r.lCllWt TCHIi ()SCIl.l..ll()l<S 519 



iii'cessitatfs a ^;i'iUTal rfadjustim-iit ol tin- nilur tlfincnl> .md liow lliis 

 riM(ljustmoiit reacts upon tlu- (iptTatiiit; point. 



l)uriiii,; thi- last few years tlir nri'd for oscill.iliiij; eircuils of ex- 

 ii'ptioiiall\- hiy;h fre{|ui"ncy stahility has Inroim.' more and more 

 pn's^ini;. Ihe retiiilrenients of multiplex telephony and telegrapln- 

 l)\ means of carrier currents si-t particularly severe limits on the 

 constancy of frequency of the alternating currents used. The efficient 

 u.so of the ether in radio communication also places a very narrow 

 tolerance upon any frequency variation in the carrier generators. 

 It may be of interest, therefore, to consider some of the fundamental 

 factors affecting the frequency stability of the vacuum tube oscillator. 



Two lines of attack are open; we can design the se\eral elements 

 so as to reduce the possibility of a change in the value of their con- 

 stants, or we can adjust the system so that una\-oidat)le changes 

 priKluce the least effect. It is in this latter connection that the 

 graphical method of analysis is particularly helpful. 



.A change in the constants of any element of the oscillating system 

 is going to result in a displacement or in a change in shape in at 

 least one of the two equilibrium ciir\-es shown in Fig. G. For a given 

 change in either curve the horizontal displacement of their inter- 

 section will depend upon the slope of the other cur\'e. The steeper one 

 curve is, the less will Ix" the frequency change resulting from any 

 variation in the other. It, therefore, follows that we should make 

 both curves as nearly vertical as possible. 



Referring to the gain and loss families, Fig. 5, it will be seen that 

 the slop)e of the amplitude equilibrium cur\-e, and consequently the 

 magnitude of the freciuency change corresponding to a given change 

 in the voltage at the reference jimction. is determined by three 

 things: 



1. The separation between the lines (letiiiiiig the i)ower gain in the 

 amplifier; the less this separation, the less will be the frcfiuency 

 change accompanying a given change in the \-oltage. 



2. The separation between the resonance curves defining the 

 p<ivver loss in the frequency' control unit; the less this separation, 

 the less will t)e the fretjuency change accompan\ing a gi\en change 

 in the voltage. 



3. The slope of the resonance curves; the steeixT these curves, the 

 less will be the frequency change accompanying a given change in 

 voltage. 



It appears then, that the change in fretjuency resulting from a 

 given change in phase displacement, that is, accompanying any 

 change in the pha.se equilibrium curve. ma\- be reduced by operating 



