I have taken transfer functions and noise measurements of the two HAM-A coil driver boxes D1100687 #S2100027 and #S2100028. All transfer functions look as expected. I'm not sure about the noise measurements. If anyone sees flaw in my measurement method, please let me know. I'm not sure why in some channels I got 10Hz harmoni peaks in the noise. That was very strange. Also let me know if my current noise estimate is wrong.

Transfer Function Measurement details

• SR785 source out was connected to the differential amplifier input of D1900068.
• The one pair of two BNC outputs of this differential amplifier goes directly to the SR785 Input 1 A and B.
• The DB9 output of the differential amplifier goes to the Coil Input DB9 connector J3.
• Header W2 was shorted to provide ground to the incoming signal.
• Header P4 was shorted to enable all the channels manually.
• Normal operation is the Acquisition mode (Acq) while when pins of header P3 are shorted, we go into the Run mode for respective channel.
• The “To Satellite Box” DB25 port at the read side was conencted to a DB25 breakout circuit and pins 1-9, 3-11, 5-13 and 7-15 were connected to 36 Ohm resistor to simulate Coil load.
• The “Output Monitor” on the rear side is then connected to the test switch DB9 port on D1900068.
• The the pair of BNCs from the test switch is connected to SR785 Input 2 A and B.
• Measurements are taken with file D1100687_TF.yml and D1100687_TF_LF.yml.
• A measurement of just cables without the DUT is taken as well.
• Commands.txt list all the commands used.
• All data is compiled and plotted in Plotting.ipynb
• D1100117_S2100027_TF.pdf and D1100117_S2100028_TF.pdf shows all the transfer functions measured.

Spectrum Measurements

• All channels were kept in disabled mode (Not shorting P4) to ensure their inputs are grounded on the board.
• I ran two BNC cables with their centers connected to output monitors V2+ and V2- and one of their shields connected to board GND.
• in SR785, A-B differential mode always runs with grounded shields mode, so effectively the board GND got grounded to SR785 GND through internal 50 Ohm resistor. But all ground loops have been evaded.
• The two BNC cables were twisted together to minimize the area between the two center cores of the cables as that is the remaining pickoff possible in this measurement.
• Instrument noise with cables was measured first but shorting the clips of the center cores and one of the shields of the two BNC cables together.
• Measurements were taken with file D1100687_SP.yml and D1100687_SP_LF.yml.
• D1100117_S2100027_Voltage_Noise_Spectrum.pdf and D1100117_S2100028_Voltage_Noise_Spectrum.pdf shows the measured voltage noise spectrum at the output monitors when loaded with 36 Ohms.
• D1100117_S2100027_Current_Noise_Spectrum.pdf and D1100117_S2100028_Current_Noise_Spectrum.pdf shows the esitmate current noise through the coil calculated by dividing the measured voltage noise by 2436 Ohms.

I took transfer function and noise measurement of satellite amplifier box's photodiode transimpedance circuit. For the measurement, I created a makeshift connector to convert backside DB25 into DB9 with the 4 channels for PDA input. The output was taken in differential form at the front PD Output port. To feed current to the circuit, I put in 12 kOhm resistors in series at the inputs, so the V/V transfer function measured was multiplied by 12 kOhm to get the transimpedance of the circuit.

Transfer Function Measurement details

• SR785 source out was fed into PDA input pins using a makeshift BNC-DB9-DB25 converter.
• The output from PDOut DB9 port was fed to test switch in D1900068 to separate differential signal.
• This differential signal was fed back to SR785 at input 2 in A-B configuration.
• Measurements are taken with file D1002818_TF.yml and D1002818_TF_LF.yml.
• A measurement of just cables without the DUT is taken as well.
• Commands.txt list all the commands used.
• All data is compiled and plotted in Plotting.ipynb
• D1100117_S2100029_TFandNoiseSpectrum.pdf shows all the transfer functions measured.

Spectrum Measurements

• Two pair of BNC cables were twisted together and clips were added at ends.
• One of the GND was connected to board GND. Rest were left unconnected to avoid ground loops.
• Each pair of signal was connected to PDOutP/N.
• The PDA inputs were shorted together to make zero input current to the board.
• Instrument noise with cables was measured by shorting the clips of the center cores and one of the shields of the two BNC cables together.
• Measurements were taken with file D1002818_SP.yml and D1002818_SP_LF.yml.
• Input referred current noise spectrum was calculated by dividing the output voltage noise spectrum by the measured transfer function.
• D1100117_S2100029_TFandNoiseSpectrum.pdf shows all the output votlage noise spectrum and input referred current noise spectrum measured.

