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AdV-SVS (Pre-commissioning)
mwas - 9:32 Friday 23 April 2021 (51505) Print this report
RF sideband frequency choice check

Following a question by Giacomo I have looked at what beat frequencies between the different RF frequencies in the interferometer and squeezing could cause problems.

The digital RF frequencies used are listed in /virgoData/VirgoOnline/DAQ_requested_modulation_frequencies.cfg

I have made a script attached to this entry to list all the possible combination, and look at those that lead to beat frequencies below 20kHz. Looking at 1f, 2f and 3f of each sidebands beating against the 1f, 2f or 3f of any other sidebands, for a total order up to 8.

The base frequencies used are the 3 iterferometer sidebands and the 4 sidebands defined for squeezing:

6270783 8361045 56436993 4006184.1 55291f427.0 8245000.0 11120000.0

The frequencies that could be generated are in Hz:

541.8       2230.7       2851.1       3395.9        11517        12059        14368        14910        17764

These correspond to the following combinations of the base modulation frequencies

0    -1    -2     2     0     0     2     2     0
2     0     1     0     2     0     1    -1    -1
0     0     0     0     0     0     0     0     0
2     3    -1     1     0    -2     1    -1     0
0     0     2     2     1     1    -1    -1     3
-3     2     1    -2     0     3    -1     2    -1
0    -2    -1    -1    -2    -2    -1    -1     0

In particular their total order is rather high.

7     8     8     8     5     8     7     8     5

The 541.8Hz line is relatively low in frequency, so could be an issues in h(t). But it is order 7, so its amplitude should be very small. It correspond to the 2f of the 6MHz ISC sideband beating iwth the 2f of the 4MHz SQZ sideband and the 3f of the 8.2MHz SQZ sideband.

The first lower order beat signal is at 11.5kHz, corresponding to the beat of the 2f of the 8MHz ISC sideband with 1f of the 5.5MHz SQZ sideband and the 2f of the 11MHz SQZ sideband.

I do not know if these sidebands are actually arriving at any point on the same beam, nor if they are in fact used. But looking at the RF spectrum of ITF PDs, the 22MHz sidebands used for the IMC control, that is in principle rejected by the IMC, is visible, as the rjection is not infinite. So we should assume a priori that any sideband can get to any beam, but the amplitude may be small.

Overall this looks good. The 541.8Hz beat signal may be an issue, so try to move it frequency higher would be nice. The 8MHz and 11MHz SQZ sidebands have 3 or 4 zeros at their end, so there should be room to adjust these by a few kHz to avoid these low frequency combinations.

Non-image files attached to this report
Comments to this report:
ciani - 22:24 Monday 10 May 2021 (51728) Print this report

Inspired by Michal's idea and after a some additional exchange with him and Marco V, I produced a similar script to Michal's, with some corrections and enhancements:

  • the sidebands Michal considered initially actually belong to groups associated to carriers which are far apart in frequency (ITF, subcarrier and green). So beat between sidebands of different carriers can be neglected and excluded from the list.
  • the selection criteria for acceptable sidebands should be two:
    • No beat note should be produced in the ITF band (up to some total sideband's order, like in Michal's script). Let's assume f >=2 kHz
    • No beat note shoudl be too close to a possible demodulation frequency (indentified as an harmonic of a sb), not to bother control loops. How much it depends on the loop. but I'm told that 50 kHz minimum distance should be safe.

The attahced python (v3) script:

  • only accounts for beatnotes between sidebands referring to the same group
  • plots all beatnotes and their order (in a quite messy but hopefully informative plot, if one is patient enough to navigate it)
  • lists all beatnotes <2 kHz (none found)
  • list all beatnotes closer than 50 kHz from a possible demodulationfrequency (there are a couple, all originating from the ITF carrier sidebands)

Max sideband harmonic to consider for beatnote generation, max total order of a beatnote (sum of the orders of the beating sb), maximum sb harmonic to be cosnidered a possible demodulation and minimum spacing of beatnotes from demodulation frequencies are all configurable. 

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