Today Keita and I spent some time thinking about OMC length noise, there will be an update coming soon with more information and a noise projection.
We spent some time looking at some nonlinear behavoir noise in the drive to PZT1. Our dither frequency is 4100 Hz, and looking at the low voltage PZT monitor we can see a small 8 Hz and a larger 16 Hz comb. There is also other non stationary noise in the monitor, and a broad peak at 12.7 kHz. We have moved the dither line frequency to 4100.21 Hz, so if this was the cause of the 16 Hz comb in DARM we would now expect it to be more like a 16.84 Hz comb. Evan Goetz tells us that we need 15 minutes or more of data in low noise to evaluate if this has changed any combs in DARM.
We have just reached nominal low noise at 3:14:34 UTC Feb 25th, although the low frequency noise (below 50 Hz) is worse than normal. I've temporarily changed the dither frequency in the OMC guardian, so if there is a longer lock later tonight it should also have this changed dither frequency. (If anyone wants to double check what the dither frequency is, the channel is H1:OMC-LSC_OSC_FREQ
Keita, Sheila
So that we can keep the OMC dither small while driving a reasonable level of counts out of the DAC, we added a voltage divider (somewhat creatively built) to the D-sub from the DAC to the driver chassis. This is a 11k/110 Ohm divider on pins 1 +6. We have increased the dither amplitude from 6 cnts to 600 cnts, so the round off errors will now be 100 times smaller compared to our signal.
The attached screenshots show the PZT1 AC monitor before and after this change. The lines below 1 kHz are always there, (even when there is 0 coming out of the DAC) and are not present on the analog signal coming into the driver chassis for the monitor.
We have reverted the frequency to 4100 Hz. If we get a long enough low noise lock tonight we can hope that the 16 Hz comb will be better. If things look good we should upgrade our voltage divider.
Thing is, our dither line used to be 12 counts pk-pk, so the rounding error was actually significant (signal/error ratio is something like 10 in RMS), and the error showed up as lots of lines because we're sending in only one sinusoidal signal. These lines actually drive the PZT length.
Making the dither bigger, the round off error RMS doesn't change much so RIN will become smaller.
We inserted two sets of 11k-110 Ohm resistive divider, one each for positive and negative input of the low voltage pzt driver input because it was easy. This is a temporary non-solution. A permanent solution is TBD.
The first attachment shows the spectrum of the DAC IOP channel for the dither, i.e. the very last stage of the digital, before we increased the amplitude. RMS of the forest of lines is about a factor of 10 below the RMS of the dither.
The second plot is after increasing the amplitude by a factor of 100, the rounding error RMS is still at the same level though you cannot tell from the plot, the dither to error RMS ratio should be more like 1000 now.
Three large lines in the second plot are not round off errors but imaging peaks that were previously buried in the round off errors: 12283.8kHz=16384-4100.2Hz, 20484.2Hz=16384+4100.2Hz, and 28667.8=32768-4100.2Hz.
Actually, the dither line is still at 4100.21 Hz for tonight, (I had forgotten that I put this into the guardian). We will revert it tomorow.