Theory of Operation
Publication 21555, Rev. E, September 2002
3-3
Each detector has a thermistor included in its housing so that the power meter can read the temperature
of the sensor. The voltage from that thermistor is applied through J7-6 (for channel 1) and amplified by
a gain of 2 by U35C (shown on Sheet 2). Channel 2 sensor’s thermistor voltage comes in through J9-6
and is amplified by 2 at U35A. The thermistor voltages are also routed to U35D and U36A&B. This
circuit is used to detect whether a sensor has been attached or disconnected. The connection or
disconnection of a sensor causes a transient voltage which is passed through C10 (for channel 2) or C11
(for channel 1). This is detected by a window comparator consisting of U35D and U36A&B. Whenever
the voltage is outside the normal DC bias range, the detector change will cause either a rise or a fall in
the voltage that will be detected by the comparator. The comparator will output an interrupt to the
processor, IRQ, which will be the output of U36A&B.
U16A and U9A provide a latch for the interrupt so that the processor will have time to respond. Pin 2
of U9A is used to enable or disable interrupts. U36C buffers the interrupt signal. The interrupt 1 signal
goes to the CPU board.
The sensors also have EEPROMs that connect to the system through a serial interface. Channel 1 uses
J8-6 for the clock and J8-5 for the data. This is a bi-directional device wherein data needs to be written
to the sensor and also read from the sensor. U24E buffers incoming data, and U24F buffers outgoing
data. Q3 provides the necessary open collector interface. U26 supplies the clock during a read or write
action. The clock signal is buffered by U24D. Channel 2 has a duplicate circuit consisting of U24A/B/C
and Q2. DC supplies of +5 V and
±
15 V are also routed to the sensors. The 15 V supplies are routed
through solid state fuses to provide protection in case of any shorts. These are resettable solid state fuses
which do not need to be replaced. The +5 V can be switched on and off. This is controlled by U26, and
buffered by Q8 and Q1.
To recap the preliminary actions of the incoming signals; they first go through the chopper, the 1st stage
amplifier, the 2nd stage amplifier, and then the filter. The signals are then sent to U23, a 16:1
multiplexer. U23 can select one signal to route to U25, the A/D converter. Most of the other signals are
used for testing purposes only. During normal operation, the signal path will be through the bessel filter.
Thermistor voltages enter on pins 24 and 25 for channels 1 and 2. The only other signal that is used for
normal operation is the V
PROP
F input that enters through J3 on the front panel. CR15 provides input
protection, and U21A is a x1 amplifier/buffer. The V
PROP
F signal can be read at pin 26 of the
multiplexer. U25 is a 14-bit A/D converter which operates at an 11
µ
s conversion time. The input is via
pin 6 on TP13. This device can measure between 0 and 10 V. U25 operating power supplies are limited
to
±
12 V (pins 11 and 5). Those voltages are derived from U39 and U43 which regulate the
±
15 V to
±
12 V.
R105, R104 and C31 configure U33 to measure from 0 to 10 V. Pin 4 of U33 provides the start convert
signal, and the processor asserts this signal to start an A/D conversion. After the conversion has
completed, pin 3 can be asserted to output the data to the bus. These two functions are supplied by chip
select ACS7 and ACS1. ACS1 is the output enable and ACS7 is the start convert. EOC is end of
convert, which occurs at pin 2 of U33, and is routed to data byte 15 so that the process can interrogate if
the data conversion is complete. D15 will be asserted if the A/D has completed its conversion.
U15 is the offset 12-bit D/A converter which provides the 1st stage amplifier with
±
5 V offset voltage.
U17 is similar but configured for 0 to 10 V. The output of U17 is buffered by U21B, current limited by
R108, and available at J4 for analog output.
U23 has a number of signals available for testing purposes. Offset voltage is available on pin 5. Output
DAC voltage is available on pin 4, and switched 5 V signals on pin 7.
Sheet 5 of DWG 21360 details the interconnections for the Calibrator circuitry and will be discussed in
Section 3.4, but U27 also provides three 8-bit ports for control of the multiplexers in the Analog
section, and an input port for the Too High and Too Low comparators.
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