The amplitudes and powers of the PC signals are not stable on time scales of 10 min, and most definitely not on time scales of hours. As the effects are the same for all sidebands and the carrier, the DC of the corresponding main diode has been used in the past to correct for the drifts in the _PWR channels, but this situation is not satisfactory. To debug the problem we added new channels to dataDisplay:
1. <EPRB/EDB>_<B4/B1p>_PC_<112MHz/56MHz>_PDH_PWR, these channels demodulate the beat of the 56 MHz upper and lower sideband / sideband with carrier in the interferometer beam as is the case for the main diodes.
2. <EPRB/EDB>_<B4/B1p>_PC_<demodfreq>_REF_PWR, these channels contain the electrical reference needed for the phase images.
In the attached Figure a comparison of the PC and the main diode signals is shown, observations so far:
1. RF channels containing the reference beam do not follow the main diodes
2. RF channels not containing the reference beam do follow the main diodes
3. DC channels do follow the main diodes
4. The electrical reference is stable
Point 1 is the problem to be solved.
Point 2 seems to indicate that it is not an electronic problem, or interferometer beam problem.
Point 3 shows that it is not the reference beam amplitude.
Point 4 shows that it is not an unstable reference beam frequency (reference beam is at 80 MHz).
Remaining suspects are:
- Reference beam polarization
- something else we did not think of?
We will contact Eric to help with further investigation of the reference beam.