CR-210 Baseline Restorer for CR-200

General Description
Cremat‘s CR-210 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 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 BLR. The input of the CR-210 BLR is connected to the output of an uncorrected Gaussian shaping amplifier (in this example a CR-200-1μs shaping amplifier module). As can be seen, the baseline of the signal at the CR-210 output remains at ground potential even as the count rate increases to 50k/sec.

Package Specifications
The CR-210 circuit is contacted via an 8-pin SIP connection (0.100“ spacing). Leads are 0.020 inches wide. Pin 1 is marked with a white dot for identification.

Equivalent circuit diagram
The figure to the right + shows a simplified equivalent circuit diagram of the CR-210.

Implemention
The CR-210 BLR stage should be configured to follow the shaping amplifier stage. The CR-210 can be used with any of Cremat’s shaping amplifiers, preserving the Gaussian signals with shaping times from 100 ns through 8 μs. Any DC offset present at the shaping amplifier output will be lost as the baseline is redefined in the CR-210 circuitry. The CR-210 works only with signals containing positive pulses. If the output needs to power a terminated coaxial cable, a line driver circuit should be implemented after the CR-210 stage.

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 μs), and as a result of decreasing count rates (correction decreases the baseline with a time constant of 20 ms).

Baseline Restoration Efficacy
The figure on the right shows the 1 efficacy of the CR-210 circuit when implemented after a 0.9 CR-200-1μs shaper amplifier. At count rates of approximately 120 kHz or more the Gaussian pulses start running together and the baseline can no longer be preserved. For other shaping times, this figure can be used with the count rate scaled proportionately.