EVOLUTION OF QUARTZ CRYSTAL CLOCK 551 



Numerous vacuum tube circuits have been proposed and used for main- 

 taining quartz resonators in oscillation, some of which are illustrated in Fig. 

 8. The one among these which at present most nearly approaches the ideal 

 is that developed by L. A. Meacham, known as the Bridge Stabilized Oscil- 

 lator.^^ This oscillator, in its original form or with slight modifications, 

 is now used almost universally in England and America where the maximum 

 stability of rate control is required. 



In the bridge stabilized oscillator, the feedback path is through a Wheat- 

 stone bridge with the crystal in one arm and with resistances in the other 

 three. The frequency of oscillation becomes that for which the reactance 

 of the crystal approaches zero; the bridge can only be balanced when the 

 crystal behaves electrically like a resistance. The unbalance voltage from 

 the bridge is fed back into the amplifier, which should provide a relatively 

 high gain, as will appear. The great frequency stability of this oscillator 

 depends upon the fact that, in the neighborhood of balance, a small phase 

 shift in the resonant elements causes an enormously larger phase shift in 

 the unbalance voltage. But the actual amount of this unbalance phase 

 shift is limited by the fact that it must be equal and opposite to that in the 

 amplifier in order for oscillations to be sustained. This insures that at all 

 times the phase shift in the crystal is much smaller than that occurring in the 

 amplifier which itself can be made small by suitable design. The ratio of the 

 phase shift of the bridge output to that of its input increases as balance is 

 approached, making it possible to practically ehminate the effect of phase 

 shift in the amplifier simply by increasing the amplifier gain. Most of the 

 variable factors in the amplifier of an oscillator circuit affect the controlled 

 frequency through the phase shifts caused by them. It is evident, then, 

 that the bridge circuit, which permits only a small fraction of such phase 

 shifts to become effective at the resonant element, will substantially free the 

 resonator from variable effects in the amplifier and allow it to control a rate 

 determined almost wholly by its own properties. 



When the above condition is attained and the crystal resonator, when 

 oscillating, acts in the circuit like an electrical resistance, it acts that way 

 because the velocity is in phase with the applied mechanical force, which, as 

 has been stated, is the condition for most stable rate control. In the crystal 

 oscillator, this ideal condition is obtained simply by the automatic balancing 

 of a bridge circuit, accomplishing in a most elegant manner the equivalent, 

 in the case of a pendulum, of applying driving pulses at the exact center of 

 swing. 



The bridge-stabilized oscillator includes also an automatic control of 

 amplitude. The variation of frequency with amplitude is very small and 

 in no way comparable with the "circular error" of an ordinary pendulum, but 

 in the quest for the highest attainable stability it must be taken into account. 



