Fluke Biomedical 440RF - Manual (Page 29)

Circuit Description
CPU Board
4
4-3
Operating power for the instrument is provided by three 9 V alkaline transistor batteries. The + 5 V
sources required for operation of the digital circuitry are provided by two voltage regulator circuits.
4.4 CPU Board
The CPU board contains the microprocessor, programmable read only memory (PROM), a user
programmable NOVRAM, and an analog-to-digital converter (ADC). (See Figure 4-2.)
ADC accepts analog data from the multiplexer and converts it to a digital format which is compatible with
the microprocessor. A Texas Instruments TMS70C00 microprocessor performs all control functions. The
operating program is permanently stored in the 27C256 programmable read only memory (PROM) which
contains 32,000 eight-bit words. Additional data are stored in the 6116 RAM.
4.5 Switchboard
The switchboard contains battery inputs, the 18 V divider circuit and a 74HC147 eight line to three-line
priority encoder. (See Figure 4-3.)
The encoder communicates the function/range switch position to the microprocessor. Battery voltages
and encoded data are then transferred to the motherboard.
4.6 Motherboard
In addition to providing interconnections between the Model 440RF/D circuit boards, the motherboard
contains the pressure sensor, temperature sensor, and meter driver circuit. (See Figure 4-3.)
The meter driver is a transistor pulsing circuit which accepts digital data from the microprocessor and
converts it to a format capable of driving the analog meter readout.
4.7 Analog Meter Readout
Meter Signal Filtration
A 300 μF capacitor, in shunt with the meter’s internal resistance, provides a long time constant RC filter
which reduces the displayed amplitude of source statistics and electronic noise.
4.8 Meter and RF Filtration
The insensitivity of the instrument to RF and electrostatic fields is obtained by completely enclosing the
instrument in a conducting, metal case. The magnesium window provides a low energy radiation
entrance to the case, while maintaining the metal shield integrity. The leads to the panel meter, which are
not included within the main shielded cavity, are routed through double-pi section filters, with each section
isolated in a separate shielded compartment. The basic case and filter design has been tested
successfully in high intensity RF fields including the microwave region on down.
Magnetic insensitivity is achieved through the use of solid-state circuitry, and a magnetically insensitive
radiation detector.