Laura, Josh, Andy, Joe, Detchar
We took a look at the 28 Mpc lock on 24th March to see if there was any evidence of arches that we have seen previously at LLO (for example LLO alog 17090).
The first attachement shows a histogram of the rate of glitches (omicron triggers) binned by the value of IMC-F at the same time. The plot looks pretty Gaussian, except for a few bins specifically between IMC-F values of -37.5 to -41.7 and -45.8 and -50. Between these values we see evidence of arches in DARM. The second attachment is a pdf file showing the arches in DARM lining up with values of IMC-F in the second bin. Here are links to many spectograms confirming the arches in the IMC-F bins outlined (spectrograms for IMC-F bin -37.5 to -41.7 / spectrograms for IMC-F bin -45.8 to -50). Each spectrogram time corresponds to a glitch as seen by omicron.
LLO has dramatically reduced the effect of these arches, see LLO alog 17191.
Also the value IMC-F has when these arches appear in DARM seems to drift, i.e. there does not appear to be a specific value of IMC-F which produces the arches (this is evident in the second attachement). This is not what we have seen at LLO, does anyone know what could be causing this?
Andy, Laura, DMac Here's a look at one of the whistles, and the putative cause as an RF beatnote with IMC-F. The idea is that the IMC VCO is beating against another (nearly) fixed RF oscillator, and that's what we see in DARM. In that case, the frequency track should look like the difference between IMC-F (calibrated in kHz) with some fixed value. In fact, the whistle clearly seems to match half of the frequency difference (black trace) rather than the full difference (red trace). Perhaps this hints at the type of coupling? Here, the fixed value is about -46.9 kHz (relative to whatever IMC-F's reference is), but we think this drifts during the lock. We can look in detail at some more of these, and find out.
Why not use the frequency readbacks of the oscillators and VCOs rather than the somewhat arbitrary IMC_F? The radbacks are only once per second, but they can still tell you which units are close in frequency.
I'd be happy to look at the readbacks, as long as they are recorded and available offsite. Can tell me what the channel names are?
I put some notes about these channels in the summary of the ISC F2F last fall. ("RF crosstalk" section)
The channel names and labels are in the attached code. There, I look at the time around the event that Andy sent around to the DetChar mailing list yesterday. It's not totally obvious what VCO is beating against the PSL VCO, the closest one is ALS COMM, but it's about 400kHz away at the time of the glitch.
The first plot shows the value of all the corner station VCOs in the ten minutes around the glitch, the second plot is a zoom around the PSL VCO frequency.
It seems like H1:SYS-TIMING_C_FO_A_PORT_11_SLAVE_CFC_FREQUENCY_5, which Dan has labeled as the PSL readback, is the best candidate. This seems to be roughly equal to (1/2)*IMC-F + 79.225 MHz, delayed by one second (i.e. I can line those two things up and they overlap). Is the one second delay due to some loop or is it just a quirk of the recording system? The whistles occur whenever this readback crosses 79.2 MHz, which is the fixed fiber AOM frequency from the talk referenced in Dan's notes from above.