NARDA 8718B - Manual (Page 123)

122 Appendices
The meter also contains built-in self-testing facilities for
checking the condition of the batteries, the operation of the
probe and for zero adjustment of the probe.
The meter provides a means for automatically zero adjusting the
meter and probe. When in an operating mode and the probe is
in a zero field, the operator presses the Auto Zero key to begin
the auto zeroing routine. The 8718B meter uses a unique digital
method to set the zero reference.
The meter provides a means of testing whether the probe is
operational using built-in test signal sources. The test is used to
prove that the probe is operational by generating upscale
indications on the Model 8718B meter.
(a) High Frequency Test Source. A 10.5 GHz (nominal)
oscillator and waveguide aperture for illuminating the
probe. Power output is capable of producing an
upscale indication on the meter of approximately
0.75 mW/cm2 (±3 dB).
(b) Low Frequency Test Source. An external test point
provides a 10 kHz (nominal) signal at the unit and test
points on the probe to excite each low frequency
element and produce an upscale indication on the
meter. The amount of deflection will vary depending
on the model probe being used.
When the Test Source key is depressed, U5 (and part of U1)
and Q1A within the meter generate test signals that are sensed
by the probe, causing the level displayed on the LCD to move
upscale. If no change in reading is observed, either the probe or
the meter may be defective. The test source automatically shuts
off after twenty seconds. The test must be performed with the
meter in the correct location relative to the meter.
Model 8718B Meter Circuits
The 8718B is comprised of five main assemblies: printed circuit
boards (PCB) A1 and A2, keypad S1, LCD display D1 and the
battery pack B1. This section provides an overview of the 8718B
circuits. There are no adjustable components (i.e., potentiometers
or variable resistors) used within the 8718B.