Filter
Top Products
Theory of Operation Teledyne API - Model 200EH/EM Operation Manual
214
10.4.5. PMT PREAMPLIFIER
The PMT preamplifier board amplifies the PMT signal into a useable analog voltage (PMT) that can be
processed by the motherboard into a digital signal to be used by the CPU to calculate the NO, NO
2
and NO
x
concentrations of the gas in the sample chamber.
The output signal of the PMT is controlled by two different adjustments. First, the voltage across the electron
multiplier array of the PMT is adjusted with a set of two hexadecimal switches. Adjusting this voltage directly
affects the HVPS voltage and, hence, the signal from the PMT. Secondly, the gain of the amplified signal can
further be adjusted through a potentiometer. These adjustments should only be performed when encountering
problems with the software calibration that cannot be rectified otherwise. See Section 11.6.3.8 for this hardware
calibration.
To
Motherboard
PMT Preamp PCA
Low
Pass Noise
Filter
E Test Control
From CPU
MUX
Amp to
Voltage
Converter/
Amplifier
D-A
Converter
PMT
Coarse
Gain Set
(Rotary
PMT Fine
Gain Set
(Rotary
Switch)
PMT
Signal
Offset
E-Test
Generator
O-Test
Generator
O Test Control
From CPU
PMT Temp
Sensor
PMT
Temperature
Feedback
Circuit
TEC Control
PCA
PMT Output
PMT HVPS
Drive Voltage
PMT Temp Analog Signal
to Motherboard
PMT Output Signal
(PMT) to Motherboard
O Test
LED
Figure 10-10-21: PMT Preamp Block Diagram
The PMT temperature control loop maintains the PMT temperature around 7° C and can be viewed as test
function
PMT TEMP on the front panel (see Section 6.2.1).
The electrical test (
ETEST) circuit generates a constant, electronic signal intended to simulate the output of the
PMT (after conversion from current to voltage). By bypassing the detector’s actual signal, it is possible to test
most of the signal handling and conditioning circuitry on the PMT preamplifier board. See section 6.9.6 for
instructions on performing this test.
The optical test (
OTEST) feature causes an LED inside the PMT cold block to create a light signal that can be
measured with the PMT. If zero air is supplied to the analyzer, the entire measurement capability of the sensor
module can be tested including the PMT and the current to voltage conversion circuit on the PMT preamplifier
board. See section 6.9.5 for instructions on performing this test.
04521C (DCN5731)