alexan.staley@LIGO.ORG - posted 22:05, Tuesday 16 December 2014 - last comment - 15:37, Wednesday 17 December 2014(15661)
Sideband Modulation Depth and RFAM
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.
I annotated the scope screenshot from alog 15639 with carrier and sideband frequencies. Just reading off the plot we have
Taking the average and assuming a 0.5 dB reading error we have Γ = 0.219(12) for the 9 MHz and Γ = 0.277(16) for the 45 MHz sidebands, respectively.
Paul, Mackenzie, Alexa
We measured the RFAM again; this time after the MC (with the PRM misaligned). Paul and Mackenzie had already aligned the beam onto the AC coupled 1811 NewFocus PD on IOT2R (same specs as before). We blocked the auxiliary laser. Again we used a TPS 30324 to measure the DC signal at high impedance, and the Agilent 43958 Spectrum Analyzer to measure the AC signal. We measured 210mV at DC, and
The PM represents the modulation depth. We repeated Daniel's calculation but wanted to collect the numbers with the Spectrum Analyzer to reduce the error. So again, we had the aux beam phase locked to the beam on IOT2R. We measured the following:
Taking the average we find. Gamma_9MHz = 0.199 and Gamma_45MHz = 0.332. This seems a bit high given our previous two measurements. I have used RFAM = 2 * (Vsb/Vcar). There is no sqrt this time because we are measuring the amplitudes; whereas the OSA measured the power. The PD has a BW of 200 MHz, but maybe we are approacing the roll-off. We tried locking the beatnote at a lower frequency so that we would not need to worry about the PD's roll off; however, we had trouble getting a clean lock even at 20 MHz which seemed to be fine yesterday. Paul will attach the raw data.
..and here are the data. It seems like we can even pick out sidebands of sidebands with this measurement technique.