andrew.lundgren@LIGO.ORG - posted 15:07, Thursday 11 June 2015 - last comment - 14:55, Friday 12 June 2015(19079)
RF beatnote whistles in PRCL
Andy, Jess Since the 79.2 MHz fixed frequency source was powered off alog, we have not seen any RF beatnote/whistles in DARM at Hanford. We see them in DARM at Livingston, however, but the mechanism is much more complicated than Hanford. The mechanism is not the PSL VCO beating against a fixed frequency. Since we still see whistles at Hanford in auxiliary channels, we thought we'd revisit them, to see if that gives us clues for L1. We looked at the lock of Jun 11 starting at 6 UTC. We see whistles in PRCL, LSC-MCL, and sometimes in SRCL. Choosing two times, we find that the whistles correspond exactly to a beatnote of the PSL VCO frequency with a fixed frequency of 78.5 MHz (or something within a few hundred Hz of that). So it's the same simple mechanism as before, just against a different frequency. Attached are plots of two times in PRCL where we predict the exact shape of the whistle, using IMC-F as a proxy for the PSL VCO frequency. SRCL and MCL are similar. We'll go back and check other locks to see if there's any evidence for other frequencies or shifts in the frequency.
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First, a question. Is there something at 34.7 MHz in the center station? I see this frequency on channel SYS-TIMING_C_FO_A_PORT_11_SLAVE_CFC_FREQUENCY_4 - the PSL VCO is number 5 on this fanout. The numerology just about works with 2*34.7+9.1 = 78.5, i.e. that frequency gets doubled and is seen in the 9 MHz demod of the POP and REFL PDs. Jeff wanted me to also post an expanded version of the whistles story that I had sent by email, so here it is: To be clear, H1 *did* have whistles in DARM. Once we got the secret decoder ring that told us how to figure out the PSL VCO frequency, we realized that the whistles in DARM were precisely a beatnote of that frequency with 79.2 MHz. As a result of that investigation, that fixed frequency was turned off, and the whistles in DARM went away. Huge success! We also see whistles in SRCL, PRCL, and MCL. We haven't been worrying about them, since they're not in DARM. But just yesterday we decided to see if this is also a simple mechanism. As you can see from the alog, it is - at least at the times we've checked, the whistles are a beatnote against something at 78.5 MHz. I realized just a little while ago that these channels all come from 9 MHz demods, so maybe the actual frequency we're looking for is actually 69.5 or 87.5. We'll check whether these signals show up on POP or REFL at either LF or 45 MHz. We know that LLO is a very different mechanism. Not only do they not have this particular fixed oscillator, but these whistles: 1. Come from multiple very different VCO frequencies. 2. The beat frequencies don't seem stable even within a lock. 3. The whistles do not follow the PSL VCO frequency. They are more like 4 to 7 times the VCO frequency. The multiplier doesn't seem stable, and sometimes the whistles seem to decouple a bit from the VCO frequency. 4. The whistles show at LF, 9 MHz, and 45 MHz PDs, on REFL and POP. Different crossings show up in different photodiodes and with different strengths. So you can see why we want to tackle Hanford first. I was hoping it would be more complicated but tractable, and that would give us a clue to what's going on in L1. In case you're wondering whether this is academic, the CBC search loves triggering on the whistles at LLO, and it's hard to automatically reject these because they look like linear or quadratic broadband chirps. I think these give the burst search trouble as well. We'll probably spend another day nailing down the case at Hanford, then look over all ER7 to figure out what was going on at L1.