joshua.freed@LIGO.ORG - posted 18:10, Thursday 18 June 2026 - last comment - 19:51, Thursday 18 June 2026(90680)
SPI, Pathfinder Install, REF Beam error found
J. Freed
Found the problem discribed in 90644, as well as the strange sawtooth beat notes we have been finding in our IFOs. It was user error on our CDS system, the double mixer (which produces our REF RF signal for AOMs) requires a sin wave and cos wave 4096Hz signal. We were sending it 2x cos signals.
This also explains why our demod systems were not working as they use the same signals.
CDS_Sin_and_Cos.jpg Shows that they were the same signal
REF_RF_Before.jpg REF_RF_After.jpg Shows the REF signal before and after. The before had a 4096Hz beat note inside itself
MEAS_IFO_BEAT_After.jpg Shows the corrected beat note on the Meas IFO after the correction. Note: There is some noise here which is partly caused by 16kHz signal that is ~13dB less than the main signal. The double mixer produces this as a byproduct and SPI is insensitive to this noise.
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The sawtooth that Josh mentions was how the poorly phased analog versions of the 4096 Hz SIN and COS LO signals manifested on the digitized versions of the IFO PD voltage, reading out that beatnote. Here, I attach what it looked like on the o-scope (before discovering the issue of phasing the two SIN and COS versions of the LO).
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