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Operating Instructions
5.5 Protection Circuit
CSPA1x is designed among others to realize better usability and trouble-free power supply.
The detector input is protected by implemented diodes network circuit. It minimally increases the internal
capacitance and thus also the values of internal noise. You can disable this function as needed ( Fig. 7.1
Jumper J2: to pos. 2-3 )
Further protection of measure is built-in input ser. resistor = 100 Ohm.This resistor is deactivated in order
to keep the noise at a low level (Fig. 7.1. JP3 pos. 1-2).
The output resistor is set to 1 Ohm by default via jumper JP9. Alternatively you can increase this value to
50 Ohm ( better output protection ). The disadvantage is the halved value of the effective output signal.
5.6 Common Operating Problems
The modern spectrometer is an extremely sensitive, state-of the-art system. Inexact performance of other
than the grossest type is generally due to subtle factors. It is the ability to determine and correct these
factors that constitutes the art in the science of spectroscopy instrumentation.
All of the many possible contributors to less than optimum performance cannot be listed here. The
purpose of this section is to note the usual causes of loss of resolution, and to suggest curative steps.
Do not expect to diagnose all problems with the detector, a preamplifier, a main amplifier, and a
multichannel analyzer. The spectroscopy system records results, it does not necessarily lead to the
identification of causes. A good, modern oscilloscope will be needed. Also a high quality tail pulse
generator will be extremely useful.
The simplest test is, of course, to connect your detector, apply bias, present a source, and accumulate a
spectrum. Be sure a pulser is not feeding the preamplifier while the spectrum is accumulating, or
resolution loss may result. If the results obtained are far different from what is expected, it then becomes
necessary to troubleshoot the system.
First observe the amplifier output on an oscilloscope at various time base and amplitude settings. Is the
amplifier properly pole/zero cancelled (do the output pulses cause undershoot that persist for longer than
two or more main pulse widths)? Set the main amplifier pole/zero cancellation (without DC resolution) to
obtain the most rapid, complete baseline recovery.
The next step is to remove all sources and, with the detector still connected and bias applied, present a
test pulse to the detector input of the preamplifier using the test input described in Section 5-2. Make sure
the pulser polarity is correct. Set the amplitude of the pulser so that its peak occurs near the region of the
peak of the source previously used.
Observe the output of the amplifier without DC restoration. Note that the amplifier is not properly
pole/zero canceled for the pulser feeding the preamp (due to the extra time constant of the pulser). This is
of no consequence for a pure pulser input. Are the baseline fluctuations of 50/60 or 100/120 Hz
frequency? A ground loop is indicated. Insert all system line plugs into the same output. Or are the
baseline fluctuations of random frequency between 10 Hz and 15,000 Hz? The area may be too noisy,
causing microphonic problems.
If high frequency noise is observed, is it random or periodic? Periodic noise is a sign of electronics failure;
isolate the cause by observing the preamplifier output. Is the same pattern observed, or is the problem in
the main amplifier? Random high frequency noise may be detector load resistor or input capacitor
breakdown.
Next, accumulate a pulser peak on the analyzer. Calculate its resolution. Repeat with the detector
removed and the input connector of the preamplifier shielded. (Wait five minutes to remove the
preamplifier from the detector after removing the detector bias.) You now have three resolution figures
available for essentially equal energy peaks:
R
: source
S
ComTec GmbH
13
F
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