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The pulse
processor
Analog pulse
shaping
In analog shaping the signal from
the preamplifier is converted to a pulse by way of analog shaping
electronics (Fig. 14). The height of the pulse is then measured and
converted into a digital signal by an analog to digital converter (ADC). A
longer peaking time (TP) is used to reduce noise and improve
resolution.
Accurate height measurement of
each pulse requires the shaping circuitry to be reset to the baseline level
before another event can be accepted for measurement. The decay time (TD)
gives the effective minimum time before another step can be measured
accurately (Fig. 14).
The challenge with analog shaping
is to compensate for changes in the baseline. To measure a 10keV photon to
within 1eV requires the baseline to be known within 0.01% of the pulse
height. The time required for the pulse to decay to below 0.01% of the pulse
height will be many times the peaking time. As the count rate increases the
residuals on the baseline left over from each shaped pulse begin to build
up, resulting in a measurable shift in the apparent baseline. In addition,
the baseline will move at high input rates due to the cumulative
after-effects of all the large restore signals. Thus, as the input count
rate increases, peaks will shift in position.
Even at low count rates, the
slope on the voltage ramp will affect the position of the baseline. This
slope, caused by leakage current from the detector (see Fig. 4), will mean
that shaped pulses will not return exactly to zero, and therefore the
baseline always has some offset.
When calibrating systems with
analog shaping two key adjustments are required. One to compensate for
detector leakage current, another to compensate for count rate-induced peak
shift. Since the degree of shift depends on the shaping time, these
adjustments have to be performed for every shaping time constant and gain
setting. Count rate effects may also vary with spectrum content and even if
exact compensation is achieved for an ‘average’ sample spectrum, peak shifts
of a few eV may still be expected when different samples are analyzed.
Furthermore, if detector leakage changes, the calibration process for
baseline offset has to be repeated.
Some pulse processors are
described as digital, because they use digital circuits and software instead
of control knobs to control the circuitry. These processors will have the
same problems with baseline measurement and calibration shown by any pulse
processor that uses analog shaping.
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