Optimization of the readout electronics for microchannel plate delay line anodes

Many current and future space missions use microchannel plate (MCP) detectors with delay line anode readouts (e.g. FUSE, GALEX and the FUV detector on the upcoming Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope). Delay line anodes are used to measure the position of the centroid of the charge clouds that exit MCP detectors. This is accomplished by measuring the time difference between the arrival of the pulse at both ends of the delay line. The spatial resolution of this position determination is dependent on the accuracy of the temporal measurement. These high frequency pulses (similar to 100MHz) are usually amplified and directed to constant fraction discriminators (CFDs) whose output pulses start and stop a time to amplitude converter (TAC). This paper reviews the optimization of these circuit elements. It includes the characteristics of various delay line types (serpentine and helical) and their effect on pulse shape. The choice of amplifier filter bandpass and optimum fraction and delay for the CFDs is also presented. Examples are taken from the MCP detectors on the missions mentioned above.