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Detailed introduction
The signal from a germanium or silicon detector at the preamplifier output is not always a signal that enables good timing resolution. The signal is usually amplified using a broadband amplifier before passing it to the timing discriminator, and additional pulse shaping may be required to minimize the impact of noise on the time resolution. The Model 863 provides a flexible way to accomplish both functions and can handle various types of solid state detectors.
Gain can be selected as inverting or non-inverting and is adjustable over a nominal range of 2 to 250. A front-panel screwdriver potentiometer adjusts the fine-tuned gain from 2 to 50, while a printed wiring board (PWB) jumper selects a coarse-tuned gain of 1 or 5. The output will drive a 50 Ω load to ±5 V with good linearity. This ensures that the full -50 mV to -5 V dynamic range of the constant fractional timing discriminator can be used. Continuous baseline restorer maintains excellent output DC stability.
Several bandpass limiting methods are used to achieve time-resolved pulse shaping. The Model 863 is a wideband amplifier with an output rise time of <10 ns when all jumpers are in the Out position. To reduce low-frequency noise and shorten the output pulse width, the CR differential time constant can be adjusted from 0.1 ms (Out position) to 200 ns using the PWB jumper. Alternatively, the two front panel connectors (using 50Ω coaxial cable) can be used to add delay line limiting. This results in an abrupt termination of the output pulse duration while reducing low frequency noise. CR differentiation and cable limiting can be used simultaneously to produce a bipolar output signal for fast, zero-crossing timing.
In some cases, a 50 ns RC integration time constant can be selected using the PWB jumpers provided for this purpose. This reduces high-frequency noise while slowing the output rise time to 110 ns.
In addition to the two standard jumper options in each differential and integral control, a third position is provided for each jumper. By adding the appropriate components to the third position, the user can select a custom set of integral and derivative time constants. To ensure that the output pulse returns to baseline as quickly as possible, the differentiator circuit includes a front-panel pole-zero trim control. This control compensates for the preamplifier's decay time constant.
Each part of the Model 863 has five sets of PWB jumpers for controlling various functions of the device. These jumpers are accessed by removing the left side panel of the module.
