Coil driver upgrade for Optomechanics experiments @ 40m

Motivation

From noise-budgeting of the DRMI, and measurements of the current suspension coil driver electronics noise, it is projected that this will the be the limiting noise source in DARM strain sensitivity at frequencies > ~100Hz. Since we'd like to measure ponderomotive squeezing, we need the limiting interferometer noise at the frequencies of interest to be quantum (shot) noise. Therefore, we need to reduce the coil driver noise in the frequency band of interest. Ideas/measurements/data towards this end are collected on this page.

Characterization of the current situation

1. Measurement of coil driver electronics noise, with input terminated to ground via 50ohms: elog13015.

2. Projected noise contribution with current de-whitening electronics, for various choices of series resistance for current coil driver topology: elog13146. Note that these plots were made with MICH in mind, so for DARM, they will have to be scaled by a factor of sqrt(4/3).

3. Impact of increasing the series resistance, and hence the reduced actuation range available: elog13417.

4. Current noise budget of MICH in DRMI configuration: elog13428.

The schematics for the suspension electronics, and associated modifications that have been implemented, are available at the 40m DCC tree.

Ideas for modeling and prototyping

1. Current topology, with increased series resistance.
   • How high can we go without compromising on actuation range?

2. aLIGO RFPD (D1101124) test circuit approach, with transistor acting as nearly ideal voltage-controlled current source.

3. Current topology, but add a series inductor in series with the series resistor, for a frequency-dependant transimpedance.