Model 8718b meter circuits – NARDA 8718B User Manual

Page 123

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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.

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