The Bridge Stabilized Oscillator* 



By L. A. MEACHAM 



A new type of constant frequency oscillator of very high stability is 

 presented. The frequency controlling resonant element is used as one arm 

 of a Wheatstone resistance bridge. Kept in balance automatically by a 

 thermally controlled arm, this bridge provides constancy of output ampli- 

 tude, purity of wave form, and stabilization against fluctuations in power 

 supply or changes in circuit elements. A simple one-tube circuit has 

 operated consistently with no short-time frequency variations greater than 

 ± 2 parts in 10*. Convenient means are provided for making precision 

 adjustments over a narrow range of frequencies to compensate for long-time 

 aging effects. 



Description of the circuit is followed by a brief linear analysis and an 

 account of experimental results. Operating records are given for a 100 kc. 

 oscillator. 



Introduction 



THE problem of improving the stability of constant frequency 

 oscillators may be divided conveniently into two parts, one 

 relating to the frequency controlling resonant element or circuit, and 

 the other to the means for supplying energy to sustain oscillations. 

 The ideal control element would be a high-(2 electrical resonant circuit, 

 or a mechanical resonator such as a tuning fork or crystal, whose 

 properties were exactly constant, unaffected by atmospheric conditions, 

 jar, amplitude of oscillation, age, or any other possible parameter. 

 The ideal driving circuit would take full advantage of the resonator's 

 constancy by causing it to oscillate at a stable amplitude and at a 

 frequency determined completely by the resonator itself, regardless of 

 power supply variations, aging of vacuum tubes or other circuit ele- 

 ments, or the changing of any other operating condition. 



This paper, concerning itself principally with the second part of the 

 problem, describes an oscillator circuit which attains a very close 

 approximation to the latter objective. The "Bridge Stabilized Oscil- 

 lator" provides both frequency and amplitude stabilization, and as it 

 operates with no tube overloading, it has the added merit of delivering 

 a very pure sinusoidal output. 



Oscillator Circuit 



The bridge stabilized oscillator circuit, shown schematically in Fig. 1, 



consists of an amplifier and a Wheatstone bridge. The amplifier out- 



* Presented at Thirteenth Annual Convention of Institute of Radio Engineers, 

 New York City, June 16, 1938, Published in Proc. I. R. E., October 1938. 



574 



