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similar result. With this in view, a general method for frequency conversion 

 has been developed by R. L. Miller^^, in which the existence of an output 

 depends directly on the presence of the control input. The basic idea in- 

 volved in this, now known as regenerative modulation, was anticipated by 

 J. W. Horton in 1919^^ but had not been developed prior to Miller's in- 

 vestigations. The circuit of a regenerative modulator in its simplest form 

 as a frequency divider of ratio "two" is shown in Fig. 12. 



Soon after the announcement in 1927 of the first quartz crystal controlled 

 clock,^^ the idea was studied and applied in many places notably in America 

 and Germany, and at the present time it forms the basis for precise measure- 

 ments of time and frequency in many government physical laboratories 

 as well as in many astronomical observatories and industrial and university 

 laboratories throughout the world. 



INPUT, 2f 



OUTPUT, f 



Fig. 12 — Frequency divider for ratio TWO employing regenerative modulation. 



Although the first results were quite satisfying, it was the immediate 

 interest of all concerned to find out what improvements could be made, 

 and these were not long in coming. As in the case of the pendulum already 

 discussed, or with any other oscillator, the constancy of rate obtainable 

 depends on two kinds of properties: those which concern the inherent 

 stability of the governing device itself, and those concerned with the means 

 for sustaining it in oscillation. Some of the factors in the two groups are 

 interrelated and must be considered together. 



The improvements in quartz oscillator stability therefore have been 

 concerned with two main endeavors, namely that of cutting and mounting 

 the resonator so as to realize effectively the unusually stable properties of 

 quartz crystal itself, and that of coupling it to the electrical circuit in such 

 a way as to avoid the effects of such variables as power voltage variation, 

 aging of vacuum tubes, and the like, on the controlled frequency. The 

 latter effects were not obvious at first because the temperature coefficient 

 and the effects of friction and change of position in the mounting caused 



