HP 85325A Operating and Service Manual 6-3
Service
Theory of Operation
Transmitter Section
The transmitter consists of a synthesized RF source, amplifier, mm-wave
source module, and transmit antenna. The RF source and transmit antenna
provide the stimulus signal for the antenna being tested. The RF source is
controlled by the receiver using the system bus. The RF source is called
SOURCE 1
in the receiver's softkey menus. HP-IB extenders are required if the
distance between the receiver and RF source exceeds IEEE 488.2 limits.
Refer to Allowable HP-IB Cable Lengths below.
The Purpose of Each Device in the Transmitter Section
Source and Amplifier
The RF source and amplifier provide the +17 dBm power level required by
the mm source module.
MM Source Module
(MM Head)
This module multiplies the RF frequency by a factor of 2, 3, 4, 5 or 6,
depending on the model used. The source module interface (SMI) allows the
source module to become an integrated part of the RF source. With the
interface cable connected, the RF source knows that multiplied frequencies
are being produced. Because of this, the source responds properly when the
receiver asks for frequencies above its normal range. The source module
accepts RF input power levels from +17 to +27 dBm.
The power output of the source module can be changed with the
STIMULUS controls on the receiver. When the SMI cable is used, the HP
source module produces leveled mm-wave power.
Allowable HP-IB Cable Lengths
You are allowed 2 meters of cable length for each instrument that is
connected to the main HP-IB bus (or to the system bus). A typical system
has a computer, printer (or plotter), receiver, and positioner-controller
connected to the main HP-IB bus (four devices). The total HP-IB cable
length allowed in this example system is 2 x 4 = 8 meters. Under no
circumstances can the total cable length ever exceed 20 meters, no matter
how many devices are in the system.
If your system exceeds the allowed HP-IB cable length, you must use HP-IB
extenders.
System Bus Cable Lengths
There are usually two instruments connected to this bus (the RF and the LO
sources). Calculate the system bus limitation separately, using the same
formula.
2 (meters/device) x 2 (devices) = 4 meters allowable length
Because the RF source is often far away from the receiver, HP-IB extenders
are commonly used. Always use the slow mode on extenders connected to
the system bus.