THE BRIDGE STABILIZED OSCILLATOR 



575 



put is impressed across one of the diagonals of the bridge, and the 

 unbalance potential, appearing across the conjugate diagonal, is applied 

 to the amplifier input terminals. One of the four bridge arms, Ri, is a 

 thermally controlled resistance; two others, R^ and Rz, are fixed re- 

 sistances, and the fourth, Z4 = Ri -\- jX^, is the frequency-controlling 

 resonant element. 



In this discussion Z4 is assumed to represent a crystal suitable for 

 operation at its low-impedance or series resonance. A coil and con- 

 denser in series could be substituted, and even a parallel-resonant 

 control element might be used by exchanging its position in the bridge 



VOLTAGE ATTENUATION 



VOLTAGE AMPLIFICATION 



Fig. 1 — Schematic circuit diagram of bridge stabilized oscillator. 



with Ri or Rz. Operating a crystal at series resonance has the advan- 

 tage of minimizing effects of stray capacitance. 



The bridge is kept as nearly in exact balance as possible. Assuming 

 that Rx, i?2 and Rz are pure resistances, we may write for exact reactive 

 balance, 



^4 = 0. 



and for exact resistive balance, 



Ri _ Rz ^ 

 R2 Ri 



In order that the circuit may oscillate, a slight unbalance is required. 

 Accordingly Ri must be given a value slightly smaller than (R2Rz)/Ri, 



