990 CLD Autoline™ Leak Detector
1-4
DRAFT 2/2/04
Signal Flow
The electrical signal that represents the concentration of helium in the vacuum system
originates in the spectrometer tube preamplifier (Figure 1-2 on page 1-5). Also, refer to
“High Vacuum System” on page 1-14.
The signal flow occurs as follows:
1. The helium ions create an electrical current on the input of an electrometer amplifier.
The voltage at the output of the electrometer circuit travels through the cable
connecting the spectrometer tube to the 990 Controller.
2. The signal flows through offset and calibration circuitry and is distributed to three main
sections of the controller: the range selection and display circuitry, the set point
circuitry, and the leak rate output circuitry.
3. The range select circuitry takes input from the front panel RANGE DOWN/UP
pushbuttons, the RANGE SELECT switch, and the remote exponent control inputs, and
determines which decade of signal to send to the bar graph display and to the audio
circuitry.
4. The set point circuitry uses voltage comparators, hysteresis circuitry, and mechanical
output relays to indicate when the signal has exceeded any one of four preset
thresholds. The leak rate output circuitry produces a linear or logarithmic voltage
proportional to the signal (see Appendix A “Understanding the 990 CLD
Analog Leak Rate Output”).
5. The leak rate output signal is buffered and clipped so as not to adversely affect
equipment that is connected to the leak rate output.
The 990 CLD controller also contains circuitry to operate the ion source and cold cathode
gauge in the spectrometer tube and to operate and protect the high vacuum pump. The ion
source support circuitry includes a multi-voltage high voltage supply and ion emission
regulator control loop. The cold cathode gauge control is a specialized 2000 V supply with
a voltage output proportional to the spectrometer tube pressure. The high vacuum pump
controller is a micro controller-based printed circuit board that monitors motor temperature
and current for protection of the pump and is capable of running the pump at one of two
selected speeds.
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