Figure 2: Block Diagram of 8-Channel ASD
A block diagram of the proposed ASD design is shown in Figure 2. Each channel consists of a charge-sensitive pre-amplifier, a programmable time-constant shaper incorporating several pole/zero stages, a timing discriminator, and a simple ADC. The most important function is leading edge pulse timing, and the design emphasizes this function. Additional measurements such as trailing edge timing and multi-track resolution may be made, and are discussed at the end of this paper.
Several critical parameters in the ASD are programmable to permit future changes in the operating point of the MDTs. These include: Shaping time (nominally 15ns) which can be adjusted to accommodate gas mixture changes; Discriminator threshold and hysteresis which can be adjusted for changes in gas gain; Pre-amp and discriminator bias current which provide a trade-off between performance and power consumption.
A test pulse injection circuit is provided, which will inject pulses of precise timing and programmable charge into each input channel.