We measured the frequency response(microphone out/signal to speaker) to see how well we can shield the outside acoustic.  The test panel did help reducing the acoustic coupling, but there is still room for improvement.

     Den lend me a blue bird microphone from 40m.  So we setup a measurement to compare two panels we have. The first one was what we made yesterday (plastic with damping pad), the second one was the aluminum panel(1/8" thick) with soft foam on the inside and foam strip on the edge where the panel met the frame.

     ==setup==

     We measured the frequency response between the microphone signal and the speaker driving signal. The source was white noise (band limit) 100Hz - 6.5kHz, 1.4V. The output has a T so that one was sent to the speaker, another one was for chA.  The SR785 chB input for microphone signal was floated  since the mic gave differential output. This should prevent the pre-amp output to see "ground" at the output and break the opamp.The measurement was average over 5000 samples.

      We measured with the speaker on and off (but the white noise ref to chA was still connected) to check we have a good SNR for every setup. Three setups were:

• 1) wihout panel (see fig1),
• 2) with plastic panel + damping pad (see fig2,left),
• 3) with aluminum panel + soft foam (fig3, right).

.

fig1: setup, with the panel remove.

fig2: two panels for testing. Left, a plastic piece with damping pad attached on (from yesterday). Right, an aluminum panel with soft foam

fig3: panel under test.

fig4: result.

  ==conclusion + plan==

    From the plot, it is not very clear if the aluminum panel (panel2) is better than the plastic one (panel1). It might be that noise coming from other panels(which we have not changed) is the dominating signal. We will put the mic in a smaller container surrounded by acoustic damping with an opening for the material/structure to be tested. Then we can test a sample easily without removing/installing the panel all the time.

     For now, we are planing to use another kind of foam to put inside the box. We check by ears and found that it is better than the current foam we use with the aluminum panel.

We are still working on the acoustic shielding panels. The work should be done by tomorrow.

Fig1: Left, one panel with damping foam + black pad on top( to prevent scattered light). Right,a panel with two layers of good damping material with a damping pad under it. This type can damp acoustic noise pretty well.

We prepared all sides of the acoustic box. However, we don't have enough damping materials, so all the panels are not similar, but they all have some soft foam to provide acoustic damping. All the holes for cables/ beam are marked and will be drilled tomorrow. 

The enclosure box for input optics are done. We still need to order more of the nuts for one panel, but the box should provide certain acoustic shielding for now.

We will measure the beat signal once the temperature stable.

    The box has four 1-inch diameter holes, 2 for periscope, one for input beam, another one is for the beam to RCAV curve mirror which we cannot fit in the box.

    We had to rearrange the cable for ACAV AOM to have fewer cables going in and out the box. The cable for driving the AOM was remade so that it did not block the panel.

I check the performance of the enclosure box for input optics. It neither improves the beat signal that much.

   ==Is the acoustic box good?  No==

   To check how good the acoustic shield can be, I measured the beat signal and feedback signal to ACAV AOM when the lid were on and off. There were no much improvement in both signals, see fig1 below.

fig1, beat signal and ACAV feedback, converted to frequency noise. Beat signals between the lid close and open (red, purple) are very similar. Feedback signal to AOM are also the same (blue, cyan). I plot the 4 traces together to see if there are any coincided peak, so I can know where it happens (beat path or input optics). Note the peak at 280 Hz in Cyan trace is not real, it pops up after ~50Avg. I could not find its origin yet.

    ==Is it really acoustic coupling?  Yeah, kind of==

    The results were so similar between the lid open and close, so I wondered if those were really acoustic. To test this, I turned off the two computer (PC and fb2) and remeasured the beat. Those computers' fans are quite loud when they are on. For fb2, the fans still work even when it is shut down, but definitely much quieter. The beat signal was improved a bit, see figure 2. The results were real, I repeated them twice. Note that the room are still not totally quiet with the two computers off, sounds from sun machine and electronic rack are still there, and they are as loud as the two computer and closer to the beat setup as well. 

fig2: beat signals when the computer are on (blue) and off (red), several peaks are obviously reduced when the computers are off.

  ==discussion and plan==

    Since the computer are sitting on the floor, it is not certain if the peaks due to the computers are from acoustic transferred through air or vibration transferred through the ground. But the peaks in question are at high frequency (almost 1kHz), and we have 3 stage seismic isolation on (except floating table). It is very likely that these peaks are caused by acoustic. To make sure that they are really acoustic, I'll float the table and repeat the measurement again.

