MM0474
4.
MODEL 478A THERMISTOR MOUNT.
a. Introduction.
(1) The thermistor mounts used with models 431C and 432A power meters
are models 478A and 486 series. The description and specifications are covered
in the following paragraphs.
(2) The 478A coaxial thermistor mount is designed for use with models
431 and 432 power meters to measure microwave power from 1 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 and the results are marked on the label of the
instrument. The model 478A can be used over the 10 MHz to 10 GHz frequency
range. Throughout the range, the mount terminates the coaxial input to 50 ohm
impedance, and has a standing wave ratio of not more than 1.75 without external
timing.
(3) 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 that
thermal noise or shocks applied externally to the mount such as those from
handling the mount manually, cannot significantly penetrate to disturb the
thermistors. This manual immunity enables the thermistors to be used in the
measurement of microwave power down to the microwatt region. See Table 5.
b. Precautions. Do not drop or subject to severe mechanical shock. Shock
may destroy the match between thermistors and increase susceptibility to drift.
When connecting the Model 478A to the 431 or 432 power meter, set the mount
resistance switch to 200 ohm position. Connecting a 200 ohm mount to a power
meter set for a 100 ohm mount can result in thermistor damage.
c. Maximum input. The model 478A/431 or 478A/432 combination responds to
the average RF power applied.
The maximum signal applied to the thermistor
mount should not exceed the limitations for (1) average power (2) pulse energy
and (3) peak pulse power.
Excessive input can permanently damage the Model
478A by altering the match between the RF and compensation thermistors
(resulting in excessive drift or zero shift) or cause error in indicated power.
d. Average Power.
The 478A/431 or 478A/432 combination can measure
average power up to 10 mW.
To measure power in excess of 10 mW, insert a
calibrated attenuator between the mount and the source. Under no circumstances
apply more power than 30 mW to the mount.
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