Tektronix 1S1 Instruction Manual page 40

circuit are applied to the sampling gate, strobing the gate
into conduction for a brief period, during which the input
signal is passed through the sampling gate. The signal
attempts to charge up the circuit capacitance to the level
of the input signal at the moment of sampling, but the
strobe pulses end and the gate stops passing signal before
the capacitance has had time to charge completely.
The small error-correction sample pulse that was passed
by the sampling gate is amplified by the sampler pre-
amplifier, then passed through a portion of the mVOLTS/CM
switch and sent to the ac amplifier to be amplified and
inverted again.
By the time the signal pulse reaches the memory gate, the
gate has been pulsed into conduction by the memory gate
driver, so the sample is passed from the ac amplifier to the
memory circuit. The memory reinverts the signal and stores
the value of the pulse on the memory capacitor. The memory
output level is then attenuated and applied through the out-
put cathode follower to the vertical amplifier of the display
oscilloscope.
Fig.3-3. A single sample signal traced through the feedback loop (see text).
Circuit Description – Type 1S1
A portion of the memory output signal is also sent back
through a positive feedback loop to the sampling bridge.
Since the memory gate stops conducting immediately after
the sample pulse has passed, the feedback signal is not
regenerated through the main signal path. The signal in
the feedback loop brings the charge on the input circuit
capacitance up to the input signal voltage level that was
present at the instant it was sampled. The vertical system
is then ready for the next sample to be initiated by the next
trigger pulse. Fig. 3-3 traces a single sample pulse through
the sampling channel, showing the effect of feedback. In
the illustration, the mVOLTS/CM switch is set to 200.
(a) The input signal is at a level of 100 mv at the time
the sample is taken, having risen from zero volts, the level
of the previous sample.
(b) A very fast rise is seen at the output of the sampling
gate while the gate is forward biased. The pulse amplitude
reaches about 30 millivolts before the gate stops passing
signal, then the level remains nearly constant while the
amplifiers process the sample.
3-3