We started installing the new acoustic enclosure box for the beat path. It covers the whole beat setup and the part for power detection. The walls are installed. The lid will be made later.

We started installing the new acoustic enclosure box for the beat path. It covers the whole beat setup and the part for power detection. The walls are installed. The lid will be made later.

The mirror mount in front of new focus 1811 PD has long knobs which extend into the wall, so I replace it with another mount that has no knobs, and the panel can fit without obstruction.

The current box has only three holes, 2 for input beams, one for output beam at the beam splitter. Windows might be installed in these holes for better acoustic enclosure.

The beat signal is measured with an adhoc lid covering the enclosure on the beat part. There are no significant improvement in the acoustic region of the beat signal yet.

     The closing lid is the only missing piece for our acoustic box, so I just use smaller pieces of acrylic planes I can find to cover the top, and measure the beat signal. There is not much improvement in the acoustic region, see fig2 below for a closer look.

     There might  be dust on the optics somewhere, because the signal at low frequency is not as stable as it used to be. I could see scattered light bump coming up when I averaged for too long (this did not happen last time). I'll check all optics and make sure that they are clean.

     NOTE: The temperature servo is back on. We had to disconnect it in order to install the acoustic box. I set C3:PSL-RCAV_TEMPAVG to 32.5 ( the previous value was 35.03). I changed it because I noticed that as the temperature reached up to the set point, the beat frequency went up as well.  I'll check if this set point will reduce beat frequency down to ~ 160 MHz or not.

- personal notes -

New Focus PD:

power from ACAV: 0.958mW
power from RCAV: 0.967mW

DC-OUT: 1.94V
AC-OUT: 1.27Vpp @160MHz in 50R

changed the ACAV temp to 37.3 degrees. Both cavities are locked now.

I'll monitor the VCO input signal to center the range by adjusting the temp a little bit more.

i lost lock of both cavities for some unknown reason. Also the crate seemed to stop working as some channels are dead now...

relocked both cavities. The second crate containing the output card to control the ACAV power supply stopped working and i can't login from here. It's blocking the telnet request.

So Tara, plz reboot the crate in the other "rack", above the SR785. Simply push the reset button... Thanks

rebooted the crate.

 A tentative layout for almost symmetric layout for beat measurement. The double 1/4 objects  in the beam paths should be 1/4 and 1/2 waveplates.

Lenses might be used to focus the beam somewhere.

the calibration value is 71.3 kHz/Volt

for the following setup

center freq = 159.294 MHz

FM DVN = 100 kHz

This calibration will be used to convert the V/rt Hz unit from spectrum analyzer to Hz / rt Hz unit for beat measurement.

Plot from yesterday measurement. The new result is in red.

The data is measured from the feedback signal in PLL loop. Its UGF is about 53 KHz.

The data is calibrated to Hz/ rt Hz unit by a factor of 71.3 kHz/ volt.

I measured the beat note signal from two different setup (f modulation) and plot the result below

We want to see where our beat signal is, and compare it to the noise budget, and improve the sensitivity.

I'm using the same noise budget for now, because

my noise model from RIN has not been finished yet. I'll try to finish it soon.

For beat measurement, I measured the feedback signal of the PLL loop,

since the UGF of the loop, which is 53 kHz, covers the region of our interest.

I used two frequency span on marconi, 20 kHz and 100 kHz.

I checked the calibration for each frequency spans which are 14.31 kHz/V and 70.8 kHz/V respectively.

The results are the same at low f to ~ 200 Hz, at higher f, 20 kHz span(brown line) has better sensitivity.

*b100 and b20 in the data.mat files are the result of beat at fmod 100kHz and 20 kHz in the format of

frequency, V/rtHz , f / rtHz

--------------

Setting

PMC

power input: 16mW

V_RF: 5.7 V

Phase ADj: 2.23 V

Gain: 13 dB

RCAV

power input: 2.86 mW

V_RF: 10

Phase ADJ: 4.2 V

Common gain: 18.9 dB

Fast Gain: 20 dB

ACAV

power input: 2.0 mW

 I realigned the beam to PMC, ACAV, RCAV, optimized gain, and find a significant coherence between ACAV_RCTRASPD and RCAV_TRANSPD.

