Matt Daniel Fil (WP 6294)
The MCL and MCF readbacks have a 10/100 Hz Sallen-Key whitening stage which amplifies the high frequency spectrum to get above ADC noise. Since a while we have observed a 20-50 mHz/√Hz flat noise level in the these spectra when we are locked with the IMC only. Looking with the oscilloscope we estimated about 10m V signal between 100 kHz and 1 MHz, before the whitening. This seems too much for the AA board, so we included additional low pass filters in the readbacks with cut-offs around 15 kHz. A 15/150 kHz pole-zero was added to the Sallen-Key, and another 15 kHz pole was added to the output stage.
In detail (common mode board, IMC, s/n 1102626):
- Stuffed a 3.3 nF into C293 and C241
- Mounted a 330 pF on top of C295 and C243.
The attached spectra now show a frequency noise level which is compatible with the one observed in full lock. The coherence is also improved. The ADC noise is not too far away in regions with reduced coherence.
Here is a comparison between MCF fully locked and 2W IMC only (REF traces). The changes are much smaller now, indicating that MCF sees frequency noise from the laser.
The IMC shot noise limit here should be about 1 mHz/Hz1/2, assuming 0.3 mW of light (mostly carrier) on the PD with the IMC locked, 5 mW of light on the PD with the IMC unlocked, and a modulation depth of 0.01 rad.