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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Appendices