671 
=e 
Several means of synchronizing linearssweep and rotating—drum re- 
cording of underwater explosions have been used at this laboratory, involving 
either the firing circuit or the pressure wave from the explosion, The details 
of these methods are described at the end of this sestion, 
(a) Linear time base, ~- The main funetion of a linear time base is to 
move the spot of a cathode-ray tube across the screen at a constant and pre- 
determined rate, This forms the time axis for the recorded transient. Other 
functions include the initiation of the sweep by a trip pudse, turning on the 
spot or beam at the same time, and turning off the beam at the end of the 
time, For convenience, a continuous sweep is available, as well as a self- 
tripping single sweep. 
The operation of the sweep may be understood by reference to Figs. 15 
and 16. A positive pulse (2 volts or greater) applied at the trip input 
terminals is amplified and operates a trigger circuit of the "flip-flcp" 
els This cuts off a switch tube, which allows the timing condenser to 
charge through a highly degenerated triode. With the aid of feedback from 
the succeeding cathode follower through a 200,000-chm restatare one output 
is a voltage whose decay is highly linear with respect to time. 
A cathode-follower output circuit is used to reduce the possibility of 
modulation by outside sources. For continuous sweep 2 portion of the sweep 
Signal is fed back to make a sclf-oscillatory circuit. Self tripping at 
2=sec intervals is obtained from the pulses of a neon-tube oscillator, 
Turning the cathode-ray tube beam on and off is accomplished by means 
of two signals fed into a mixer stage, The first pulse turns the beam on 
when the sweep starts, and the second turns the beam off when the sweep has 
reached its end, The regulation of the voltage supplies is rather critical, 
and vacuum-tube regulators are used for both positive and negative supplies. 
The troubles that have been encountered in the course of a year's use 
are as follows. If the tripping pulse is followed by a large negative signal 
(about 10 volts at full gain) the sweep can be stopped in midrange, This 
aa nn 
“This and other principles are described in an excellent book by O. S. 
Puckle, Time bases (New York, 19))3). 
10/ this circuit is essentially an integrator, and the feedback method 
applied to it is the analogue of a differentiator circuit described by 
Schmitt and Tolles, Rev. Sci, Inst. 13, 115 (192). 
