Paul, Sheila, Mackenzie, Alexa
We measured the RFAM and the modulation depth for 9 MHz and 45 MHz sidebands. This measurement was previously done by Volker (see alog
3693,
3695). We followed a similar procedure as explained in our setup alog
15625. During this measurement, the IFR modulation frequency was set to 9.100229 MHz as read from the timing comparator read back.
45 MHz RF Input:
12.6dBm out of RF balun on the ISC field racks
3.3dBm into the RF amplifier with -8dB attenuator in the path (with RF power meter).
11.32dBm without the -8dB attenuator (with RF power meter).
21.5 dBm with the -8dB attenuator and with the RF amplifier (with Spectrum analyzer)
3.2dBm into the RF amplifier with -8dB attenuator in the path (with spectrum analyzer).
This is consistent with Filiberto's alog
14251 when the RF amplifier was installed.
9 MHz RF Input:
17.27dBm measured at the RF balun on the ISC field racks. Based on what we saw for the 45 MHz there might be a 1dBm loss in the cable to the PSL enclosure.
Modulation Depth:
With the OSA aligned we found,
|
|
RF Input Power |
Peak Height measured with OSA relative to noise floor |
Modulation Depth |
|
45 MHz |
21.5dBm |
79.2mV |
0.284 |
|
9 MHz |
17.27dBm |
41.2mV * |
0.205 |
* For the amp of the 9 MHz we had to estimate a bit because the sidebands were laying on the carrier. We estimate that the carrier was about 5.2mV at the peak of the 9 MHz sideband with respect to the noise floor. The 9 MHz sideband peak is then 46.4mV with respect to the noise floor. So the approx 9 MHz sideband is 41.2mV.
Meanwhile we measured, the carrier Amp = 3.92 V with respect to midpoint of noise floor. Similary to alog
8867 we measured the modulation depth using: Gamma = 2*sqrt(V_sideband/V_carrier). The 24 MHz sideband is barely visible. These modulation depths are fairly consistent with what Dan measured using the OMC scan (see alog
14801)
RFAM Measurement:
We used an 1811 NewFocus PD (125 MHz Bw) which has a 50 Ohm output impedance for both AC and DC. Our PD is the AC-coupled version the transimpedance gain is 40V/mA (AC) and 1V/mA (DC). So the AC gain is amplified by 40.
DC was connected to a TPS 2024; we measured 28.8mV with high impedance. AC was connected to Agilent 43958 Spectrum Analyzer with a 50 Ohm transimpedance. (Note: this gives another factor of 2 for DC).
Taken from Volker's old alog: the RFAM is then measured via, RFAM = (V_AC/40) / (V_DC/2) where V_AC denotes the Vrms as measured with the spectrum analyzer and V_DC the voltage read from the oscilloscope. The PM value below is the modulation index as measured previously.
Spectrum Analyzer peak height for
45.5 MHz = -70.6dBm = 65.99 uVrms, RES BW is 10 Hz, Range is 45.9875 Mhz to 45.50375 MHz (5kHz span)
9.1 MHz = -48.36dBm = 854.05 uVrms, RES BW is 10 Hz, Range is 9.09975 Mhz to 9.1025 MHz (5kHz span)
24 MHz = -98.8dBm = 2.57 uVrms, RES BW is 3 Hz, 24.0784 MHz is the peak which is about center on 5kHz span
|
|
RFAM |
RFAM/PM |
|
45 MHz |
1.146e-4 |
4.0199e-4 |
|
9 MHz |
1.486e-3 |
7.233e-3 |
The RFAM has degrared over time since Volker measured it in 2012 as expected. Kiwamu tells me that these values are still pretty good.
Tomorrow we also want to measure the RFAM after the MC. We are all set up to do this on IOT2R, which already has a PD1811 aligned (this just involves blocking the aux beam). Another thing we might consider doing is installing a long cable to the 1811 on the PSL table and send it to a spectrum analyzer outside the enclosure so that we can monitor the RFAM over time. Also, we need an extention for the 1811 PD power cable; at the moment it can't reach the power supply on the floor unless the box is elevated. We did not want to leave the power supply elevated, so at the moment the 1811 PD is not powered.
The plot attached shows the Mich Out signal:
- in the first box, HEPI Z sensor correction is ON, Stage 1 X abd Y are in low blend, the IPS are off centered. The low frequency amplication is huge.
- in the second box, HEPI Z sensor correction is ON, Stage 1 X abd Y are in high blend, the IPS are still off centered. The low frequency amplication is gone.
- in the third box, HEPI Z sensor correction is ON, Stage 1 X abd Y are in low blend, the IPS are re-centered. Our current understanding/aseumption is that the Z to RX and RY coupling on HEPI has been well reduced.
The latest configuration is likely the best compromize:
- good micro-seism atenuation thanks to the low blend on X and Y
- low vertical to pitch coupling thanks to the Z feedback
- little RZ amplification at the micro-seism thanks to the Z sensor correction to HEPI (that offloads Stage 1 Z drive at the micro-seism)
- amplification acceptable at very low frequency, now that the IPS have been re-centered. We'll try to further improve it.
An ASD plot of the MICH_OUT channel is attached under different configurations. The first (RED) is with no sensor correction on BS, ITMX and ITMY. The second (BLUE) is with X, Y sensor correction signals to all three BSCs. The third (GREEN) is with Z sensor correction to HEPI for the BS chamber, showing the large low frequency amplification. The fourth (BROWN) shows the MICH_OUT with the IPS recentered and same configuration as the third. Tilt-decoupling on HEPI ought to reduce the amplification further.
Z sensor correction has been turned OFF on BS and ITMY. X and Y sensor corrections seem to be working fine and can be left ON.
Could be that I'm missing something but it sounds to me like at least one of the IPS is not working properly (ie broken). They are supposed to be linear to within 0.1% over the full range (+/- 0.05 inches)