Daniel, Keita, Sheila, Thomas, Hang
1,
Because the 118.3MHz and 9.1MHz&45.5MHz SBs' phase are not sync'ed perfectly, we need to use a double demodulation scheme to sense the AS72 signal, see T1700324. Previously, we offset the first demodulation frequency by 205Hz (demod at 72.8M + 205 Hz for the first demod). However, with this config we could not engage the PLL loop to stabalize the secondary demod freq.
Daniel pointed out the if we domod at 72.8M + 205, we would down convert both <45.5M, 118.3M> and <2x45.5M, 2x9.1M> to 205Hz, and the two signals could beat against each other since 118.3M SB's phase drifted relative to 9.1M. Therefore the PLL loop could not be locked.
To solve it, we instead demod'ed at exactly 72.8M for the first demod loop, but offset the 118.3M by 205Hz. After this modification we could have a stable PLL loop to derive the 72.8M Hz signal.
2,
We preformed some preliminary AS72 vs AS36 comparison. Under DRMI config (10 W input),the SRM pitch respose in cnt/rtHz were (also the first plot)
| A_I | A_Q | B_I | B_Q | |
| AS36 | 253 | 207 | 944 | 360 |
| AS72 | 2.5 | 0.19 | 1.4 | 1.2 |
In order to see the signal, we had to have a very narrow bw of 0.01Hz. For reference, the noise level for AS72 is about ~0.1 cnt/rtHz.
Then in the NLN w/ full IFO, we did the measurement again (second plot)
| A_I | A_Q | B_I | B_Q | |
| AS36 | 318 | 77 | 786 | 223 |
| AS72 | 1.6 | 0.39 | 1.1 | 0.7 |
The noise level for AS72 is still about 0.1 cnt/rtHz.
In addition we report the sum signal under the full IFO
| A_I | A_Q | B_I | B_Q | |
| AS36 | 25 000 | -4 300 | 33 000 | -14 000 |
| AS72 | 169 | -3.3 | 211 | -1.8 |
We also tried to see the BS repose but could not see anything even if the line we drove dominated the BS rms motion...
Note that the 72 MHz signals is tiny because the modulation index for the 118 MHz is puny. See alog 37042. Since we are adding the new modulation to the EOM for the IMC, it is completely off-resonance. Obviously, we would need a dedicated EOM, if this installation should become permanent.