ORTEC DSPEC Pro, Hardware User Manual

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794380D / 0914 1. INTRODUCTION
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Figure 4. LFR-Enabled Digital Filter.
The ORTEC LFR filter removes most of the microphonic noise by estimating the microphonic-
induced error signal on a pulse-by-pulse basis and subtracting the estimated error signal from the
trapezoid output. As noted above, the error signal is proportional to the slope of the baseline
during the energy measurement. If the slope is known, then so is the error introduced by the
microphonics. An excellent estimate of the slope can be obtained by using the trapezoidal filter
itself to measure the slope both before and after the energy measurement. Since the digital filter
is always sampling the input signal, it is only necessary to store (1) the values measured before
the event is detected, (2) the gamma-ray energy measurement, and (3) the values measured after
the event is detected. The modified trapezoidal digital filter for LFR from an InSight Virtual
Oscilloscope trace is shown in Fig. 4. A suitably weighted and averaged value of the before and
after slope measurement is then subtracted from the energy measurement producing a measure-
ment essentially free of microphonic noise. Although the inherent increase in the pulse proces-
sing time increases the dead time of the system, the resolution can be greatly enhanced when
periodic noise is present.