NARDA 8718B - Manual (Page 125)

124 Appendices
Calibration is performed using the RS-232 port, a voltage
source and a computer.
PCB A1 is the primary circuit board within the meter. The
majority of the components are surface mount. It is located in a
heavily shielded area beneath the keypad. Operating power
from battery B1 is routed through Q3 and Q4 that form a
momentary switch for the micro controller U5. U5 provides a
control signal to the +5V, -5V and +2.5V power supplies made
up of U12, U13 and U14. The DC voltage output (two voltages if
a ultra-broadband probe with a two channel amplifier is used)
from the field sensing probe is coupled through the A2 PCB into
an input amplifier made up of U2 and then input into the Analog
to Digital converter U3. U3’s digital output is applied to the
micro controller U5. The on-board data bus is used to drive the
LCD display. The data bus interfaced with latch U8, static
Random Access Memory (RAM) and Time-of-Day U1O and the
program memory U11. The LCD module receives input from the
data bus, along with a contrast control input from U8 through
U2A, R2 and R21. The LCD back light control is provided from
P/O Q4, which is controlled by U5. The address bus circuit is
made up of U5, U10, U11 and the glue logic circuit comprised of
U4, U7 and parts of U1, U6 and U4. The circuit provides RS-232
interfacing through U8, which also controls A2U2, the RS-232
UART. The 10 GHz test source is controlled by U8 in
conjunction with Q2 and Q3A. Micro controller U5 also provides
Pulse Width modulated (PWM) signals that are used by the
8718. The 10 kHz source control is provided by U5’s PWM
along with U1. A PWM line and Q1 drive the audible alarm
circuit. A PWM line and U1 and U2 also control the Recorder
Output.
This small circuit board primarily handles input/output (I/O)
functions for the meter. It is located in a shielded area behind
the probe connector. A2 contains the RS232 circuit and
interfaces with the charger jack, the recorder output jack, the
probe and fiber optic inputs, and the high frequency test source.
A2 connects to A1 via a filtered connector mounted on A1.
PCB A1
PCB A2