Fortress Technologies An E1T Timepiece, Руководство пользователя

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amplitude of at least 24V so that it is large enough to completely cut off the electron beam. The
main anode current falls and therefore the main anode and x" electrode potential rises. The
change of the x" electrode potential causes the beam to be deflected towards it so that 'zero' is
indicated. This method of resetting the tube takes a comparatively long time and cannot
therefore be used in high speed circuits. The circuitry required is, however, simpler than that used
in the higher speed resetting circuits. An example of a practical circuit involving beam cut off will
be given in the circuit of Figs. 5.10. In the second method of resetting the tube, a positive pulse is
applied to the x' electrode and deflects the beam to the zero position. This method is suitable for
high speed circuits operating at up to one million pulses per second.
5.5.1 - Reset Involving Beam Cut Off
When an E1T tube is cut off, its anode voltage will rise exponentially as the stray capacitance C
(shown dotted in Fig. 5.7) charges through the resistor R
a
. The time taken for this capacitance to
charge limits the maximum frequency of operation of the tube. The minimum reset time may be
estimated by the method discussed below. It is important to ensure that the duration of the cut
off pulse fed to the tube is great enough (with an adequate safety margin) to allow the stray
capacitance, C, to charge to a potential which is enough to cause the beam to return at least as
far as the zero position. Otherwise the beam may come to rest at any intermediate position. If the
cutoff time is too long, however, the reset time will be increased and the maximum counting rate
will be reduced. If the beam is deflected too far, it will be in an unstable state and will quickly
return to the zero position at the end of the cutoff pulse.
It can be estimated from Fig. 5.6 (allowing adequate safety margins for normal tolerances, etc.)
that the maximum voltage swing of the anode a2 ever likely to occur in practice is from V
a2
(9) =
95V in position 'nine' to V
a2
(0) = 240V at the 'zero' position. The maximum stray capacitance, C, in
parallel with R
a2
can be taken as 16.5 pF. If a close tolerance 1 % high stability resistor is used for
R
a2
, the maximum possible value of this resistor will be 1.01M Ω . In addition a 10k Ω resistor is
normally placed in series with R
a2
for test purposes (as shown in Figs. 5.13 and 5.15). The
maximum value of R
a2
is therefore 1.02M Ω . The capacitance C charges from the H.T. supply
voltage V
b
from the initial anode voltage of V
a2
(9) volts to V
a2
(0) volts during the cut off pulse. It is
shown in many elementary text books on electricity that if a capacitor C is charged from a source
of voltage V
b
via a resistor R, the voltage V across the capacitor after a time t is given by the
relation:
where e is the base of natural logarithms. The above equation may be altered to:
This equation applies only if V= 0 when t = 0. In the case of the stray capacitance C charging
through the resistor R
a2
, however, V= V
a2
(9) initially. If C had charged to a potential of V
a2
(9) from
an initial potential of zero through Ice the time taken, t
1
, would be given by:
Page 118 Version 1.0 Copyright Grahame Marsh/Nick Stock 2019
V
b
− V
V
= e
− t
RC
V = V
b
(1 − e
− t
RC
)
V
b
− V
a 2
(9)
V
= e
− t
1
Ra
2
C
(1)