Operation & Service Manual
Section III
Model 1805B RF Control Unit
Theory & Operation
SECTION III, THEORY & OPERTION
GENERAL
This section contains all the theory and operating instructions necessary to perform
power measurements and calibrations using the Model 1805B RF Control Unit.
OPERATION
The primary purpose of the Model 1805B RF Control Unit is to provide fast, reliable, and
accurate leveled RF power when used in a closed-loop feedback arrangement. The unit
provides automatic dc substitution at 0.5 mW and 1 mW to 10 mW in 1 mW steps using
local or IEEE-488 bus control for easy and accurate transfer of calibration factors.
When used with the TEGAM Model 1109 RF power standard, the 1805B permits the
accurate transfer of up to 132 calibration frequencies traceable to NIST from 0.01 to
18.0 GHz. The 1805B is also compatible with other TEGAM System IIA components
including the 1109H, 1116, 1117A, 1119, and 1119H series RF Transfer Standards (refer
to applicable mount manual for specifications).
An internal temperature controller raises and maintains the temperature of the mount
chamber above ambient. This minimizes the effects of changes in ambient temperature
for all TEGAM RF Transfer Standards.
The 1805B operates in a closed loop configuration for leveling the RF output of compati
ble signal sources. They are controlled directly using an analog signal applied to a dc
coupled AM input connector.
Ultra stable dc high precision metal film resistors provide dc power increments across a
TEGAM patented self-balancing bridge. The front panel bridge meter provides visual
indication that closed loop stabilization has been achieved and enables rapid system
operation. Coarse and fine adjustments are provided to obtain a meter null reading with
no RF applied.
In addition to the bridge balance and temperature indicator, the 1805B contains several
operational and performance checks. A front panel Mount Error indicator blinks if the
voltage across the mount is not within a specified range. This alerts the operator of a
mount fault such as open or shorted leads, or improper mount temperature. DC supply
voltages are monitored by PC B-mounted LEDs which illuminate with an active power
supply.
A standby mode switch allows mount heater circuit operation to maintain mount temper
ature at all times. This ensures full performance capability and long term stability of the
bolometer mount.
The Model 1805B provides automatic dc level control to within ± 0.1 %, +1
μ
W, with an
RF source variation of ±3 dB and an ambient temperature range of +10 to +40°C. The
dc substitution level is held to an accuracy of +0.1%, +5
μ
W. Maximum accuracy is
achieved by using DC power for both bias and substitution signals.
This instrument employs LEDs to inform the user of RF power ON/OFF condition, ac
power on/standby, mount error, and remote/listen functions. Easy-to-read meters are
used to monitor temperature stability and bridge balance.
For more information on the use of the 1805B in a typical system, refer to the TEGAM
System IIA Automatic Power Meter Calibration System operation and Service Manual
(IM-198).
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