THE BRIDGE STABILIZED OSCILLATOR 



587 



In Fig. 8, the gain of the amplifier and the output level of the oscil- 

 lator are plotted against plate battery voltage, while in Fig. 9 the same 

 quantities are related to filament potential. These curves show that 

 although power supply variations change the amplifier gain, they have 

 but slight effect upon the amplitude of oscillation. This stabilization 

 is produced, as explained heretofore, by the action of the lamp. 



The oscillator was designed to work into a load of 150 ohms, its 

 output impedance approximately matching this value. It might be 

 expected that variations in the magnitude or phase angle of the load 



(D 53 



O 



> 51 



■7^, 



-8 S 



60 



80 



100 120 140 160 180 200 220 

 PLATE BATTERY POTENTIAL IN VOLTS 



240 260 



-10 



Fig. 8 — Amplifier gain and oscillator output level vs. plate battery potential. 



would affect the frequency materially even though a certain amount of 

 isolation is provided by i?7 and R^. However, measurements made 

 with (1) a series of load impedances having a constant absolute magni- 

 tude of 150 ohms but with phase angles varying between — 90° and 

 + 90° and (2) a series of resistive loads varying between 30 ohms and 

 open circuit, showed less than one part in a hundred million frequency 

 variation. Graphs of these results have not been included, since they 

 practically coincide with the axis of zero frequency deviation. 



The tuned transformers Ti and T^ in this experimental model pre- 

 cluded the suppression of harmonics by negative feedback, |/x| being 

 small at the harmonic frequencies. The tuning itself provided sup- 

 pression, however, so that the measured levels of the second and third 