I haven't re-aligned the beam to each cavities for awhile, and the alignment was quite bad.

PMC_RCTRANSPD: 9.4 - > 10.9 V

RCAV_RCTRANSPD: 1.7 ->2.07 V

ACAV_RCTRANSPD: 0.8 -> 1.3 V

So I need to optimize the gain setup again, see detail below. 

I measured beat signal and coherence before and after realigning. 

For coherence, I see nothing significant except <beat | ACAV>, see fig1, so I did not save the rest of the measurement.

After I realigned the beam, there is a big coherence between ACAV andRCAV see fig 2.

the coherence between PMC and RCAV follow the same trait as that of PMC and ACAV, but slightly less, so I show only PMC and ACAV.

However,  the beat note before and after realigning the beam are still the same, see fig3.

I'll add RIN from  RCAV/ACAV/PMC .

-----------------

gain setup

--------------

PMC: Gain 16

         RF_ADJ 5.7 V

FSS: Common gain 21.5

         FAST gain 21.2

        RF_ADJ  10

These values allow maximum Transmission power and stable lock (I checked this by unlock and lock the cavity and see if the signal is stable after loss lock)

I measured the TF between VCO feedback and PLL feedback. The result agrees with the TF of the VCO.

  The frequency noise measured from PLL feedback and VCO feedback do not have the same shape, even though

the coherence is ~1. There might be some devices that do not have flat frequency response and change the shape

of the frequency noise. 

    So I measure the TF between the feedback signal to VCO (ref, chA) and the feedback signal to Marconi (resp, chB).

The excitation is sent to In test 2 ch on FSS servo input which modulates the frequency of the laser to RCAV.

Then I measure the TF of the LIGO's VCO box.

The shape of the transfer functions agree well except a resonance peak around 100 Hz and a pole at 7 kHz.

 This TF can be used to calibrated the feedback signal to the VCO to real frequency noise between two cavities.

Then calibrate the feedback signals correctly. The plot in this entry is totally unacceptable to show to anyone.

I measured the beat signal from 3 different power input levels, the signal goes down with the power mostly at higher frequency.

When HEPA filters above the table are off, the noise also goes down, this might suggest that we still have scattering light problem.

 We are hunting for the noise source for our experiment. We want to see if our setup is limited by intensity noise or not

so, we try to change the power level and see the beat signal.

 I used 3 levels of power (measured after PMC) to be 10 mW (original value), 20 mW , 2mW, (the settings are listed below.)

The reason that the level at 10 mW decreases from previous beat msmt is that the gain for FSS and AOM loops are optimized

PDH box for AOM loop is also modified (R12, 1k -> 10k), the power into both cavities are adjusted to be about the same.

 The noise level at 10 and 20 mW are almost the same except a hill at 60 kHz on 20mW, and

  the noise from 2mW setup is lower than other settings at high frequency, starting ~ 20kHz.

 At 1 mW, I can't lock the PMC (the transmitted beam on the PD is so faint), so I increase to 2 mW to make locking easier.

At 2mW power, the range for feedback signal for PLL loop becomes smaller.

If it exceeds approximately +/- 50 mV [3.5kHz], the lock loses, but I still keep the frequency range at 100kHz.

I tried to increase the gain on SR560 which is a servo for PLL loop, but

the beat signal was worse at high frequency, starting at 2kHz.  Fortunately, the temperature drift at night (~10mV/min)is much smaller than

the drift during the day(~5mV/sec), so I can wait long enough to measure the beat at low frequency, with the gain level of 1, as usual.

At 2mW setting, I have the HEPA filter on and off in comparison. The noise level decreases a bit when the fan off, this might suggest that

scattering light is still a problem.

Even though the noise from beat measurement change with the power, the amplitude does not go with the power change.

i.e, the noise in the beat note does not go up with the same factor of the power change.

So the setup is not limited by intensity noise.  

setting for the measurement ["-" means the same as previous value]

                                      10 mW              20 mW             2mW

PMC: common gain         16 dB                 -                     30dB                       

PMC:RF                            5.7V                 -                       -

FSS:FAST gain               18.5 dB              -                      -

FSS:common                  22 dB               18 dB             20dB   

FSS:RF                           10 V                  -                      -

PDH for AOM:GAIN        4.45 (knob)       3.2                 7.0

Pin RCAV/ACAV            2.1/2.1 mW        4.8/3.9 mW    0.4/0.4 mW

I added a Faraday isolator after PMC and 35.5 MHz broadband EOM, now the noise becomes less susceptible to the

input power to the cavity.

