NARDA 8718B - Manual (Page 126)

125 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