CRYSTAL OSCILLATORS 



between grid and earth, and between anode and earth. Putting these in 

 {Figure 14.22) and remembering that the crystal is to act as a parallel tuned 

 circuit, we see that we have a modified Colpitts oscillator in which some of the 

 tuning capacitance is direct across the coil and some of it is split, Colpitts- 

 wise, to effect the virtual tap on the tuned circuit {Figure 14.23). 



The output from a Pierce oscillator can be taken out directly to a high- 

 impedance load such as another valve as shown, or by electron coupling, in 

 which case the oscillator circuit undergoes the same kind of transformation 

 that occurred between Figure 14.6 and 14.12. The circuit as published will 

 have the appearance of Figure 14.24a, but a truer representation is Figure 

 14.24b. The purpose of the choke inductance is to carry the cathode current 

 while allowing the cathode to be an active electrode driven by the stray 

 capacitances as shown. As usual with electron coupled schemes the purity of 

 the output is poor and the anode load is often made a tuned circuit, where 

 this matters. 



TATG oscillator 



This has the form of Figure 14.25, from which it will be seen that an LC 

 tuned circuit is required as well as the crystal, but the circuit has the merit 

 of giving rather greater output. The mode of operation is complicated; the 

 name arises from an equivalent LC oscillator which is of little interest to 

 electrobiologists and has not been mentioned, the tuned anode, tuned grid 

 {Figure 14.26). The feedback is via the valve grid-anode capacitance, and can 



HT+ 



Figure 14.25 



Figure 14.26 



be shown to be positive at a frequency just below that at which the tuned 

 circuits are resonant. In the crystal oscillator, setting-up procedure involves 

 monitoring the anode currents as the tuned circuit is adjusted. As adjustment 

 proceeds it will be found that the anode current passes through a minimum 

 having one steep and one gradual side. The tuned circuit should be left on the 

 gradual side of the minimum as shown in Figure 14.27. The oscillations are 

 maximal when the anode current is adjusted to the bottom of the trough, 

 but this is an unstable point, since any upward drift in the tuned circuit 

 resonant frequency will cause the circuit suddenly to cease oscillating. 



219 



