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AdV-DET (Commissioning)
masserot, gouaty - 15:35 Friday 19 March 2021 (51147) Print this report
Change of VGA gain on B2 photodiodes
Following the increase of power on the B2 photodiodes (logbook entry https://logbook.virgo-gw.eu/virgo/?r=51145) the ADC of the B2_PD1/3 RF channels were saturating.
Thus we decreased the gain of the demodulation mezzanine VGA to prevent this saturation:
- Gain of B2_PD1 changed from +10 dB to +4 dB
- Gain of B2_PD3 changed from +10 dB to -6 dB
After this adjustment, the RF channels are no longer saturating. However there is only a factor 2 of margin on B2_PD1 and a factor sqrt(2) on B2_PD3. This was with the PR mirror aligned, but CITF unlocked.

We may have to reduce further the gains or to go back to lower power on the PD in case the RF channels saturate during the lock acquisition.

One can notice that the 169 MHz signal is visible on B1_PD1 but with a quite low SNR (Fig.5).
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masserot, gouaty - 19:18 Friday 19 March 2021 (51154) Print this report
At the beginning of the afternoon shift, we reduced the VGA gains of the B2 quadrants in order to prevent them from saturating (Fig.1):
- Gain of B2_QD1: reduced from 9.86 V/V to 0.7 V/V (corresponds to a reduction of 22.7 dB).
- Gain of B2_QD2: reduced from +20dB to +3dB (reduction of 17dB).

We also further reduced the VGA gains of the B2 photodiodes (PD1 and PD3) as we noticed that they were at the limit of the saturation with the CITF aligned:
- B2 PD1 gain reduced from +4dB to +1dB
- B2 PD3 gain reduced from -6dB to -9dB.

The B2 PD3 gain is now at its minimum.
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