The isolator is installed between the 35.5 MHz EOM and a lens just before the beam splitter that splits the beam into

ACAV and RCAV path. I measured the beat signal at 10mW (measured in front of the BS, as before).

The beams going into both cavities have the same power level ~0.4 V, see blue plot.

  Then I realized that I had not re-aligned the beams into both cavities. The isolator significantly

alters the beam paths, I aligned the input beam, then measured the beat noise again, at 4mW and 2 mW, see green and red plots.

*I'm very surprised to see no significant difference between before and after alignment.

All new three results are very comparable to 2 mW measurement before the isolator was installed, see pink plot.

However, if we compare 10mW from today and yesterday measurements, there is a significant broad band effect at high frequency.

By adding the isolator, we reduce the back reflected power to the PMC, and the laser. The power dependent noise we saw before

might come from this back reflected beam to the laser.  Tomorrow, I'll try adding an EAOM.

The current PD for the 160 MHz beat signal is 120MHz. We use a 2 GHz PD to compare the results between two PDS,

and there is no difference. 120 MHz PD seems to be working fine for us. However, the beat signals at freq above ~5 kHz we have seen so far are not real signal from cavities' noise.

We have checked several parts on the PSL setup to search for excess noise, we have not checked the PD for beat signal, so

we try this measurement.

We use a 2 GHz PD to see the beat signal from another port of the BS.  The attached figure has

two traces of the beat signal, the one on top from 120MHz PD and the one on the background from 2GHz PD . 

The results are similar up to ~10 kHz.

The difference at high f comes from different bandwidth and gain setup for PLL loop, because

it changes with gain setup on SR560. So, the beat noise results shown so far are valid only up to 10kHz.

At higher f, it just the PLL loop.

some additional information:

the beat noise was measured as the feedback signal to the VCO of the PLL, so the calibration factor does not change with changing optical power, alignment, mixers etc.
It's a convenient way to change individual things in the setup and be able do directly compare the measurements without lots of calibration.
We checked the following things:

• power level on beat PD
• different PD with much more bandwidth
• different mixers

The feedback signal is only valid until about 10k. Tara will measure the UGF again on Monday but the signal above 10k exactly scales with power on PD or gain settings while below it stays constant which is exactly what we expect when having enough loop gain in the PLL loop.

Looking more into detail in the spectrum we looked from some tens of Hz to 10k and then tried to excite the spectrum. We could clearly identify the individual resonance peaks from e.g. the beam splitter mount of the beat setup. We know that the way it is set up is very bad but nevertheless we expect mainly lots of resonance peaks but not this hump shaped spectrum.
Now the interesting part is that if we excite the surface of the optical table by touching it softly with e.g. a balldriver we can excite those resonances. What i found very interesting is if you excite the bottom of the table we excite the hump very broadband. You only have to touch it barely like tipping with your fingertips and the whole hump increases. So my guess is that we have a scatter source somewhere which would also explain the shape (at least from my experience).

So the plan for Monday is to have a closer look on that, then checking all the wholes in the foam insulation and probably make them little bigger (right now they are 1/2 inch). Other things are reducing the power from the laser (something we wanna do anyways in the long term), replacing the PMC which scatters a lot of light (you actually don't need an IR viewer to see that, detector card is enough) by a good one and getting a symmetric layout with two periscopes (simple ones like we have now on the other side of the cavity, we casn replace them later by real good ones) for the beat at a lower beam height  to reduce all resonances to better see where/what the underlying noise floor looks like.

Today we 1)replaced out PMC with DMASS' PMC and get better transmission efficiency, 2) added EAOM to modulate the laser intensity

3) measured TF to see how RIN couples into laser frequency shift, it is small and not the current limiting source for now.

Our PMC is not very clean and get the transmission only ~60-70%. The PMC we got from DMASS is much better, now the transmission is up to ~80-90%, we have not

align it carefully yet.  After replacing the PMC, the beat noise did not change.

The Faraday isolator was re-installed and optimized, I used the wrong side before and dumped the beam inside the isolator, instead of outside.

