Tektronix ECO 422, Instruction Manual

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Theory of Operation
ECO 422
4–3
Primary and R261 for Backup) AC coupled with 47 and 0.01 F parallel
capacitors (C194 and C132 for Primary and C195 and C131 for Backup) samples
the input signals. This signal then goes to the Input Buffer.
Input Buffers.
The Emitter-follower Input Buffer (Q32 for Primary and Q33 for
Backup) reduces the input capacitance that could degrade the return loss at high
frequencies. The signal may be attenuated by Q36 for the Primary circuit and
Q37 for the Backup circuit. If the ATTEN1 from the Selector Switch signal is
high, it turns on the transistor that reduces the amplitude of the signal. This
allows checking a broader range of signals without the Negative Peak Detectors
being overdriven or saturated. The output of the Input Buffers goes to the
Negative Peak Detectors.
Negative Peak Detectors.
The Negative Peak Detector circuit begins with a
transistor that inverts the input signal. The Peak Detector circuit picks off the
input signal peaks, which charge the memory capacitor, C140 or C139. When no
peaks are present, the memory capacitor is discharged by R260 or R259. The DC
output of the peak detector is buffered by a low drift op-amp, U11A and B.
Voltage Comparator. The Voltage Comparator takes the output of the Negative
Peak Detector and compares it to a DC reference voltage, VREF1, selected from
eight possible choices: off, five presets, or two user-defined values. When the
input negative peaks generate a voltage less than the selected reference, the
output of the comparator goes low, signaling a fault to the control circuit. The
resulting signal is (P_AND_C) for the Primary and (B_AND_C) for the Backup.
(P_TEST1) and (B_TEST1) are only for automated testing purposes. All the
Primary signals are wire-ANDed together as the (P_AND_C) signal and sent to
the Main board via J19. The same is done with all the Backup signals.
Control and Remote 8
The Primary and Backup signals from the Voltage Comparators on the Main
board are ANDed with like signals coming from the Connector board and passed
to the Controller PLD that then generates the control signals for the switching
relays.