Circuit Descriptions
5–24
1502C MTDR Service Manual
Regulator
± 12V
± 12V± 15V
On board
supply
25 µs
One Shot
5 µs
One Shot
Step trigger
Inverted
Video Out
–1.6
Sample trigger
0.6 V
Adjustable
source
Step
Generator
Sampler
strobe
generator
2nd
Sampler
Hybrid
Sampling
bridge
+B
–B
Bridge
bias
Front
panel
cable
connector
Figure 5–12: Driver/Sampler Block Diagram
The bridge bias for the first sampler is set by U1070. With a zero voltage input
signal, the circuit holds
"
2.0 V on the bridge inputs. As the input signal moves, the
4 V window also moves to stay centered around it. This centering is accomplished
by feeding part of the output of U2050A into the bridge bias circuit. The outputs of
the bridge bias circuit are available on TP1020 and TP1021.
There a\re two incoming triggers: the sample and the step. Both require modification
before they are usable by the hybrid. The sample trigger is a 30-to-50-ns negative
TTL signal. This pulse is buffered by Q2030, then coupled to the hybrid through
T1020. This provides a differential drive that can have common-mode voltage on
it. The sampler pulse is also stretched to 5
m
s by U3030B to strobe the second
sampler. The step trigger, GEN TRIG
, is a 3-to-5-
m
s negative TTL signal, stretched
to the proper 25
m
s pulse length by U3030A. CR3020 and CR3021 provide a logic
OR of the incoming signal and the output of the one-shot. This prevents the
introduction of jitter on the trigger signal. The OR output can be observed at
TP3020.
There are seven power supplies for the hybrid:
"
5 V
S
,
"
5 V
P
,
"
12 V, and +0.6 V.
The
"
5 V supplies come on board as 5 V, so they require no regulation, but are
merely filtered before being used by the hybrid and the board. The
"
12 V supplies
enter the board as
"
15 V, so the necessary filtering and regulation is accomplished
by U3070 and associated circuitry. The +0.6 V supply is used by the hybrid to set
Bridge Bias
Trigger Pulse Shapers
Power Supply Conditioner
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