Teledyne T200, Operation Manual, Page: 319 / 425

Teledyne API
– T200 NO/NO
2
/NO
X
Analyzer Operation Manual Principles of Operation
317
Figure 13-18: Basic PMT Design
A significant performance characteristic of the PMT is the voltage potential across the
electron multiplier. The higher the voltage, the greater the number of electrons emitted
from each dynode of the electron multiplier, in effect, making the PMT more sensitive
and responsive to smaller variations in light intensity but also more noisy (this is
referred to as “dark noise”).

The gain voltage of the PMT used in the T200 is usually set between 400 V and 800 V.

This parameter is viewable through the front panel as test function
HVPS
(see
Section 4.1.1).

For information on when and how to set this voltage, see Section 12.8.4.
The PMT is housed inside the PMT module assembly (see Figure 12-9). This assembly
also includes the high voltage power supply required to drive the PMT, an LED used by
the instrument‟s optical test function, a thermistor that measures the temperature of the
PMT and various components of the PMT cooling system including the TEC.
13.5.1. PMT PREAMPLIFIER
The PMT preamplifier board provides a variety of functions:

It amplifies the PMT signal into a useable analog voltage
(PMTDET)
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.

It supplies the drive voltage for the HVPS.

It includes the circuitry for switching between the two physical ranges.

It amplifies the signal output by the PMT temperature sensor and feeds it back to
the thermoelectric cooler driver PCA. This amplified signal is also sent to the
Motherboard to be digitized and forwarded to the CPU. It is viewable via the front
panel as the test function
PMT TEMP
.

It provides means for adjusting the electronic signal output from the PMT by:

Adjusting this voltage directly the sensitivity of the PMT’s dynode array and
therefore the strength of the signal output by the PMT through the use of two
hexadecimal switches.

Directly adjusting the gain of the output signal.
06858E DCN7057