ORTEC DSPEC Pro, Hardware User Manual

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794380D / 0914 2. GETTING STARTED
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Patent number 5,821,533.
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Gain — Set the amplifier coarse gain by choosing from the Coarse droplist, then adjust the Fine
gain with the horizontal slider bar or the edit box, in the range of 0.45 to 1.00. The resulting
effective gain is shown at the top of the Gain section. The two controls used together cover the
entire range of amplification from 0.45 to 32.
Input Polarity — These buttons select the preamplifier input signal polarity for the signal from
the detector. Normally, GEM (p-type) detectors have a positive signal and GMX (n-type) have a
negative signal.
Baseline Restore — This is used to return the baseline of the pulses to the true zero between
incoming pulses. This improves the resolution by removing low frequency noise from dc shifts
or mains power ac pickup. The baseline settings control the time constant of the circuit that
returns the baseline to zero. There are three fixed choices (Auto,
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Fast, and Slow). The fast set-
ting is used for high count rates, the slow for low count rates. Auto adjusts the time constant as
appropriate for the input count rate. The settings (Auto, Fast, or Slow) are saved in the DSPEC
Pro even when the power is off. The time constant can be manually set on the InSight display
(see the discussion beginning on page 27).
You can view the time when the baseline restorer is active on the InSight display as a Mark
region (see the discussion on Marks, p. 29). In the automatic mode, the current value is shown
on the InSight sidebar (Fig. 15). For a low-count-rate system, the value will remain at about 90.
Preamplifier Type — Choose Transistor Reset or Resistive Feedback. Your choice will
depend on the preamplifier supplied with the germanium detector being used.
Optimize — The DSPEC Pro is equipped with both automatic pole-zero logic
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and automatic
flattop logic.
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The Start Auto (optimize) button uses these features to automatically choose the
best pole-zero and flattop tilt settings. Note that if you select Transistor Reset as the preamp-
lifier type, the optimization routine does not perform the pole-zero.
During optimization, pole zeroes are performed for several rise-time values, and the DSPEC Pro
cycles through all the rise time values for the determination of the optimum tilt values. As all of
the values for all the combinations are maintained in the DSPEC Pro, you do not have to repeat
optimization for each possible rise time.
Optimization requires a source with sufficient activity to register between 800 cps and 5000 cps.
Arrange the detector in its final configuration. Unmark the LFR checkbox on the Amplifier PRO
tab (you cannot optimize with LFR mode enabled; see Section 2.3.3), turn on the HV so the
instrument is processing pulses, then place the source on or near the detector endcap. A count