ORTEC 109A - Suggestions for Troubleshooting

If a multichannel pulse height analyzer is used following
the main amplifier, the noise performance can be tested
merely by using a cal ibrated test pulse generator with
charge terminator, as outlined in Section 6.1.1. With only
the charge terminator connected to the DET. INPUT jack,
the spread of the pulser peak thus analyzed wil l be due
only to the electronic noise contribution of the preamplifier
and main amplifier. The analyzer can be calibrated in terms
of keV per channel by observing two different pulser peaks
of known energy, and the FWHM of a peak can be taken
directly from the analyzer readout.
It is also possible to determine the noise performance of
the preamplifier by the use of a wide-bandwidth rms ac
voltmeter such as the Hewlett-Packard 400D, reading the
main amplifier output noise level and correlating with the
expected pulse ampl itude per keV of input signal under the
same conditions. Again, a cal ibrated test pulse generator is
required for an accurate measurement. In this method the
preamplifier and main amplifier are set up as they would be
used normally, but with a dummy capacitor (or no capacity)
on the DET. INPUT jack, and with the ac voltmeter
connected to the ampl ifier output. The noise voltage
indicated by the meter, designated Efms, is read and noted.
Then, a test pulse of known energy, Ejn (in keV), is
applied to the input jack, and the amplitude of the result
ing output pulse, Eout. is measured in volts with an
oscilloscope. The noise spread can then be calculated from
the formula
FWHM (keV, Si det)
2.66 (Erms) (Ejp)
Eout
where Epms is output noise in volts on the 400D meter,
Ejn is input signal in keV particle energy, and Egyt is
output signal in volts corresponding to the above input. If
the gain of the shaping amplifier is adjusted so that the
output voltage is 2.66 V, then the meter reading will be
directly in keV FWHM except for a scale factor. [The
factor 2.66 is the product of two relations: correction from
rms to FWHM (2.35), and correction of the 400D meter
from sinewave to white noise (1.13).]
The noise performance of the preampl ifier, as measured by
the above methods, should not differ significantly from that
given in Section 2.
6.1.4 When testing the preamplifier and detector, if the
noise performance of the preamplifier has been verified
as outlined in the preceding section or is otherwise not
suspected, a detector may be tested to some extent by
duplicating the noise performance tests with the detector
connected in place, and with normal operating bias applied.
The resulting combined noise measurement, made either
with an analyzer or by the voltmeter method, indicates the
sum in quadrature of the separate noise sources of the am
plifier and the detector. In other words, the total noise is
given by (Ntot)^ = (Ndet)^ + (Nampl)^- Each quantity is
expressed in keV FWHM. The quantity N^et is known as the
"noise width" of the detector, and is included as one of the
specified parameters of each ORTEC semiconductor de
tector. By use of the above equation, and with a knowedge
of the noise of the preampl ifier, the noise width of the
detector can be determined. The significance of this noise
width in evaluating the detector is subject to interpretation,
but generally the actual resolution of the detector for pro
tons or electrons will be approximately the same as the noise
width; the resolution of the detector for alpha particles will
be poorer than the noise width. The most useful application
of determining the noise width of a detector is in the
occasional monitoring of this quantity to verify that the
detector characteristics have not undergone any significant
change during use.
6.2 Suggestions for Troubleshooting
If the preampl ifier is suspected of malfunctioning, it must
be isolated and tested alone, not in a system involving other
units such as a source of particles to be analyzed, the de
tector, the preamplifier, a main ampl ifier, and subsequent
sealers and/or analyzers. Such logical isolation and individual
testing of components wil l be the most productive approach.
6.2.1 Charge-Sensitive Loop The function of the preamp
lifier is simple and lends itself to relatively easy scrutiny.
The charge-sensitive loop performs a charge-to-voltage
conversion on the input signal. It has an output signal
that manifests itself as a fast rise (~15 ns at 0 pF external
capacitance) step of voltage whose height is determined by
the input charge, fol lowed by a 400-/is decay back to the
baseline. This signal can be observed at the emitter of Q4
while impressing a signal, as described in Section 3.4. The
ampl itude of thissignal should be45 mV per MeV equivalent
input signal.
6.2.2 Voltage Amplifier To reduce pulse pileup in the
voltage amplifier and subsequent stages, the output signal
from Q4 is differentiated with a 50-/xs time constant by
C12. Transistors Q6 and Q7 provide voltage amplification
of 0.34 or 3.4 for the XI or X10 gain switch positions,
respectively. Accordingly, the output signal at the 81
wiper arm should be a fast rise with 50-;tis time constant
decay, with amplitude either 0.34 or 3.4 times greater than
that at the Q4 emitter.
6.2.3 Cable Driver The cable driver, consisting of Q8
through Q11, is simply an impedance converter, and the
output signal should look exactly l ike the input signal. No
gain is obtained in the cable driver.
6.2.4 Table of DC Voltages The following l ist of voltages
will help to locate defective components. Exercise extreme
caution in making these measurements, because a probe
shorting two points on the printed board can cause great
damage.