Cremat's CR-210-R0 is a single channel baseline restoration (BLR) module, intended for use with pulsed signals such as those produced by nuclear detection instrumentation. The CR-210-R0 corrects the baseline shift occurring in AC-coupled pulse amplifiers that may degrade the resolution of the acquired pulse height spectra. An illustration of the problem can be seen in the scope traces below, showing the output of a Gaussian shaping amplifier detecting pulses from a tail pulse generator. Most pulse detection electronics utilize AC-coupled amplification stages.
As a result, the DC gain of these shaping amplifiers is zero and thus the time averaged output from the amplifier is at ground potential. While this is generally not a problem for detection instruments at low or perhaps medium count rates, at high count rates the baseline of the signal becomes significantly depressed, reducing the pulse height of the signals with respect to ground and degrading the pulse height spectrum. The scope traces below show the effect of the baseline depression with increasing counting rate, and the restorative effects of the CR-210-R0 BLR. The input of the CR-210-R0 BLR is connected to the output of an uncorrected Gaussian shaping amplifier (in this example a CR-200-1ms shaping amplifier module). As can be seen, the baseline of the signal at the CR-210-R0 output remains at ground potential even as the count rate increases to 50k/sec.
The CR-210-R0 circuit is contacted via an 8-pin SIP connection (0.100" spacing). Pin 1 is marked with a white dot for identification.
The CR-210-R0 BLR stage should be configured to follow the shaping amplifier stage. The CR-210-R0 can be used with any of Cremat’s shaping amplifiers, preserving the Gaussian signals with shaping times from 50 ns through 8 ms. Any DC offset present at the shaping amplifier output will be lost as the baseline is redefined in the CR-210-R0 circuitry. The CR-210-R0 works only with signals containing positive pulses.
Response to changing count rates
In situations where the count rate is changing, the baseline is of course redefined to accommodate this. The scope traces below show how quickly the baseline is redefined as a result of increasing count rate (correction increases the baseline with a time constant of 200 ms), and as a result of decreasing count rates (correction decreases the baseline with a time constant of 20 ms).