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STJ Quick Start Guide v 2.01
April 1, 2021
Page 14
•
The PULSERATE setting controls the rate at which the DAC is updated: update rate = 20 ns *
2^PULSERATE. Note that this setting affects the rise time and decay time! With the default
value 0, the waveform spans 80
µ
s, with a 1
µ
s risetime and 5
µ
s decay time. With
PULSERATE=1, the waveform spans 160
µ
s, with a 2
µ
s risetime and 10
µ
s decay time, etc.
•
The PULSEWAIT setting, ranging from 0 to 255, controls the time that the circuit waits before
cycling back to the beginning of the waveform data, i.e. it can be used to reduce the count rate.
Its units depend on PULSERATE: wait time = 256 * update rate as defined above. For example,
with PULSERATE=0, the units are 5.12
µ
s.
Now proceed to the Scope Panel section below to explore the Decay Time setting in the Detector tab.
When you are ready to connect to actual STJ detectors, you will typically want to disable the pulser. Note
that it is safe to enable the pulser while the STJ Relay is closed, in case you want to combine the periodic
signal with the detector signal.
DETECTOR TAB
The Detector Settings need to be edited. Start by first setting the Polarity and Preamp Gain to these
recommended settings:
•
Set the Polarity to “-“
•
Set the Preamp Gain to “150.000 mV/keV”
POLARITY
For a positive STJ bias voltage, the Preamplifier: Polarity setting should be “-” (negative), and vice versa.
When the polarity is set correctly, ADC pulses should be positive as viewed in the Scope panel.
DECAY TIME
When using STJ detectors, the preamplifier Decay Time can only be determined after the STJ bias voltage
has been set and x-rays are present. In our experience the Decay Time ranges from a few 10’s to 100’s of
microseconds, depending on the detector. It can be measured directly using the cursors in the Scope
panel (with Trace Type = ADC, Trigger Type = Threshold). Note that the Sampling Interval can be adjusted.
The minimum value is 20 ns, corresponding to the 50 MHz ADC clock. The trace buffer is 4000 points,
yielding an 80
µ
s waveform with the minimum Sampling Interval. You may need to increase the Sampling
Interval for longer decay times.
•
When using the built-in Pulser, the Decay Time should be set to 5.0
µ
s * 2^(PULSERATE). For
example, with PULSERATE=0 (default), it should be set to 5.0
µ
s.
PREAMP GAIN
The Preamp Gain setting, along with the Dynamic Range setting on the Acquisition tab, determines the
MPX-32 analog and digital gain settings. The exact relationship is a bit convoluted, due to the fact that
each bank of 8 channels share the same analog gain setting due to hardware limitations.
•
We recommend starting with a value of 150 mV/keV along with a 2 keV dynamic range, and
then using the ROI Calibration tool and/or the GainMatch tool to refine the Preamp Gain
setting on a per-channel basis.