Kiwamu, Sheila, Evan
Durring the calibration activities last week Kiwamu noticed that there was upconversion of the drive from ETMY. Specifically when he drove in the L3 LOCK filter, he saw a second harmonic. When he drove each stage individually, through the test filter bank with a similar amplitide he saw no evidence for upconversion. This suggests that the upconversion might come about by driving mutliple stages, or through the length 2 angle paths.
We went back through the data and looked at the relative heights of the peaks. We also calculated a ratio of the amplitude of the second harmonic to the square of the amplitude at the fundamental :
ASD(2f) = alpha * (ASD(1f))^2
| drive Frequency (Hz_ | amplitude at drive frequency (m/rt Hz) | amplitude at second harmonic (m/rt Hz) | alpha (1/m) |
| 4.98 | 5.8e-14 | 2e-15 | 5.9e11 |
| 5.9 | 5.6e-14 | 9.8e-16 | 3.1e11 |
| 6.4 | 3e-14 | 2.8e-16 | 3.1e11 |
| 10 | 3.6e-15 | 2.8e-18 | 2.2e11 |
If we ignore other frequencies which could mix and only consider the second harmonics of DARM control, we would expect this upconversion to be something like a factor of 10 below our measured noise at 20 Hz, and near the measured noise at 10 Hz.
The next step is probably to identify which stages contribute to this upconversion, for example it seems probable that this is only noticeable for frequencies where L2 get a significant fraction of the DARM control signal.
We made some injections into ETMY to check when exactly this upconversion shows up. It is much smaller today than it was durring Kiwmau's measurement.
An injectionat 5 Hz (500 counts in ISCINF) that increased the DARM noise there by a factor of 67 produced a peak at 10 Hz that is a factor of 2.6 above the noise floor. We also injected in L2 L2L (which will bypass the L2P and L2Y filters) to produce a simlar peak, and got a similarly low level of noise at 10 Hz.
It may be that part of the problem durring Kiwamu's measurement was that some ASC loops were accidentaly off.