microDXP Technical Reference Manual
Version 3.15
October 7, 2019
55
Baseline
Average
Length
=
32768
BLFILTER
Equation 4-7
Physically, the baseline is a measure of the instantaneous slope (Volts/second) for a pulsed-
reset detector, and a measure of the DC offset for an RC-feedback preamplifier. For a
perfect detector and preamplifier the baseline value is independent of time. In fact, the
variation in leakage current of the detector and offset drift and 1/f noise of the preamplifier
contribute to a baseline value that wanders at low frequencies. The goal is to achieve a
baseline average that has a sufficient number of samples to average out the high frequency
noise, but which still reflects the ‘local’ instantaneous baseline. Generally speaking,
Baseline Average Length
is set to achieve the best energy resolution performance over
the desired range of input count rate. There are two considerations worth emphasizing:
1.
Excess detector/preamplifier noise and pickup: A high-frequency noise peak can
result in poor relative performance at the corresponding ‘resonant’ peaking time.
Often this problem can be mediated, though not eliminated, by increasing the
number of baseline samples in the average for the affected peaking times. On the
other hand, excess low-frequency noise, i.e. wandering, can be remedied by
reducing the number baseline samples in the average.
2.
High rate performance: At higher rates, i.e. > 50% deadtime, the slow filter returns
less and less often to baseline, thus the time between baseline samples grows
longer. This is the primary cause of degraded energy resolution at high rates. The
microDXP now employs a proprietary circuit that virtually eliminates this
problem, resulting in industry-leading count rate stability.
4.5 X-ray Detection & Setting Thresholds
Before capturing a value of V
x
we must first detect the x-ray. X-ray steps (in the preamp
output) are detected by digitally comparing the output of a trapezoidal filter to a threshold.
In the DXP up to three trapezoidal filters are implemented:
fast
,
intermediate
and
slow
;
each with a threshold that can be individually enabled or disabled. A fast (trigger) filter
very quickly detects larger x-ray steps. A slow (energy) filter averages out the most noise
and can thus detect smaller x-ray steps, but has a response that is much slower. An
intermediate (baseline) filter provides a balance between the speed of the fast filter and the
noise reduction of the slow filter.
The fast filter is used solely for x-ray detection, i.e. a threshold crossing initiates event
processing. Its short base width (2L+G) means that successive pulses that would ‘pile-up’
in slower filters can be resolved in the fast filter and rejected from the spectrum (see Figure
4-9 below). Conversely, little noise reduction is achieved in the fast filter, thus the fast
threshold cannot be set to detect particularly low x-ray energies.
The intermediate filter threshold is applied as part of the baseline acquisition circuitry, i.e.
baseline measurements are taken when the signal is below this threshold. Intermediate
threshold crossings by default also trigger event processing, extending the detectable
energy range significantly below the fast filter threshold. Note that this threshold is
initialized to the maximum, i.e. most conservative, value, and should be adjusted
downward by the user for best performance.
