Sampling oscilloscopes 99
a.c. amplifier, which explains the second, longer, lower bulge in
its output waveform. But as the memory gate is not conducting
during this period, it is of no significance. It is worth noting that
the combination of a.c. amplifier (acting as a differentiator) and
memory (acting as an integrator) ensures that the d.c. compo-
nent of the signal will in fact be passed by the circuit.
Figure 6.5 shows a second sample then being taken, and since
at this time the signal is at 3 vertical units and the sampling gate
output already sits at 2 vertical units, the circuit sees a potential
difference across the gate of only 1 unit. With a sampling
efficiency of 25 per cent, the output moves only a quarter of a
unit before the sampling pulse ends, but with the same circuit
gains as before this results in just the right amount of change to
bring the memory output to the correct level.
Looking now at the solid-line drawing of Figure 6.6, the more
common case is shown where, at the time of the second sample,
the signal is still at the same voltage as on the first. There is
therefore no voltage across the sampling gate when it conducts,
no energy need be transferred, no kickout occurs, the a.c.
amplifier sees no change at its input and thus produces no
output, and the memory remains at the same level. All is well in
the best of all possible worlds.
But Figure 6.6 also illustrates with dashed lines how the
feedback loop takes care of departures from this ideal. As an
example, it has been assumed that the a.c. amplifier gain is
excessive. This means that the memory output will be too high,
and the dot will appear too high on the c.r.t. Because, in Figure
6.6, the signal level for the second sample is unchanged, the
action of the feedback loop can be seen very readily. When this
second sample is taken, the voltage at the gate output is in fact
(erroneously) too high, so energy will be transferred in the
opposite direction and the gate output voltage will drop down (by
the usual 25 per cent of the difference). This negative change is
seen and amplified and added to the memory, but since the a.c.
amplifier gain is excessive, it will again result in too much
movement. The original overshoot is overcorrected, giving an
undershoot of small amplitude. On the third sample the
overshoot is reduced still further and on successive samples the