andrew.lundgren@LIGO.ORG - posted 21:50, Sunday 24 January 2016 - last comment - 14:49, Tuesday 26 January 2016(25134)
Small glitches on second boundaries
There are small glitches very close to each second boundary in the ETMY drive signal and the ETMY SUS ad SEI rack magnetometers. In order to investigate the half-Hz combs in DARM (see alog 20790), I took an hour of data and folded it with a four-second period. If there is a repeated glitch at any multiple of this period, it should become far more visible. The result is that in several channels, there are glitches very near the boundary of each GPS second. The peak time of these glitches seems to be about 10 milliseconds after the start of the second. The glitch does not repeat identically every second. There is one shape in the first second, then one with an opposite polarity in the next second. The first two attached plots are for the SUS and SEI racks, which are shown with a 40-Hz zero-phase lowpass. The SUS has a narrow spike, and the negative spike is larger than the positive one. The SEI signal is more complicated. The ETMY L3 MASTER signal, which is the DARM output to the ESD, is shown with a 10 to 50 Hz bandpass. These glitches are more like sine-Gaussians, but the even and odd seconds still seem to have opposite polarities. There are more channels with similar glitches. We can make a more thorough investigation, and use more data and more times, to try to track down the origin of these glitches. Hopefully these glitches are responsible for the 0.5 Hz combs such that removing them will improve those.
Images attached to this report
Comments related to this report
J. Kissel
Tagging CDS in this entry.
I'd recently taken a look at the requested output of the ETMY L1 stage,
${IFO}:SUS-ETMY_L1_MASTER_OUT_*_DQ
and was interested to find ~1 [Hz] combs in the requested output. Though this isn't the 0.5 [Hz] combs that Andy mentions above, I think it's an excellent place to take the investigation further in a more focused manner -- with the point being that even the SUS's *requested* signal has a comb.
Attached is a 100-sec FFT ASD, of a typical, 1000 [sec] stretch of observation-ready data during the run (2016-01-04 04:00 UTC). Here, to give a feel for the physical amplitude of these signals: at 30 [Hz] the noise amplitude of one of these comb peaks is roughly 1e-3 [ct/rtHz] of requested DAC output, which corresponds to
1e-3 [ct/rtHz] * (20.0 [V] /2**18 [ct]) = 7.6e-8 [V/rtHz] @ 30 Hz ( * sqrt(2 * 0.01) = 1.1e-08 [V_pk])
Potentially verifiable / refutable Crack-pot Theories / Wild guesses:
- Perhaps there is some of this glitching in the inter-process communication (IPC) on the reflected memory (RFM) data transfer from the corner to the end station, that's only exposed for requested drives that have such a huge dynamic range? For whatever channels in the signal chain that are stored, can you reproduce the same combs by filtering those channels offline?
- Recall that the power supplies for the Hartmann Wavefront Sensor (HWS) were replaced some time ago, see Integration Issue 1062. Has anyone made a before-and-after comparison on this searched other sources for such combs in auxiliary channels? Perhaps forming a BruCo-like search where this UIM / L1 stage control signal is the response instead of DARM?
- Keith has already done a long-term study of the analog-to-digital converters (ADCs), looking for combs: see, eg. pg 44-49 (yeah!) of G1300997. He found no-such combs. Perhaps we should do a similar study on the digital-to-analog convert (DAC) side of things? I could also imagine a similar set up for a set of RFM channels that make the 4km journey along the arms.
Images attached to this comment
In response to Jeff: The SUS-ETMY MASTER signal is just a filtered version of DARM, so if DARM has the comb so do the drive signals. I don't think that tells us where in the loop they originate. But you're right, this could be a digital problem or an electronics one involving something synched to GPS. Keith T. and Annamaria both suggested that the power supply of the timing fanout might be involved. That of course can be perfectly synched to the GPS second. Annamaria showed me an ADC in the L1 corner station being used as a temporary monitor of one of these power supplies. That signal (attached) jumps downward one second and upward the next, matching what we see in the magnetometers and DARM. Could we check if there's such an effect at the H1 Y-end?
Note that the small glitches in Andy's post are exactly synchronized to GPS; this makes coupling to many power supply glitches (HWS, or trickle chargers for magnetometers, etc.) an unlikely source.