A PBS and a 1/2 wave plate were installed after the PMC to adjust the power without changing the power input of the PMC, the beat noise gets higher

a bit.  I mounted the beam splitter for beat signal on a more rigid post, and aligned the two beams, then measure the beat noise.

See plot.

 Intensity modulation set was installed. The set consists of a 1/2 WP, an EAOM, a PBS. Then we amplitude modulated by sending a sine wave to

the EAOM, and measured the TF between (The excitation is sent to the EAOM)

1) intensity modulation and ACAV_trans_PD

2)intenisity and RCAV_trans_PD

3) intensity and beat noise

4) ACAV_trans_PD and VCO feed back to AOM

extra: we measure the TF between PMC_trans_PD and ACAV/RCAV_trans_PD, since the line width of PMC is much larger than that of RCAV/ACAV,

we can measure the TF of their poles.

 [The plot will be posted soon]

I measured the open loop gain TF of the phase lock loop for beat measurement, at the current setup, the UGF is 40 kHz.

setup 

Gain on SR560 is x5 , power input for each cavity is 1mW.

Today we

1) checked if LO phase noise dominates at high f. It turned out that it is dominate at frequency above 1kHz.

2) found out that  RFPD for current RCAV has no 35.5 resonsnce,The calibration for input range at 10kHz is 7.55 kHz/ V

3) tried to float the table, but the pressure is not enough. The gauge reads 2.5 bars.

As we are hunting down various noise sources in our setup, we have to check every components of the setup.

1) As LO phase noise for the current setup (carrier:160MHz, input range:100kHz )has never been measured,

   we tried to see if it actually the limiting source. By  reducing input range of the Marconi, we might see the change of the beat spectrum.

     The regular setting for input range is 100kHz, which was verified to produce real beat noise upto 10kHz.

we changed to 10kHz input range, and measured the beat noise at gain x5, x10, and x20. 

 Since we checked that at gain 10 and gain 20, the noise spectrum, upto 3kHz, does not change with the gain setup.

We can be sure that, at gain x20, the signal we measure up to 3 kHz represents the real beat noise, see plot 1,

the red circle shows where the noise spectrum at gain x20, still similar to that of x10..

The calibration for 10kHz input range is 7.55 kHz/V.  We see that at high f from 500Hz and above, LO phase noise is dominating.

[Wed Dec 08 13:39:24 2010 ]

I measured the beat noise spectrum from 1 - 3 kHz and plotted it on top of the usual noise budget. see fig 3.

2) We measured the TF of RFPD for PMC, RCAV, and ACAV. 

 By modulating the intensity of the laser with the EAOM, we can measure the RF response at the RFPD for each cavity.

The source out is split into two paths, one to EAOM, another one is used for reference.

PMC-RFPD has a resonance peak around 18.2 MHz.

ACAV_RFPD has a resonance around 36.3 MHz,

RCAV-RFPD does not have a peak at 35.5 MHz as it should have.

We will try to switch RCAV RFPD with that of ACAV to get some extra gain. This should give FSS loop some extra gains.

I'll bring it to 40m and use Jenne's laser to test it again and measure a proper TF. Besides, EAOM might not work well at that high frequency.

2.1) Just for fun, we measure the beat noise spectrum without the PMC installed (we removed it when measuring the TF of ACAV/RCAV RFPD), there is no significant change between PMC and no PMC. see plot 2

3) we tried to float the table. Alas, the 2.5 bar pressure from the valve is not enough. Plus one of the table leg (the one that is close to the fume hood) seems to be broken.

The air suddenly leaks from that leg after we turned on the pressure for 30 mins.

I got Marconi noise data from Frank, and plotted it on the noise spectrum to show that we are limited by LO noise at high f.

Note that I use old RIN approximation as Frank suggested that  RIN noise and beat noise have a same feature.

I plot marconi noise with 100kHz input range and compare it with the bea noise with 100kHz input range, on fig2.

We are cleaning optics to reduce possible scattering light sources. Optics behind PMC are quite dusty. Improvement in the beat measurement was observed during the process, the

actual data is yet to be measured.

 Dirty optics might cause the beam to scatter, and the scattered light could end up in the RFPD for PDH locking. Its random phase will

mess up the error signal causing extra noise in our measurement. So we try to clean the optics and see if we can improve our measurement.

The circled optics in the picture are cleaned already. Aligning and optimizing is yet to be done.

Red plot was taken today after cleaning a few lens, blue plot was taken after cleaning all the lens shown in fig1.

