F.l.EC I KIC.il tests ./.\/> llllilK .iri-l.lC.ITIOXS ,W) 



an- i-mploNoil. ll may, liowcNrr, In- usod lor dtlu-r kinds of iiK-asiiri-- 

 im-nts whiTc siiiglo frixiuonry rurrrnts of fairly j;oo<l wavf form aro. 

 ri'<|iiiri'<l. Other forms of vibrator oscillators are availahle, par- 

 tirularly for use in capacity and capacit\- unbalance tests and cross- 

 talk and noise tests. 



(2) Osiillators Employinii Motor Gnicrtilor Equipment. This tyjK' 

 of oscillator is illustrated iiy ordinary ringinj,; and trouble tone ma- 

 chines anil the low fre()ucncy alternating currents generated by these 

 machines are often used in tcstboard work. The circuit for an oscil- 

 lator of this tyjKN particularly designed for producing l.OOO-cycle 

 alternating current with good \\a\e form, is shown in Fig. :}(). 



A C Output 



24Volts 



Fis. 30 



In this circuit the field of an electromagnet is varied by a laminated 

 core or rotor resembling a spur gear driven by a small 24-volt d.c. 

 motor. The speetl of the motor is automatically regulated and the 

 electromagnet and rotor so designed that a l.OOO-cycle current is 

 generated. Harmonics which are inherent in the oscillator are 

 eliminated by the use of a filter. This oscillator can be operated on 

 the regular 24-volt central office battery and is compactly mounted 

 to make it readily portable. It is particularly adaptable, therefore, 

 for supplying the measuring current required to operate portable 

 visual transmission measuring sets and is now generally used for 

 this purpose in the telephone plant. 



(3) Oscillators Employing Vacuum Tubes. Fig. 31 shows the sim- 

 plified circuit arrangement of a vacuum tube oscillator. 



The Oscillating vacuum tube in this generator has its plate and grid 

 inductively connected together in a tuned circuit. Closing the fila- 

 ment battery circuit starts this tube oscillating, the freciuency of the 

 oscillations l)eing controlled by the inductance of the plate and grid 

 coupling and the variable condenser C. The current thus generated 

 is amplified by other vacuum tulx's to the values which are required 

 in the alternating current testing work. The circuit of Fig. 31 shows 



