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AdV-DET (Commissioning)
mwas - 17:47 Tuesday 20 July 2021 (52612) Print this report
B1p QD2 RF channel may be saturating

Figure 1. I have looked at the RF quadrant time series for a lock from Sunday, and B1p QD2 might be saturating when in dark fringe. It does not reach 8192 count on the ADC, but this quadrant is connected to a demodulation board with the old AD8370 VGA that has 5 times less dynamic. The gain on the VGA is set to 2.3, so the ADC range is about the same as the analog range of the VGA.

Figure 2 Compares the RF spectra and B1p QD2 seems to have more RF harmonics around 100MHz than B1p QD1, so I suspect it stops being in the linear range of the RF dynamic.

B1p QD1 should be fine as it is connected to a demodulation board with the new VGA that has 5 times higher dynamic range. The gain is set to 2.3, so if the ADC starts to saturate (8192 count, while max is at 4000 in the lock I have looked at), so if needed the gain can be reduced by a factor up to 4 to increase the dynamic range. But it does not seem needed at the moment.

B1p QD1 should give a cleaner signal than B1p QD2 for alignment purposes (no risk of signal distorted by saturation).


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letendre - 18:52 Tuesday 20 July 2021 (52613) Print this report

The VGA AD8370 used on B1p QD2 RF channels has two gain modes:
- high gain mode. -8dB to +34dB . This mode has a better noise figure, better linearity and better SFDR (Spurious-free dynamic range) at higher gains. But this mode shows saturation with signal amplitude higher than 375mVpp.
- low gain mode.
-25dB to +17dB. The low gain range has better gain resolution, higher SFDR at lower gains. This mode saturates with an input signal amplitude higher than 1.5Vpp, well after the high gain mode.

Today B1p QD2 RF channels are using the "high gain" mode, with a gain 2.3 (0x86 in the configuration file). In order to reduce the VGA saturation, these channels could be used in "low gain" mode by setting the gain 0x2A in the configuration file. This is the same gain 2.3 as before, but in low gain mode.

More info: section 8.2.2 in this document

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