Probe designs – NARDA 8718B User Manual

Page 126

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Appendix B Theory of Operation

The LCD is a self-contained display and meter circuit assembly.

The display has four lines with twenty characters per line. Each

character is a 5 x 7 dot matrix. The display is backlit for

improved visibility under low light conditions. It connects

directly to PCB A1.
The keypad has 22 keys. It is shielded in aluminum and

connects directly to PCB A1.
The battery pack is a custom package consisting of six

individual NiCd cells. It has a rated output of 7.2 volts with a

capacity of 7.2 Ah. The battery pack contains an over current

protection device. It connects directly to PCB A1.

Probe Designs

The probes used with the Model 8718B meter measure either

the electric field or the magnetic field. Some have a flat

frequency response, others have a shaped frequency response

(see Chapter 3, Probe Selection). All 8700 series probes have

square law characteristics that provide an accurate summation

of field levels present from multiple frequency sources. All

probes are internally coated with a thin film, high resistive

coating. The film is on the order of 0.5 M per square inch and

provides a shield against electrostatic charges. All 8700 series

probes utilize a preamplifier in the handle to amplify the detected

signals from the sensors. This eliminates probe-to-meter cable

modulation. The probe amplifiers are connected to the probe

elements by inflexible monolithic resistive leads. This prevents

cable modulation from affecting the signal derived from the

elements. Each instrumentation amplifier consists of three

distinct sections: two analog and one digital.
The 876xD series probes operate from 300 kHz to 3 GHz. They

use three sensors supported along mutually orthogonal axes.

Each sensor consists of a dipole-diode combination. The

dipoles are approximately 8 cm in length. The broad beam

width is achieved through the capacitive divider effect of the

dipole-diode combination. Some linearity correction is provided

for field levels that approach full scale. A patented circuit design

maintains RMS detection over the entire dynamic range of the

probe.

LCD Display D1

Keypad S1

Battery B1

Communications

Band Electric

Field Probes

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