CRYSTAL OSCILLATORS 



CRYSTAL OSCILLATORS 



In applications where extreme stability of frequency is required, such as in the 

 production of time markers, quartz crystal oscillators are used; for with these, 

 provided the crystal is of good quality, the circuit must of necessity oscillate 

 at the correct frequency if it is to oscillate at all. 



Oscillation secured with the aid of a quartz crystal is associated with a 

 wec/ja«/ca/ resonance, which, in virtue of the mechano electric coupling by the 

 Piezo effect, appears between the crystal terminals as an electrical resonance 

 of remarkable sharpness. The electrical equivalent circuit has the form of 

 Figure 14.20, from which it is clear that two forms of resonance are possible, 



CD 



T 



1 



Ci 



Figure 14.20 



one series and one parallel. In crystal oscillators we are concerned only with 

 the parallel resonance. Quartz is an almost perfectly elastic material which 

 implies that if shocked mechanically into vibration the vibrations will die 

 away extremely slowly. We have seen that in LCR circuits this is a state of 

 affairs associated with high Q, and it is to be expected that the effective Q of a 

 crystal will be large, and so it is. 



A certain 450 kilocycles crystal had an equivalent L of 12-8 henries, 

 Ci = 0-01 pF, R of 1,600 ohms and Cg of 18-3 pF. In computing the Q 

 value we can take C = 0-01 pF, since the value of capacitance formed by Q 

 and C2 in series is almost the same as Q alone. Then 



2=- (7^1 ■-r> = [wu) •r^= 22,400 



1600 



Similarly a 3 Mc/s crystal described by Scroggie* had L = 0-127 H, Q = 

 0-022 pF, Cg = 8 pF and /? = 30 ohms, and 



/ 0-127 V^ 1 

 2=(2T^n(Fn) X 3-^ = 80,000 



There are two classical crystal oscillator circuits, the Pierce and the 'TATG'. 



Pierce oscillator 



The Pierce oscillator is extremely simple and has the appearance of 

 Figure 14.21. To see how it works we have to remember the stray capacitances 



* Radio Laboratory Handbook. 



in 



