SUS Noise Monitor
Design goals
- No saturations as long as the coil driver input signals are at the 99th percentile or below.
- Provide enough gain (at 20 Hz and above) to boost the DAC noise above ADC noise (by what factor?).
- Is it possible to measure the coil driver noise? Or alternatively, with what SNR should the DAC noise be measured?
Design inputs
DAC noise model: page 6 of G1401399.
PUM coil driver transfer function. LISO models are available here.
- The worst-case dewhitening state for the noise monitor is ACQ off, LP on.
ADC noise level, about 4 uV/rtHz. (Reference: page 6 of T070213)
Coil driver input spectra, G1801540. (Jupyter notebook)
The aLIGO sensitivity curve, T1500293.
Quad suspension PUM transfer function: above a few Hz, about 3e-8*(10 Hz/f)4 m/N. (Reference: T1100595)
PUM coil driver circuit diagram, D070483
aLIGO QUAD controls block diagram, T1100378
DAC noise
The strain of DAC noise The plot is in DAC-noise-strain.pdf, compared with the aLIGO strain noise (at low frequency DAC-Low-Freq.pdf). The aLIGO-strain-requirement curve is the aLIGO noise curve divided by 10.
We calculate the strain of the DAC noise with the following steps.
Input at the Coil driver: we produce the input DAC noise at the coil driver using the model given in G1401399.
Generating the coil driver transfer function with the LISO model here (we use ACQ off, LP on coil driver state since it is the worst case for DAC noise), we calculate the current noise out of the coil driver caused by the input DAC noise.
Then we calculate how much driving force can be generated from the output current of the coil driver according to the block diagram of the PUM coil driver (0.0309 N/A).
Next we calculate how much strain noise is produced by the force noise previously calculated, according to T1100595. The rate is estimated to be about 3e-8*(10 Hz/f)4 m/N.
- Finally, we combine the noises of all the coil drivers in the interferometer: four optics with four coil drivers on each, combined to 16 incoherent noise sources. Thus, we multiply individual coil driver noise by 4.
DAC noise through the coil driver Since our noise monitor picks up the signal at the output of the coil driver, we need to know how much noise is left after the attenuation of the coil driver. We have previously known the DAC noise input at the coil driver at G1401399. We then produce the voltage transfer function using the LISO model here plot CD-transfer.pdf) to get the output DAC-noise-CD-in-out.pdf (we zoomed in since we are most concerned about DAC noise between 20Hz and 100Hz).
DAC noise and different states of the coil driver A marginal observation has been made comparing the DAC noises of different states of the coil driver. A plot comparing the noises under three different states with the aLIGO noise has been made DAC-noise-CD-states-strain.pdf. The difference is caused by different states of the coil driver has different transfer functions. For example, with the same input voltage, you get more current out of the coil driver when LP is off and ACQ is ON then LP on, ACQ off. Plots of the transfer functions: Current-TF-CD-states.pdf.
General Design Ideas
- The DAC frequencies we need to monitor is 20 - 100 Hz, above 100 Hz the DAC noise is too much attenuated by the mechanical transfer function.
The noise monitor will pick up differential signal from the output voltage of the coil driver. This is computed using the input data of channel L1:SUS-ETMY_L2_MASTER_OUT_LL_DQ, given in the design input G1801540. The input and output of the coil driver is plotted as CD-in-out.pdf.
- We need to avoid saturation. In order to see if the design saturates, we compute the RMS of the output at each opamp and make sure it is far enough from ± 15 V. A plot should be made on the accumulative RMS to infinity.
We want enough gain in the passband such that the DAC noise (originally 300nV/rtHz but attenuated by the coil driver, plot DAC-noise-CD-in-out.pdf) overwhelms the ADC noise (4uV/rtHz). However, we want to allocate the gain reasonably in the circuit to avoid both saturation and too much noise.
According to CD-in-out.pdf, most of the input signal lies below 20 Hz. Thus, to avoid saturation, we need high-quality high-pass filtering at 20 Hz.
- We calculated the input RMS from the coil driver to be 0.83V. We need some filtering before the instrumental amplifier to avoid saturation.
Design
Schematics: noisemon.pdfMonitorBoard.SchDocMonitorCircuit.SchDoc
LISO model: Gitlab
Transfer function transfer-function.pdf
Gain: we applied a gain of roughly 125. At 100Hz (the worst), the DAC noise is amplified to 5.76 times of the ADC noise. Plot of the amplified DAC noise by our noise monitor and ADC noise: GAIN.pdf.
Electronic noise of the monitor: NOISE.pdf (zoom: NOISE-zoom.pdf). It is at most 1/16 of the DAC noise throughout the band. The main contributor is the current noise of U5 (the op amp of the second HP filtering stage) noise-components.pdf.
The RMS of the output is 2.45V SATURATION.pdf.
Prototype
Constructed the instrumental amplifier in the EE shop.
Transfer function: PROTOTYPE-TF.pdf. The measured is almost identical to the LISO calculations.