PCA Timing Signatures

Several high frequency time signatures due to the PCA/EDS instrument have been identified. We illustrate a broad noise component at 4 kHz and 5 narrow components near 194 kHz.

The Good Xenon data from a long observation of MCG 6-30-15 have been separated by PCU. The unnormalized auto-correlation functions (ACF) are shown in figure 1. For each detector, there is a noticable increase in the ACF near 4 kHz. Also visible is the ~10 micro-sec deadtime (i.e. no events at very short separations), and in the case of PCU 2, a beating of these two effects. The origin of these features is not understood in detail.

Event data obtained with 1 micro-sec time resolution during observations of Cygnus X1 reveals a sharp feature at 194 kHz in figure figure 2a. The expanded view in figure 2b reveals that 5 separate narrow lines are involved. These lines are identified as the beat frequencies between the spacecraft 2^20 Hz oscillator (used by the EDS to time tag PCA events) and the five separate 4 MHz clocks used by the individual PCUs to drive event data over the serial line to the EDS. The PCA clocks are not controlled nearly as well as the spacecraft clock, and it is quite possible that these features could move slightly over the course of the mission as the temperature of the PCA electronics goes up and down.

These plots were made by Ed Morgan (MIT Center for Space Research) from data obtained in 1996.
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