jeffrey.kissel@LIGO.ORG - posted 16:09, Monday 27 February 2023 - last comment - 09:18, Friday 10 May 2024(67644)
Diagram of all the different Digital Versions/Channels of the OMC DCPD Signal Chain
J. Kissel As we continue to investigate both ADC noise (both broadband, in-general behavior) as well as down-converted aliasing noise in the new fast 524 kHz OMC DCPD signal chain, I needed to wrap my head around all the available channels in order to begin thinking about how to compare / take advantage of the differences between the channels. I don't understand anything until I diagram it, so here's my diagram of the existing OMC DCPD signal chain as I understand it, hoping it helps others. As mentioned in each of the previous aLOGs where I've started the investigation (LHO:67552, LHO:67530, LHO:67465, LHO:67297, LHO:67439), studies of the 524 kHz portion of the system are severely limited in that (a) no 524 kHz channel stored in the frames, (b) one can only look at 3 test points at a time, (c) we don't have any analog measure of the noise between 524 kHz and 102.4 kHz (the upper limit of the SR785) to compare against But, we'll do what we can! Hopefully this visual aide will help understand future studies. For example, because we *don't* have any digital anti-aliasing filters in the OMC-PI SIG filter bank, we can compare these test point channels, - H1:OMC-DCPD_A0_OUT (524 kHz, w/ digital anti-aliasing) - H1:OMC-PI_DOWNCONV_SIG_OUT (524 kHz, w/o digital anti-aliasing) - H1:OMC-PI_DPCD_64KHZ_AHF (65 kHz, w/o digital anti-aliasing) - H1:OMC-DCPD_A_IN1 (16 kHz, w/ digital anti-aliasing) to explore the effectiveness of the digital anti-aliasing filters to better understand the aliasing that Evan Hall found in LHO:67328. Note -- and this is something that I'm still learning (via conversations with Erik von Ries and Daniel Sigg and their aLOGs, including but not limited to LHO:67587, LHO:67560, LHO:67291) -- the h1iopomc0 model actually runs at 65 kHz (really, 2^16 kHz). In order to achieve a pseudo-524 kHz data stream, there's a for loop within the h1iopomc0 model that's able to complete 8 iterations (thus 2^16 kHz * 8 = 2^19 kHz = 524 kHz) during any given 65 kHz clock cycle.
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Sorry team. There's a typo in the 65 kHz version of the DCPD channel in the PI model in both the above list, as well as in the diagram.
The channel list to accompany the diagram should be
- H1:OMC-DCPD_A0_OUT (524 kHz, w/ digital anti-aliasing) [[NOT STORED IN THE FRAMES, ONLY LIVE AVAILABLE]]
- H1:OMC-PI_DOWNCONV_SIG_OUT (524 kHz, w/o digital anti-aliasing [[NOT STORED IN THE FRAMES, ONLY LIVE AVAILABLE]]
- H1:OMC-PI_DCPD_64KHZ_AHF (65 kHz, w/o digital anti-aliasing) [[STORED IN FRAMES at 65 kHz as H1:OMC-PI_DCPD_64KHZ_AHF_DQ]]
- H1:OMC-DCPD_A_IN1 (16 kHz, w/ digital anti-aliasing) [[STORED IN FRAMES at 16 kHz as H1:OMC-DCPD_A_OUT_DQ]]
Careful here: In O3, H1:OMC-DCPD_A_IN1 was NOT equivalent to H1:OMC-DCPD_A_OUT_DQ, since H1:OMC-DCPD_A_IN1 *used* to be the "raw" (down-sampled, and digital AA filtered) ADC 16 kHz channel, and then the DCPD_A bank applied all of the the calibration and frequency response compensation. Now, as of O4, with the 524 kHz system, that's done in the DCPD_A0 bank, and the DCPD_A bank is an "empty" gain of 1.0 "passthrough," so DCPD_A_IN1 is equal to DCPD_A_OUT.
Further, a reminder that we've installed a "plug" that shorts the ADC inputs of channels 17 through 20 of this new 524 kHz system, so looking at the channels might also be interesting if we're looking for noise that might be present on the ADC channels alone (i.e. without any signals going into it). See LHO:67465 for details, but in short, you want to look at the (live, not stored in the frames) 524 kHz channels, - H1:IOP-OMC0_MADC0_EPICS_CH17 - H1:IOP-OMC0_MADC0_EPICS_CH18 - H1:IOP-OMC0_MADC0_EPICS_CH19 - H1:IOP-OMC0_MADC0_EPICS_CH20 for a "shorted" version of the OMC DCPD ADC card channels. This would be useful to, say, investigate how / why the DuoTone signal shows up in the DCPDs (see LHO:77579), and to compare and contrast against the L1 DCPDs which also see it (see LHO:70961), but they've not yet segregated their OMC ADC card, and (I think) have a different, older version of the Timing Interface card.