Although blue curve has higher noise upto 200 Hz, this might come from beam misalignment after removing lens for cleaning.

I'll try to re configure gain setup to see improvement.

I'm continuing with cleaning optics. The optics in cyan circles in the attached picture are cleaned today.

One of the RFPD (currently, for RCAV), has no protecting glass, and it is very dirty. I used an air can to blow some dust away.

 Most of the improvement in the noise spectrum comes from the cleaning of the RFPD, and it is still dirty.

I notice that there is a peak at 60 Hz coming up in the measurement, there was none before. I have to look into this

and make sure that no electronic noise is coming up.

The picture is posted on the board outside the lab as well.

This morning I was able to measure the beat noise down to 12.5 mHz. So I plot it together with the noise budget here.

We also added channel for frequency counter,C3:PSL-FSS_FREQCOUNT which will allows longer data acquisition time for lower frequency.

Then we will be able to see the temperature effect at lower frequency(~ 10 mHz.)

Fig1: beat signal at 12.5 mHz to 10 Hz

We are also working on PID thermal control for refcav.

Perl script for PID thermal control won't work on Solaris because it doesnt have ezcaread/write command, we will get that from op40m machine at the 40m.

(We can't run Perl script on Linux because it complains that it fails to read / write data from C3:PSL-FSS_HEATER.)

found a nice calculator here:

http://vk1od.net/calc/tl/tllc.php

which has a huge amount of different cable types in it's database. Checked/compared the calculated values for some examples given in several datasheets found on the web and they are close within each other.
Here an example for our current case for the 500ft spool of RG58C/U we have in the lab. Loss is only 32dB which looks pretty good to me for that cheap cable.

We measured beat noise from the frequency counter(FC) instead of the feedback from PLL. The result is plotted below

We switch to use the FC because we hope it will allow us to measure the signal down to lower frequency (~10 mHz) which is not quite possible

for PLL because of its small input frequency range for the acceptable phase noise level.

The FC can measure the relative frequency from the chosen center frequency and give a voltage output (from 0 to 8 V.)

For example if we set the center frequency to be 160 MHz, with 1MHz/V gain, signals at 156 MHz and 164 MHz will correspond to 0 V and 8 V respectively.

Since the LIGO VCO range is ~ 10 MHz, we tried measuring the signal with 500 kHz/V setup which is equal to 4 MHz range. However the noise is too high,

so we have to choose a new gain setup, and  20kHz/V setup is still acceptable for our signal compared to the signal from PLL.

 (It turns out that data from 50kHz/V is too noisy)

Attachment 1:

Blue and Green show the beat noise measurements from PLL and FC. They agree well. The gain for FC is set to 2kHz/V.

When the temperature became more stable, the data could be acquired to down to ~ 3mHz (red).

We used AC coupling for FFT measurement, so the TF for AC coupling is measured. There is a 160 mHZ high pass, and the data is corrected accordingly.

Attachment2:

Time series for ACAV's temperature, RCAV's temperature, VCO mon, and Frequency count Vout during the FFT measurement.

Attachment3: show TF measurement for AC highpass and fit with 160mHz high pass.

The plot is flipped because chA is AC couple and ChB is DC couple during the measurement, and

The output is B/A.

I used PLL feedback to measure beat noise from dc to 3 Hz and dc to 1.5 Hz to compare with the data taken yesterday by FC.

There is no peak around 3 Hz (probably a peak from cavity suspension)in FC data, I'm not quite sure if we what we see is real or not.

2 stretches of data taken at the following UTC times:

11/2/2 23:18:19  duration: ~9min

11/2/2 23:22:50  duration: ~9min

units for data: volts  SCALE: 10kHz/V

Here a screenshot for the first stretch from dataviewer:

 here the data:

 we didn't take longer timeseries with more range as the resolution is so bad that it couldn't be used to extract a spectrum down to mHz .

RAW data can be accessed via 131.215.114.84:8088 or SSH-login and doing the usual.
Channel name is C3:PSL-FSS_FREQCOUNTER

changed the frequency noise readout to the cable-delay version to see how it works.
As the loss of the cable is very large and the signal from the photodetector not very strong i had to add some amplifiers for signal conditioning (see figure below).
Data is acquired with channel C3:PSL-FSS_FREQCOUNT in replacement for the frequency counter. Started taking data around 11/2/4 4:16:30 UTC.
Will do final calibration tomorrow. First test gave signal from mixer changes from peak-to-peak for about 600kHz (+/-10%) in frequency change.

