(5) The MOUNT RES switch on the front panel permits the use of three
types of thermistor mounts with the 431C. Model 486A waveguide mounts can be
used by setting the MOUNT RES switch to the 100 ohm or 200 ohm position,
depending on the microwave band used (refer to Table 2). The 200 ohm position
is used with Model 478A thermistor mounts and the 200 ohm BAL position is used
with a balanced thermistor mount such as the 8478B.
To avoid severe damage to the thermistor mount, be careful not to move the
MOUNT RES switch while operating the RANGE switch.
(6) Two output BNC type jacks are provided on the rear panel of the
instrument, labeled DVM and RECORDER/LEVELER. The DVM jack provides a voltage
linearly proportional to the meter current; 1 volt equal to full scale meter
deflection. A DVM connected to the 431C must have an input impedance greater
than 500 k ohms on the range used. The RECORDER/LEVELER jack furnishes a DC
voltage of low source impedance necessary for isolation between a recorder or
leveler amplifier and the metering circuit of the power meter.
voltage is proportional to the power measured and is offset 40 mV or less from
its nominal value, depending on the load impedance. This output voltage allows
the Model 431C to be used in a number of additional applications.
c. Controls, Connectors, and Indicators.
The front and rear panel controls, connectors, and indicators are
explained in Figure 2. The descriptions are keyed to the corresponding items
which are indicated on the figure. Further information regarding the various
settings and uses of the controls, connectors, and indicators is included in
the applicable procedures of this lesson.
Figure 3, Turn-On and Nulling Procedures, present step-by-step
instructions for operating the Model 431C.
Steps are numbered to correspond
with the appropriate control, connector, or indicator on the power meter.
e. Major Sources of Error in Microwave Power Measurement.
(1) A number of factors affect the overall accuracy of power
measurement. Major sources of error are presented in the following paragraphs
to show the cause and effect of each error. Particular corrections or special
measurement techniques can be determined and applied to improve overall
The following are the major sources of error to
consider: 1) mismatch error, 2) RF losses, 3) DC-to-microwave substitution