SINE WAVE OSCILLATORS 



Modern crystal oscillators 



Crystal oscillators of the type so far discussed are primarily intended for 

 radio work and the frequencies of crystals suitable for such circuits are 

 rather high to be of direct use as timers in electrophysiological circuits. Thus 

 it was at one time usual to employ 100 kc/s crystal oscillator and 'count 



Mean 



anode 



current 



Suitable 

 working 

 point I 



Crystal 

 frequency 



Figure 14.27 



Frequency 

 to which 

 anode circuit 

 is tuned 



down' 10 times to produce 10 kc/s (100 /^sec marks), count down 10 times 

 again to produce 1 kc/s (1 msec mark) and so on. Since marks are seldom 

 required at 10 [xsqc intervals, it is wasteful to produce an unnecessarily high 

 frequency such as 100 kc/s and then have to cut down 10 times before pro- 

 ducing a useful frequency. Modern four-terminal crystals such as the 

 Standard Telephone and Cable 4023 can be had which oscillate at frequencies 

 as low as 4 kc/s. The circuit recommended by Standard Telephones for 

 crystal oscillators of this type is given in Figure 14.28. The feedback path is 



lOk^Sg^F ^^^^ 



STC 

 type A023 



■o250V 



8aF 



hH' 



A7kft< O'OlpF 100 kn. 



II /A— * 



O-Ol^FJii ■ 



°K <A7kSl 



.220kft 



Figure 14.28 



mechanical through the crystal itself. The circuit has the D'Arguimbeau 

 amphtude regulating device to be discussed under RC oscillators. 



Timers for electrobiological work employing crystal oscillators have been 

 described by Dickinson^ (TATG), Kay^ (Modern) and Attew'^ (electron 

 coupled Pierce). 



220 



