Circuit Descriptions
1502C MTDR Service Manual
5–23
Driver/Sampler
The front-end consists of:
H
Hybrid Sampler/Step Generator
H
Second Sampler
H
First Sampler Bridge Bias Generator
H
Trigger Pulse Shapers
H
Power Supply Conditioning
The function of this board is to generate the step test signal and to sample and hold
the reflections from the cable under test. A block diagram of these circuits is shown
in Figure 5–12 (next page).
Most of the primary active circuitry is located within the hybrid. The balance of the
Driver/Sampler Board is dedicated to interfacing with the rest of the instrument.
The step generator is triggered by a negative pulse from the Main Board. One of the
trigger pulse shapers stretches this to 25
m
s to set the length of the output step. The
0.6 V adjustable power source sets the “on” voltage for the output step.
The sampler is also triggered by a negative pulse from the Main Board. Inside the
hybrid, this trigger causes the strobe generator to apply 50-ps pulses to turn on the
bridge, capturing a portion of the input waveform. This sample is stored outside the
hybrid in the second sampler to reduce droop rate. The stored signal goes two places:
back to the Main Board as the video output, and to the bridge bias circuit, which
holds the sampling bridge off between samples.
The video signal from the hybrid is sent to the second sampler. The second sampler
reduces the droop rate to about 1 LSB/ms. This is accomplished by buffering the
signal through U2050B and storing it in C2053 via the FET switch, Q1060. The FET
is strobed by the one-shot U3030B for 5
m
s after the sample is taken. The voltage
stored on C2053 is buffered by op-amp U2050A, then inverted and amplified by
U1050A. The strobe signal for the FET can be observed at TP2060, and the inverted
video output at TP1060. The signal from the second sampler buffer, U2050A, is also
fed to the bridge bias amp, U1070, via R1060.
Introduction
Second Sampler
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