RC OSCILLATORS 



Danger — cracked crystals 



It must always be remembered that the resonance of the crystal is mechanical 

 and, since quartz is a brittle material, that if the oscillations are allowed to 

 become excessive the crystal will break. There is no need for this, since the 

 function of a crystal oscillator is primarily to give the correct frequency, not 

 to supply large powers ; power can always be had by appropriate subsequent 

 valve amplification. In developing crystal oscillators the HT supply should 

 initially be low, and the a.c. voltage across the crystal monitored to ensure 

 that it does not exceed that recommended by the manufacturers. 



RC OSCILLATORS 



The RC oscillators are closely related to the RC tuned acceptor amplifiers 

 employing positive feedback; virtually they are tuned amplifiers with the 

 input terminal arrangement left out and AB made equal to one. 



It is therefore to be expected that there would be two of them, a 'phase 

 shift' oscillator and one involving the RC band-pass filter, a = 1. In fact 

 nearly all RC oscillators are of one of these two kinds, though the latter type 

 are known rather indiscriminately as Wien bridge oscillators. The author 

 feels that this is a bad name, since the RC band-pass filter is only half of a 

 Wien bridge, and is an acceptor network, whereas the Wien bridge is a null- 

 transmission rejector network. Only in some circuits, such as Figure 14.35, is 

 there an actual Wien bridge. However, the name is brief and seems to be 

 generally accepted. 



RC oscillators are nowadays almost universal for the generation of test 

 oscillations in the frequency range 100 kc/s-1 c/s or even lower, their advan- 

 tage being the cheapness with which such wide ranges may be covered. The 

 method is to have either R (or C) variable in switched steps, and C (or R) 

 continuously adjustable. If the lowest frequency to which the oscillator is to 

 tune is of the order of 20 cycles, design usually proves easier if a variable C 

 is used — a radio type multi-gang tuning capacitor — with switched R. If the 

 apparatus is to go down to a much lower frequency, then the R values 

 required become undesirably large since the capacitors usually have a 

 maximum value of some 500 pF per section; if the R values are large the 

 frequency determining elements in the circuit are prone to interference from 

 stray electric fields and elaborate screening becomes necessary; also there 

 may be transgressions of the valve maker's rules about the maximum value of 

 grid-cathode resistance. 



Thus for very low frequency RC oscillation it is better to have C switched in 

 steps and R continuously variable. If this is done the ganged variable 

 resistances must be of exceptionally good quality. The ganging of ordinary 

 double potentiometers is not as accurate as that of ganged capacitors and 

 when the frequency of an oscillator employing one is altered it will be found 

 that the automatic level control device is kept hard at work. 



In outhne the phase shift oscillator is shown in Figure 14.29a and the 

 Wien bridge type in Figure 14.29b. For a fixed frequency oscillator the phase 

 shift version is preferable on the ground of simphcity, but the Wien bridge 

 type is more usual in wide-band oscillators because fewer frequency deter- 

 mining components are required. The Wien bridge type requires an overall 



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