Tektronix WFM601, Руководство по обслуживанию

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WFM 601 - Service Manual
3–7
Circuit Theory
The following discusses circuit theory for the WFM 601 Serial Digital Compo-
nent Monitor to the component level. It is arranged on a diagram-by-diagram
basis for the schematic diagrams located at the rear of the volume.
Diagram 1 Serial Inputs
The serial inputs are high return loss, 75 compensated (external termination
required) passive loop-through inputs. Q1 and Q2 are buffers that, in conjunc-
tion with the printed circuit inductors ( located on the BNC board), keep the
return loss relatively constant across the frequency range.
Q3, Q5, Q6, and Q8 form a switching circuit. When the base of Q8 goes low,
Q5 is turned on and Q3 is turned off allowing signal current from Q2 (Serial A)
to flow into Q4. When the base of Q8 goes high the opposite condition occurs
Q5 is turned off and Q3 is turned on allowing the signal current from Q1 (Serial
B) to flow into Q4.
Q4 drives the 75 input of the Deserializer on diagram 2. The gain from input
(bnc connector) to J1 is 0.85.
The reclocked serial signal, from the Deserializer (Diagram 2) passes back
through the Input board to the rear panel Serial Out connector. J8, J9, and J10
(bnc connectors on the rear panel) are the analog component outputs normally
used to drive picture monitors. The component analog outputs originate on
Diagram 8 (Transcoders and Picture Monitor Outputs).
The EXT REF input is a high impedance bridging loop through compensated for
operation in a 75
W
environment. Q7 is an emitter follower to provide the high
input impedance. CR7 and CR8 prevent the power supply impedance from
affecting the return loss characteristics of the loop through input when the power
supply is turned off.
Diagram 2 Deserializer
The serial signal from the input is applied to the AIX (pin 26) input of the
Deserializer, U3. It equalizes the signal for coax cable loss, recovers the data
clock, and converts the serial data to parallel data. R2 through Q1, an emitter
Input Amplifiers
Serial Out & Mon Out
External Reference Input
Deserializer