MM0474
LESSON 4.
POWER MEASUREMENT EQUIPMENT PART III
AIPD Subcourse MM0474........................ Power Measurements
Lesson Objective ............................. After studying this lesson, you
should be able to describe the
purpose,
characteristics
and
operation of the power standards
set and the NBS power bridge. You
should also be able to answer all
exercise question without error.
Credit Hours ................................. Three
TEXT
1.
INTRODUCTION.
At the Army Calibration Laboratory a requirement exists to certify
thermistor mounts used in the Army Calibration System. To perform the power
measurements at an accuracy sufficient to certify thermistor mounts the Army
Calibration System uses an NBS Type II Power Measurement System.
2.
DESCRIPTION.
a. The NBS Type II Power Measurement System, Figure 1, consists of two
units: a self-balancing bolometer bridge, and a reference voltage generator.
The bridge is a self-continued unit, but the reference generator draws its
power from the bridge and can only be used with it or some other external power
source.
b. The bridge is designed to measure microwave or high-frequency power in
conjunction with an external bolometer mount (either a thermistor or a
barretter) and some means of accurate voltage measurement.
The reference
voltage generator may be used for this purpose, although this does not
generally result in the best accuracy for RF power levels less than about 80%
of the normal DC bias power.
Use of the reference voltage generator as a
stable voltage source in conjunction with an external voltmeter produces the
most accurate measurements.
This technique is particularly important at low
power levels, i.e., less than 10 milliwatts.
c. The reference generator is also designed to be used as a precision
power stabilizer. It has two output connectors; one with a limited
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