THERMIONIC VACUUM TUBES 



91 



53. Oscillator for A.C. Measurement Purposes. For many a.c. 

 measuring purposes an oscillator whose output is both free from 

 harmonics and constant in frequency is desirable. Such a circuit is 

 shown in Fig. 66. the design of which is radically different from the 

 oscillator circuits already discussed. It possesses a tuned input LC 

 and coupling is supplied by the resistance R. R is usually given a 



Output 



Fig. 66 



value between 100,000 and 400,000 ohms. A negative grid battery 

 fixes the average grid voltage, the emf, of this battery being about 

 8 to 10 volts. It is customary to make the resistances r\ amd r 2 

 several thousands ohms apiece, r x being perhaps 5 times r?. The 

 condenser C\ is merely a blocking condenser and should offer little 

 impedance to the a.c. 



It is not difficult to make the oscillator of Fig. 68 maintain a fre- 

 quency that is constant within 3/10 of 1% and when the feed-back 

 is not too large the harmonics in the output will comprise only 5% 

 or even less of the total a.c. output. 



54. Range of Frequencies Obtainable with Vacuum Tube Oscillators. 

 Circuits have been constructed whose frequency is but a fraction 



Fig. 67 



of a cycle per second. The requirements of such a circuit are large 

 inductance and capacity and very close coupling between input and 

 output. A satisfactory circuit for low frequencies is that shown in Fig. 

 67; the two inductances taking the form of an iron core transformer. 



