was important that 

 the frequency remain constant during a series of measurements as even 

 slight variations, of a fraction of a per cent, would change the attenua- 

 tion of the 800 cycle tuned circuit to the sideband current. Stability 

 in this case depended mainly upon the "A" and " B" battery voltages, 

 since the output load consisted of a pure resistance and there was no 

 reaction back to the oscillator due to a variable output impedance. 



TO MODULATOR 



I'ig. 7()s(illatoi' for liettTodyiie current analyzer 



The oscillator circuit is shown in Fig. 7. This shows two tubes, one 

 as an oscillator and one as an amplifier. The coupling consists of a 

 20,000-ohm resistance used as a potentiometer, which is placed in 

 series with a 100,000-ohm resistance and the two used as the oscillator 

 load. This makes the coupling impedance only one sixth of the total 

 oscillator output impedance and therefore reduces the effect which the 

 amplifier tube might have on the frequency. The change in frequency 

 due to the "A" and "B" voltage can also be controlled by inserting a 

 high resistance in the feed-back path between the plate and oscillation 

 circuits. This should be several times that of the tube impedance so 

 that any change in the latter would then be a proportionately smaller 

 part of the total impedance and hence have a less effect upon the fre- 

 quency. 



The selection of tuning coils for various frequency ranges is made by 

 keys which at the same time select the proper feed-back resistance. 

 Only three coils are used to cover the frequency range between 3,000 



