I calculated the DAC noise for L1. Attachment 1 has all the plots and data. Attachment 2 is the result in strain.
We have noisemon at all four stations: ITMX, ITMY, ETMX, ETMY. DTT gives me the CSD, the coherences and the ASD of all the channels at all the four stations. I use this to calculate the transfer functions of the coil driver and the noisemon.
where D(f) is the drive and CSD is the CSD between the drive and the output of noisemon. The absolute value of H(f) will be the gain of the circuit, in ADC counts / DAC counts. Then I use the coherence to calculate the total noise
This noise has DAC noise, ADC noise, noisemon noise in it. This noise is in DAC counts. I will call this "*DriveNoise*".
Then I picked another time when the interferomter is not running and the drive is zero. I measured the noisemon spectrum N(f) at that time. The plots and data of these spectrum can be found in attachment 1. The plots are considered to be a result of the noisemon noise and ADC noise, which I will call "*NoDriveNoise*" (in ADC cts). Since the drive is zero, there is no DAC noise in it - just ADC noise and noisemon noise. I use the transfer function to covert it to DAC counts
Then I subtract the noise drive noise from the drive noise to get the DAC noise to get the DAC noise, which is then converted to DAC volts
Then I find the current on the coil using the transfer functions of the coil driver, assuming the coil driver is in LP OFF and ACQ OFF state. The transfer function can be found in attachment 1.
where the transfer function H is a voltage-to-current transfer function.
Then we have the force
Lastly we have the displacement and strain
For each station I summed all the four channels, LL, LR, UL, UR and then I calculated for all the four stations and summed as
I tried to compare this with the GWINC model - it is much higher. I do not have real L1 noise at the moment. I will see once we have real noise data. |