Rick, Evan
Summary
This evening we went into the PSL and examined OLTF of the FSS.
Since we want to increase the FSS gain, but cannot turn the common gain slider up any further, we looked for other ways to squeeze more gain out of the loop.
Rick had the idea to try to increase the error signal slope by adjusting the demod phase using the delay line. Indeed, we were able to increase the loop gain uniformly by 3 dB. The phase remained more or less unchanged below 700 kHz.
We now have a UGF of about 350 kHz with 50° of phase. The gain margin is about 3 dB.
Details
Delay line switch positions (up/down) are as follows:
| Delay (ns) | Old | New |
| 1/16 | D | D |
| 1/8 | D | U |
| 1/4 | D | U |
| 1/2 | D | U |
| 1 | D | U |
| 2 | D | U |
| 4 | D | U |
| 8 | U | D |
| 16 | U | U |
| 1/16 | D | D |
| 1/8 | D | D |
| 1/4 | D | D |
| 1/2 | D | D |
| 1 | U | D |
| 2 | U | D |
| 4 | U | D |
| 8 | D | D |
| 16 | D | D |
| Total | 32.9 ns | 40.0 ns |
So the phase change at 21.5 MHz is 56°. That seems like quite a lot, so perhaps we should take a closer look at the error signal with the FSS unlocked to make sure it's reasonable.
Also, on the manual FSS MEDM screen, we found that the TEST2 enable/disable button didn't really work; we seemed to get a sensible transfer function no matter what.
Peter K, Jeff B, Evan H
We did some FSS diagnostics today in and around the PSL:
- We measured the LO going into the FSS tabletop box; it is 20.0 dBm.
- We also measured the refcav powers: with 28.3 mW incident, we have 11.8 mW transmitted immediately after the refcav. The transmission PD voltage was 1.15 V. But I think this must have been during the time at which the ISS AOM was diffracting an anomalously high amount of power, since the best transmission PD voltage we've observed is more like 1.5 V.
- By driving the NPRO PZT with a triangle wave, we took sweeps of the error signal, the cavity transmission, and the cavity reflection.
- We were able to entirely remove the delay line phase shifter. We replaced it with about 20 cm of extra RG58.
- For a stable lock with a >300 kHz UGF, we had to adjust the common (PZT+EOM) and fast (PZT) gains to 26 dB and 21 dB, respectively (they were 30 dB and 15 dB) before. As we were leaving we noticed the loop started to oscillate, so we backed off the common gain to 24 dB instead.
Originally there was 14.5 dBm of 21.5 MHz drive going into the delay line, and 8.1 dBm coming out (and thus going to the EOM). So we have won back almost 6 dB of drive to the EOM. That's roughly consisent with the extra headroom we now have on the common gain slider.
However, I do not understand why we had to adjust the fast gain after removing the delay line. With 26 dB common and 15 dB fast, we saw a broad peak in the transfer function around 50 kHz or so, and we increased the fast gain to 21 dB to suppress it. So perhaps removing the delay line shifted the crossover frequency.
A new OLTF is attached (at 26 dB of common gain), along with the error signal and cavity sweeps that we took (which are now outdated).
Using data (scope_7.csv) in the above attachment, we find that the PSL NPRO PZT actuation coefficient is 1.3 MHz/V [ = 21.5 MHz / (7.11 V + 9.09 V)].