General Information
1
Keysight 478A Operating and Service Manual
15
Introduction
The Keysight model 478A coaxial thermistor mount is designed for use with
Keysight model 431 and 432 power meters to measure microwave power from 1
μ
w to 10 mw. Design of the mount minimizes adverse effects from environmental
temperature changes during measurement. For increased measurement accuracy,
Effective Efficiency and Calibration Factor are measured for each mount, and at
selected frequencies across the operating range; the results are marked on the
label of the instrument (see “Mount Calibration Data” in
). The model 478A can be used over the 10-HMZ to 10-GHz
frequency range. Throughout the range, the mount terminates the coaxial input in
a 50
Ω
impedance, and has a SWR of not more than 1.75 without external tuning.
Each mount contains two series pairs of thermistors, which are matched to cancel
the effects of drift with ambient temperature change. Thermal stability is
accomplished by mounting the leads of all four thermistors on a common thermal
conductor to ensure a common thermal environment. This conductor is thermally
insulated from the main body of the mount so the thermal noise or shocks applied
externally to the mount, such as those handling the mount manually, cannot
significantly penetrate to disturb the thermistor. This thermal immunity enables
the thermistors to be used in the measurement of microwave power down to the
microwatt region.
Instruments Covered by Manual
Instruments with serial numbers 79401 and below are documented directly by this
manual. Some instruments with serial numbers lower than 79401 may be
documented in
; other instruments with serial numbers higher than
79401 may be documented in a MANUAL CHANGES supplement.
Incoming Inspection
Inspect the model 478A upon receipt for mechanical damage. Also check it
electrically; if the mount was subjected to severe mechanical shock during
shipment, the match between thermistors may be affected. To check the
thermistor match, proceed as described in
page 34.
If any damage is found, inform the carrier and your nearest Keysight Sales and
Service Office immediately.
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