LIGO Document T1700332-v1

RF Leakage Studies & GPS Signal Phase Jitter Analysis

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T - Technical notes
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RF signals are mainly responsible for putting the interferometer into lock, an important task that allows LIGO to be sensitive enough for gravitational events. However, RF signals can cause leakage that can interfere with other electronics. One culprit is DC ground isolation units. When performing a spectrum analysis using an RF analyzer and measuring cabling ground to cabling ground, it was found that there were significant signal leakages as high as ~80 mV when it should ideally be 1 mV. Upon investigation and testing, it was determined that the leakages were due to small or absent capacitance along the unitsí enclosures. After modifying the capacitance of the units and then testing for insertion loss and phase delay, an order of magnitude decrease in RF leakage was observed.

Timing, from both GPS and a cesium clock, is another important aspect of LIGO which dictates much of how measurements are taken. When several atomic clocks are tuned to be in phase with each other and triggered on a 1 PPS GPS signal, the clock signals jitter on the order of nanoseconds. Given that this is tested against multiple clocks, GPS must be the source of the jitters. This may not have a large effect on short term measurements, but for long term integrations such as continuous wave searches, this error may become a problem. After long term data acquisition and analysis, it was found there were distinct periodicities in the jitter. Further analysis was also done to determine whether these jitters were random or deterministic.

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