Optical Timing Receiver for the NASA Laser Ranging System Part II: High Precision Time Interval Digitizer

摘要

Position-resolution capabilities of the NASA-Goddard Space Flight Center satellite laser ranging systems are essentially determined by time-resolution capabilities of its optical timing receiver. The optical timing receiver consists of a fast photoelectric device, primarily a standard or microchannel-plate-type photomultiplier or an avalanche photodiode detector, a timing discriminator, a high-precision time interval digitizer, and a signal processing system. The time resolution capabilities of the receiver are determined by the electron time spread of the photoelectric device, the time walk and resolution characteristics of the timing discriminator, and the time interval digitizer. It is thus necessary to evaluate fast photoelectric devices with respect to their time-resolution capabilities, and to design a very low time walk timing discriminator and a high-precision time digitizer which are used in the laser ranging system receiver. The development of a high precision time interval digitizer is described. The time digitizer is a 10 psec resolution stop watch covering a range of up to 340 msec. The measured time interval is determined as a separation between leading edges of a pair of pulses applied externally to the start input and the stop input of the digitizer. Employing an interpolation technique and a 50 MHz high precision master oscillator, the equivalent of a 100 GHz clock frequency standard is achieved. Absolute accuracy and stability of the digitizer are determined by the external 50MHz master oscillator, which serves as a standard time marker. The start and stop pulses are fast 1 nsec rise time signals, according to the Nuclear Instrument Module Standards. Each digitizer input is made level sensitive by means of tunnel diode discriminators. Firing level of the discriminator define start and stop points between which the time interval is digitized. (ERA citation 02:052821)