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AdV-ISC (Commissioning up to first full interferometer lock)

casanueva - 15:32 Thursday 12 July 2018 (42109)

6MHz SNR study

I have taken a look to the two days of data with the 6MHz on also in Dark Fringe. We let the 6MHz at 0dBm of modulatio depth.

First interesting thing is that for B2 6MHz error signal MICH and SSFS are not separated exactly by 90deg but by 50deg. (for B4 56MHz they are separated by 85deg). However, the loss of SNR for MICH if we use the demodulation phase optimal for the SSFS is only of 10%. If we compare the SNR of both error signals we have (see Figure 1 and 2):

	B2 6MHz	B4 56MHz
SSFS	156.4	666.7
MICH	78.9	355.6

The difference is of a factor 4, which means that both error signals are comparable for a modulation depth of 12dBm. Notice that these measurements were done for 3mW in Dark Fringe, so there were non-linearities on the RF channel at the moment of the measurement.

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casanueva - 11:59 Friday 13 July 2018 (42115)

Michal noticed that the spectrum of B2 6MHz which is not flat at high frequencies, suggests that the signal does not reach the sensing noise but still contains information. Making the coherence between the B2 8MHz (which is used for correcting the frequency noise since it has better SNR) and the B2 6MHz shows that at low frequencies and at high frequencies there are coherence between both, even though not at all bandwidths (Figure 1).

Figure 2 shows both the B2 6MHz and 8MHz error signal superposed. Notices that the B2 8MHz has a SNR for the SSFS of ~ 145, less than B2 6MHz, suggesting that with the 6MHz at a normal modulation depth the B2 6MHz error signal would be better than the 8 one.

Everything points towards the B2 6MHz as a promising error signal.

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