At these unstable points the anode current decreases with increasing anode voltage, thus giving
a negative resistance effect over this portion of the curve. The criterion of stability for any
operating point in Fig. 5.6 is that the anode current of the E1T must increase as the anode
voltage increases. That is, the point at which the load line cuts the characteristic of the tube is
stable if the characteristic at that point slopes upwards from left to right. If for any reason (such as
a change in the supply voltage) the potential of
x' alters fairly slowly, the anode current/anode
voltage characteristic will maintain the same
general form as shown in Fig. 5.6, but will be
moved horizontally along the x axis (anode
voltage axis) of the graph. This is because the
stabilising effect discussed above alters the
voltage of the anode and x" electrodes to
maintain the beam deflection almost constant.
Fig. 5.8 shows the anode characteristic for an
ElT with a potential of 170V applied to the x'
deflector electrode. It can be seen that the
same system of stable and unstable operating
points will be present and the general
operation of the tube is unaffected by this
voltage change.
5.4 THE COUNTING PROCESS
A very different process occurs when a positive going pulse with a very short rise time is fed to
the x' electrode. The beam will be deflected to the left and the potential of the anode and x"
electrode will again tend to rise by the process discussed previously. The capacitance C (shown
dotted in Fig. 5.7) prevents any very rapid change in the potential of the anode and of the
electrode x", as time is taken for C to charge through R
a
. The capacitance C is merely the inter-
electrode and stray wiring capacitance of the tube circuit. The beam is therefore deflected to the
left before the voltage of x" has time to rise appreciably. If the pulse is rapid enough and of a
suitable amplitude, the beam will therefore move to the next stable position to the left of the
initial position in Fig. 5.6 and a count will have been registered. The stabilising mechanism of the
tube circuit cannot work more quickly than is permitted by the anode resistance R
a
and the
unavoidable stray parallel capacitance, C. The pulse rise time and amplitude are quite critical. If
the pulse is of too small an amplitude, the beam will not be deflected as far as the next stable
position and no count will be registered, whilst if the amplitude is too large, the beam may pass
through one stable position and register two counts for only one input to x'. The amplitude of the
input pulse should be approximately equal to the difference of the tube anode voltage between
two adjacent working points, e.g. a and c in Fig. 5.6. The geometry of the tube and the shape of
the electrodes are carefully chosen so that the voltage difference between each of the stable
working points (a to c, c to e, etc. in Fig. 5.6) is constant (about 13.6 V). The input voltage required
to cause the tube to register one additional count is therefore independent of the digit being
indicated. It is most important that the input pulse amplitude to the x' plate of the tube should be
13.6V ± 15% (that is, 11.5 to 15.5V). An additional requirement is that the trailing edge of the
pulse must not be too sharp or it will deflect the electron beam back to its initial state and no
count will be registered. If the slope of the trailing edge is not very great, the stabilising effect
discussed previously will prevent the tube returning to its initial state when the trailing edge is
applied to x'. If the time of fall of the pulse is too long, however, the maximum counting speed is
reduced. It might be thought that if the stray capacitance, C, could be made very small, the
maximum counting rate could be increased. In actual practice, however, the reset time is usually
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Fig 5.8 - The E1T Anode Characteristic for Vx’ = 170V
Summary of Contents for An E1T Timepiece
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