6-4
HP 85325A Operating and Service Manual
Service
Theory of Operation
Extender Recommendations
If using high speed “TURBO” (fast CW) modes, use fiber optic extenders
for distances greater than 250 meters. Using coax at distances over 250
meters greatly slows the whole HP-IB bus. This slows the data transfer to the
system computer which results in slow measurements.
Always use Fast Mode on coax extenders connected to the main HP-IB bus.
Always use Slow Mode on coax extenders connected to the HP 8530A local
bus.
Receiver Section
The receiver section consists of the antenna under test, reference antenna (or
directional coupler), frequency converter, mm-wave mixer modules,
mm-wave mixers, isolators and the receiver (HP 8530 or HP 8510).
Purpose of Each Device in the Receiver Section
Isolators
There is an isolator on the input of each mixer. The isolators perform two
functions:
•
They improve the impedance match between the test or reference
antenna and the mixers.
•
The isolators stop mixer harmonic signals from radiating back toward
the antennas, reducing crosstalk.
Mixers
The HP 8510 or HP 8530 receivers cannot accept RF frequencies directly.
Mixers are required to downconvert the RF frequencies to a 20 MHz IF
frequency. The test and reference mixers are identical, but are driven by
different types of mixer “modules.”
Mixer Modules
Mixers must receive proper LO power (+16
±
2 dBm) or they will not work
properly. The Reference Module has an internal detector that measures the
amount of LO power it receives. The detector's output voltage is sent to the
HP 85309A LO/IF unit, which controls the LO power level. This system
does not measure power to the test module. It is assumed that the electrical
distance to the test module is the same as the electrical distance to the
reference module. Thus, the power arriving at the test module will be equal
to the power arriving at the reference module. The system cannot control test
and reference LO power independently.
The test module has a diplexer, which allows the LO and IF signals to travel
through the same cable. This allows you to use a single-ring rotary joint for
the antenna under test.