 This setup has too much gain (i.e. not enough range). Please reduce the arm length asymmetry by a factor of 10 so that we can monitor over 24 hours.

Also the temperature channels ought to be calibrated (via the EPICS .db) so that the readout is in degC instead of ARB.

exchanged the 500ft spool by a shorter cable to get more range (but less resolution).
Due to the lower losses of the cable i also removed the 2W amplifier.
Right now both cavities can't be locked at the same time, they are slightly out of range.

RCAV is resonant at 0.1958 for the slow actuator, ACAV is resonant at 0.1930

max range for VCO is reached at 0.1948

i've changed the RCAV settemp a little bit and will keep an eye on that and we will hopefully be back online tonight

stopped taking data for data calibration at 11/02/07 23:35:00 UTC

used two frequency ranges to calibrate the mixer signal

1. calibration factor is 6.85MHz_pkpk measured between 150MHz and 160MHz
2. calibration factor is 6.42MHz_pkpk measured between 147MHz and 154MHz

those numbers should be sufficient for comparison with the VCO feedback signal.

i analyzed  the data taken with the short cable and compared it to the signal from the tuning input to the VCO.
I used the 9.order polynomial fit from yesterday to convert the VCOMON voltage into absolute VCO frequency.
The beat frequency is then twice that frequency.

On the other hand the cable-delay technique was sampled with channel FREQCOUNT.
I've measured the peak voltage of the amplified mixer response to be about 4.52V.
The frequency change which corresponds to peak-to-peak change in output signal (see yesterdays entry) i've used is the smallest value i measured ( 6.42MHz).

Using the sampled data, divided by 4.52V, taking sin^-1 of that and multiplying it by 6.41MHz /2 gives the measured frequency change.
As i don't get an absolute frequency from that measurement i added an offset of 151.5MHz (this information is from the VCO's absolute frequency) in order to compare the fluctuations with the VCO signal.

RESULT:  beat signal fluctuations over a long period of time can be measured using the VCO feedback signal.
                Both signals absolutely agree except for the region where the mixer signal is at it's peak and so the slope is close to zero and so the uncertainty is too large.

Here the plot:

Matlab-code:

t0 = tconvert('02/06/2011 10:10:00');
dur = 3600*24*1;

chans = {...
'C3:PSL-ACAV_VCOMON',...
'C3:PSL-FSS_FREQCOUNT',...
};

y1 = get_trend(chans{1},'minute',t0,dur);
y2 = get_trend(chans{2},'minute',t0,dur);

y1.data = y1.mean;
y2.data = y2.mean;

y1.data = 2*fittedmodel2(y1.data);
y2.data = 151.5-asin(y2.data./4.52).*3.21;

t1 = linspace(0,dur,dur*y1.rate)';
t2 = linspace(0,dur,dur*y2.rate)';

figure(1)
plot(t1,y1.data, 'r',t2,y2.data, 'b')
xlabel({'time [s]'});
ylabel({'beat frequency [MHz]'});
grid

Changed the setup to be used with two different cables at the same time, but only using one right now.

mixer signal calibration with PD signal ~4.7dBm:  5V = 157.295MHz  -5.09V=150.725MHz    df_pkpk=6.57MHz

channel names:

• C3:PSL-GEN_DAQ15 : SR560, DC-coupled signal, gain somewhere between 100 and 200, LP30KHz
• C3:PSL-GEN_DAQ16 : SR560, AC-coupled signal, gain 1000, LP30kHz

data valid from 11/02/09 6:20:00 UTC

will post schematic later...

shorter cable: cable length 62 inches

amplified (DC-coupled) signal from mixer using SR560, LP@30Hz, gain20 in channel C3:PSL-GEN_DAQ15

185.0MHz :  -2.405V
154.2MHz :   0.005V
123.2MHz :   2.263V

max range of double-passed VCO signal: 142MHz-170MHz

142MHz :  1.38V
170MHz : -1.685V

As the FET preamp need some more time to set up i added the AC-coupled signal using a SR560, gain 10k, LP30Hz into C3:PSL-GEN_DAQ16

FET pre-amplified signal will be connected to C3:PSL-GEN_DAQ14   preamp broken

data taken at 02/06/2011 10:10:00 UTC, duration = 24h

updated plots:

data files:

structure of data files like this:

y1 =
   name: 'C3:PSL-ACAV_VCOMON'
   min: [1440x1 double]
   mean: [1440x1 double]
   max: [1440x1 double]
   rate: 0.0167
   start: 981022215
   duration: 86400
   data: [1440x1 double]

raw data in min/mean/max
calibrated (mean) data in data

I plot beat noise PSD, with estimated H1 arm noise, and SR560 noise converted to frequency noise in the current beat measurement. From the approximation, SR560 noise will be smaller than H1 noise.

