The attached plot shows the frequency of the IMC VCO, when the common tidal feedback path to the ETMs is running. At the beginning of the plot the offset in the IMC_F filter module was set to zero. This then puts the IMC VCO in average ~150 kHz below the fixed frequency oscillator which drives the fiber EOM. In the second half of the plot the offset was changed to -3500 counts and the IMC VCO shifted to ~200 kHz above the AOM frequency. Under nominal conditions the offset is at –1750 counts, which corresponds to roughly zero offset at the output of the filter module.
The important thing is to avoid a frequency crossings of these two RF sources, since this is one of the reasons for RF crosstalk into the laser frequency noise. We should probably try to move the IMC VCO frequency up by another 100 kHz to make sure we stay away from zero.
The COMM and DIFF VCOs are running at nominal 78.92 MHz. Since they implement two stages of the frequency-difference-divider, their range is no larger than ±150 kHz. Hence, they cannot reach the AOM frequency—or the IMC VCO frequency when it is above the AOM one. Once we are in full lock and the green beams are turned off, we need to make sure that these two VCOs are at least 10 kHz apart from each other. This should then prevent RF crosstalk by the VCOs from reaching the detection band.
Here is a plot from yesterday's lock showing a maximum frequency excursion of no more than 150 kHz over a 2 hour stretch.
The end station VCOs were using a lot of their range at low frequencies after switching to the common tidal feedback path. This was traced to a factor of 2 error in the Hz-to-µm conversion. I also increased the gain of the integrator to 0.03 Hz from 0.02 Hz to make it the same as the local feedback path. This reduced the required VCO range by roughly a factor of 2.
To set the DIFF/COMM VCOs to a predefined frequency, disable the input switch of the PLL and adjust its input offset. Both common filters need to be on for this to work, since we need a high DC gain. The input offset is divided by ~200 before it is applied. The mid range set point can be selected by enabling the optional daughter board and tweaking the VCO offset. Since we don't have a daughter board, enabling it will simply disable the output instead.
Parking the DIFF VCO (input enable: "H1:ALS-C_DIFF_PLL_INEN"; input offset "H1:ALS-C_DIFF_PLL_OFS"):
- ~77.87639 MHz: Disable the input and set the offset to –2.0V or lower.
- ~80.41885 MHz: Disable the input and set the offset to +6.0V or higher.
- ~78.92 MHz: Turn on the optional daughter board (assumes that the VCO offset is trimmed to 78.92 MHz).
The VCO offset for the DIFF VCO was set to –1.864V.
Parking the COMM VCO (input enable: "H1:ALS-C_COMM_PLL_INEN"; input offset "H1:ALS-C_COMM_PLL_OFS"):
- ~78.79464 MHz: Disable the input and set the offset to –6.0 or lower.
- ~78.99198 MHz: Disable the input and set the offset to +5.0 or higher.
- ~78.92 MHz: Turn on the optional daughter board (assumes that the VCO offset is trimmed to 78.92 MHz).
The VCO offset for the COMM VCO was set to +0.083V.
Daniel also adjusted the tune voltage we use for the DIFF VCO as a starting point, before we lock. We used to use 0, now we use -1.9V, which is included in the gaurdian now. This means that the locations where we can normally fin the DIFF beatnote have changed, now one is at around 1370-1386 while the other was at 0 - 20 counts offset in the DIFF PLL CNTRL offset. I've added these to the hardcoded list that the guardian uses.