691 
= 52 = 
from photographic records is also desirable for some uses, To cover the 
range of time scales employed at this laboratory, several types of time 
standards have been usede 
(a) Multivibrators, -- For time scales requiring time standards of 
frequency greater than 1000 cycle/sec, the multivibrator is the most reliable 
and convenient. In this circuit, a frequency standard, usually a crystal 
oscillator, generates an accurate time signal, whose frequency is divided 
down to the values desired for calibration. 
The circuit, shown in Fig. 2), is a multivibrator designed at this 
laboratory to generate output frequencies of 1, 5, 10, and 25 ke/sec, ob- 
tained from two multivibrator stages synchronized with a 100-kc/sec ecrystal- 
controlled oscillator, The output signal consists of very sharply spiked 
pulsese 
The fundamental oscillator is a modified Colpitts circuit, electron- 
coupled and controlled by a Bliley SOC-100 crystal. Small variable ceramic 
condensers afford small adjustments of the oscillator frequency over a range 
of approximately 100 cycle/sce, 
The multivibrators are thc conventional type employing 6SN7 tubes; the 
first multivibrator stage, operating at 25 and 10 ke/sec, is synchronized 
with the 100-kce/sec oscillators; the second, operating at 5 and 1 ke/sec, 
with the 10-ke/sec output of the first. The synchronization pulses are 
sharply peaked and applied across a resistor common to both multivibrator 
grids, in order to assure equal synchronization of both sides, The fre-= 
quency=range switching is accomplished by changing the grid circuit elements, 
while the fine adjustments of each range utilize variable resistors in the 
gridse 
The output signal of each multivibrator is peaked by a grid-leak biased 
amplifier. Variable negatively and positively spiked output voltages are 
available with a maximum output impedance of 500 ohms at the ends of the 
rangee 
The oscillator is adjusted by beating its signal on an ordinary receiver 
with the 5-mc/s& carrier of Station WVV of the National Bureau of Standards, 
The variable trimmer condensers should be adjusted together, to keep the two 
capacitances equal. 
