HP 70427A User Manual page 451

Block Diagram
100 MHz Reference Loop (A7A1)
80 dB Isolator
The 80 dB isolation amplifier provides the isolation necessary to prevent
multiplier spurs, from the 10 to 100 MHz multiplier, from leaking onto the
100 MHz clean-up loop main output. The leakage path being eliminated is
from the multiplier output, through the phase detector, through the 100 MHz
phase detector amplifier, through the couplers and into the output amplifier's
output.
The 80 dB isolation amplifier is a tuned three stage amplifier with
attenuators placed between each stage. The attenuators are necessary to
maintain the amplifier's stability and to provide an additional 26 dB of
isolation. Each stage has a 17 dB gain with about 20 dB of reverse isolation
in the amplifier passband. Outside the 40 MHz passband the reverse
isolation increases at 6 dB/octave. Inside the passband the input and output
return losses are about 10 dB. The output return loss drops to 0 dB outside
the passband. The input level to the 80 dB isolator is 0 dBm, ±2 dB. The
output level is +5 to +7 dBm into the phase detector.
Digital Control
The digital interface with the rest of the module is through a serial three line
data bus. The interface is implemented using a custom I/O interface chip.
The chip requires a bidirectional data line, a clock line and a chip
enable/interrupt line from the microprocessor. The I/O chip generates 22
parallel outputs and controls all the functions on the A1A7 board. The I/O
chip also latches and transfers the interrupt status of the out-of-lock detectors
to the microprocessor.
13-16 HP 70427A/HP 70428A User's Guide