Symmetricom 5071A, Сборка и сервисное обслуживание

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3. Theory of Operation
Functional Block Descriptions
141
Analog Signal Chain Block
The analog signal chain block consists of the A14 signal-amplifier module
and A6 servo assembly. This block has three inputs and three outputs
providing a means of converting the CBT error-signal output into the EFC
voltage signal required by the A19 module EFC input.
This block amplifies low-level current output from the CBT, conditions the
signal, and converts it to a digital count which can be processed by the
feedback algorithm that runs on A3. The algorithm output is then transferred
back to the block to generate the EFC signal fed to A19.
The CBT output signal is a current with a peak value of about 100 nA. The
current carries low-frequency signals caused by the frequency and amplitude
modulation of the microwave probe signal processed by the CBT's transfer
function. Useful information is contained in the frequency range from dc-900
Hz.
The CBT output is fed to A14, a low-noise transimpedance amplifier with a
frequency response of dc-900 Hz. The push-pull output (A14 Vout) of the
amplifier is coupled to the differential signal-channel input of A6 which also
supplies it with power. The single-ended-to-differential transimpedance of
A14 is 50 M
Ω
.
The input stage of A6 feeds a programmable amplifier (gain set by A3 via I-
bus). The amplified CBT signal is then converted to a chain of pulses by a
linear voltage-to-frequency converter (VFC). The VFC output pulses are fed
to A3, and the number occurring during a predetermined gate time counted.
The number of pulses corresponds to the integral of the CBT output during
the gate time, and is the input to the servo algorithm.
The servo algorithm (P/O operating firmware) processes the signal counts
during successive gate times using the predetermined microwave probe
signal modulation sequence to generate estimates of the frequency, C-field,
and microwave probe signal amplitude errors.
The updated servo loop output is fed via the I-bus to a 16-bit DAC on A6. The
output of the DAC is low-pass filtered and fed by a special amplifier to the
EFC input of A19, closing the main servo loop. The A6 assembly also
contains power conditioning circuitry for A19.