A HIGH PRECISION STANDARD OF FREQUENCY 



501 



Since the frequency varies with the pressure surrounding the crystal 

 an approximate adjustment of the frequency may be made con- 

 veniently by an adjustment of pressure. 



The circuit of the crystal-controlled oscillator and the first amplifier 

 stages is shown in Fig. 11. The oscillator is of the familiar type in 

 which the crystal electrodes are connected to grid and ground and in 

 which a tuned plate circuit is used. The great advantage in being 

 able to ground one electrode was the major consideration in choosing 

 this circuit. With this circuit, as in the one described a year ago,^ 

 it has been found possible to choose plate tuning elements such that 

 slight variations in either the inductance or the capacity have little 

 effect upon the frequency. For certain values of inductance and 



Fig. 11 — Circuit of standard frequency oscillator. 



capacity the frequency, as a function of their product, takes on a 

 maximum value. The adjustment that gives the maximum value of 

 frequency is used, therefore, so that slight variations, such as those 

 due to temperature coefficient and aging of the tuning elements, will 

 have a negligible effect on the frequency. 



The output circuit of the oscillator is very loosely coupled to three 

 independent output amplifiers. This arrangement provides three 

 independent output circuits free from mutual interference and unable 

 to react to an appreciable extent on the crystal oscillator. 



The final adjustment of frequency is made with a small cylindrical 

 condenser, having a capacity of about 5 mmf., connected in parallel 

 with the crystal electrodes. The size of this condenser is chosen 

 such that an adjustment of one division on the dial corresponds to a 

 change of frequency of about one part in a hundred million. There 

 are 100 divisions on the dial and a total of 10 turns may be made 