 The conversion for SR560 noise to frequency noise is computed by finding the slope of signal from self beat measurement.

parameters are from Frank's entry below

amplified (DC-coupled) signal from mixer using SR560, LP@30Hz, gain20 in channel C3:PSL-GEN_DAQ15

185.0MHz :  -2.405V
154.2MHz :   0.005V
123.2MHz :   2.263V

The pkpk values is then (2.263V - -2.405V)/gain20 = 0.2334 V over 185 MHz - 123.2 MHz = 61.8 MHz range.

Thus the signal is = (0.2334V)/2 sin ( pi df/ 61.8 MHz)

This gives the maximum slope = 0.2334 V* pi /2 /61.8 MHz, or  ~ 170 MHz/V.

SR560 noise is estimated to be flat, 5nV, at high f, a corner at 10Hz which gives 5 uV at 10mHz.

multiplied by 17 MHz/V to get frequency noise from SR560.

 Here's the plot from the H2YAC doc that explains the idea.

If we can get down to ~1 Hz/rHz at 1 Hz, it would be nice for measuring the suspension performance at that point.

plots are not taking the change of transmitted PD power when changing VCO frequency into account !

setup and cal data from post #485

I checked my below entry, and I found a mistake in the conversion factor. It should be ~170 MHz like Frank's result in the quote. So the noise from SR560 in my plot has to be corrected.

looks like a fu**** up the data aquisition with the short cable, but i don't know how.

When trying to calibrate the data taken i realized that something was totally wrong as i got some khz/rHz but i couldn't find the mistake.
So i thought the gain setting if the preamps must be different than i wrote down but they are not.
So i checked the calibration again, this time in 1MHz steps all the way through the system but everything was/is OK.
Got almost exactly the same mixer response vs frequency tuning and also the dc-coupled signal was what i measured before.

Here the problem:

The VCO feedback signal gives us a rough idea what frequency we have, which i checked over and over again and matches the frequency counters we have.
The problem is that the DC signal (already amplified) for pi in phase change goes from something like 2.3V to -2.4V.
The same signal from end to end of the VCO goes from -1.7V to 1.4V, see here.
I measured this several times, last week and today and this is fact whatever equipment i use (scope, multimeter, DAQ).

So now the funny part:

The DC signal and the VCO monitor signal look almost identical (shape, uncalibrated).
As i checked  the calibrated (fitted) VCO monitor signal reflects the beat signal within 100kHz or so, but 1MHz for 100% sure.
Now, taking the recorded DC-signal from the mixer from 35h of data and using the calibration (which i did several times) the beat frequency is out of range of the VCO, totally different !
The recorded signal does not match the VCO signal at all! Using the same coefficient to calibrate the spectrum recorded with the AC-coupled amplifier the noise is way to high, higher than everything we had before.
So i took the VCO monitor signal, assuming it is right and showing me the absolute beat frequency and calculated the right coefficient for the mixer signal, which is something like 5MHZ/V instead of 170MHz/V.
Done that, comparing both time series looks fine.
Taking this coefficient to calibrate the spectrum then gives, at least for a the features around a couple of Hz the right level we measured several times before.
The lower frequencies are dominated by the noise of the preamp, so no comparison possible.
So i don;t know what's wrong because i can't reproduce what we recorded. Everything looks the same as last week, checked over and over again. But the DAQ shows something different.

RESULT:

As we can't trust whatever we recorded we redo the measurement. After bringing back the 4-way splitter to 40m i'm using 2-way splitters now instead, increased the signals where possible and the gain of the preamp. re-calibrated everything and triple checked, taking another measurement over night. We will see tomorrow....

Plots, 32h data stretches.
noise plot shows spectrum of data taken with and without 30mHz high pass correction