Edit Wed Feb 10 15:14:13 2021 :

THE NOISE MEASUREMENT WAS WRONG HERE. SEE 40m/15799.

Taking inspiration from SR785 on how it reads differential signal, I figured that acromag too always need a way to return current through RTN ports always. That must be the reason why everything goes haywire when RTN is not connected to IN-. Now for single ended signals, we can always short RTN to IN- and keep same GND but then we need to be careful in avoiding ground loops. I'm gonna post a wiring diagram in next post to show how if two signal sources connect to each other separately, a GND loop can be formed if we tie each IN- port to RTN on an acromag.
Coming to the issue of reading a differential signal, what SR785 does is that it connects 50 Ohm resistance between Earth GND and differential signal shields (which are supposed to signal GND). In a floating GND setting, SR785 connects a 1 MOhm resistor between input shield and Earth GND. This can be used to read a differential signal through a single BNC cable since the shiled can take arbitrary voltages thanks ti the 1 MOhm resistor.

We can do the same in acromag. Instead of shorting RTN to IN- ports, we can connect them through a large resistor which would let IN- float but will give a path for current to return through RTN ports. Attached here are few scenarios where I connected IN- to RTN throguh wire, 820 Ohms, 10kOhms and 1MOhms in two sub cases where RTN was left open or was shorted to Earth GND. In all cases, the signal was produced by a 9V battery outputing roughly 8.16V. It seems that 10kOhm resistor between RTN and IN- with RTN connected to Earth GND is the best scenario noise wise. I'll post more results and a wiring diagram soon.

Here I present few wiring diagrams when using Acromag to avoid noisy behavior and ground loops.

Case 1: Only single-ended sources

• Attachment 1 gives a functioning wiring diagram when all sources are single ended.
• One should always short the RTN to IN- pin if the particular GND carried by that signal has not been shorted before to RTN for some other signal.
• So care is required to mark different GNDs of different powersupply separately and follow where they inadvertently get shorted, for example when a photodiode output is connected to FSS Box.
• Acromag should serve as the node of all GNDs concerned and all these grounds must not be connected to Earth GND at power supply ends or in any of the signal sources.
• I think this is a bit complicated thing to do.

Case 2: Some single and some differential sources

• Connect all single ended sources same as above keeping care of not building any ground loops.
• The differential source can be connected to IN+ and IN- pins, but there should be a high resistance path between IN- and RTN. See Attachment 2.
• Why this is the case, I'm not sure since I could not get access to internal wiring of Acromag (no response from them). But I have empirically found this.
• This helps IN- to float with respect to RTN according to the negative signal value. I've found that 10kOhm resistance works good. See 40m/15778.
• If RTN is shorted to nearby Earth GND (assuming none of the other power supply GNDs have been shorted to Earth GND) shows a reduction in noise for differential input. So this is recommended.
• This wiring diagram carries all complexity of previous case along with the fact that RTN and anything connected to it is at Earth GND now.

Case 3: Signal agnostic wiring

• Attachment 3 gives a wiring diagram which mimics the high resistance shorting of RTN to IN- in all ports regardless of the kind of signal it is used for reading.
• In this case, instead of being the node of the star configuration for GND, acromag gets detached from any ground loops.
• All differences in various GNDs would be kept by the power supplies driving small amounts of current through the 10 kOhm resistors.
• This is a much simpler wiring diagram as it avoids shorting various signal sources or their GNDs with each other, avoiding some of the ground loops.

Edit Wed Jan 27 13:38:19 2021 :

This solution is not acceptable as well. Even if it is successfull in reading the value, connecting resistor between IN- and RTN will not break the ground loops and the issue of ground loops will persist. Further, IN- connection to RTN breaks the symmetry between IN-  and IN+, and hence reduces the common mode rejection which is the intended purpose of differential signal anyways. I'll work more on this to find a way to read differential signals without connecitng IN- and RTN. My first guess is that it would need the GND on the source end to be connected to EarthGND and RTN on acromag end to be connected to EarthGND as well.

I took some steps to reduce the coupling of 60 Hz harmonics in noise measurement. The box was transferred to the floor instead of on top of another instrument. Measurement was immediately converted into single-ended using SR560 in battery mode with a gain of 10. All of the setups was covered in aluminum foil to increase isolation.

Spectrum measurement details

• Boards were powered by sorenson power supplies with +/- 18V DC.
• 36 Ohm load was connected to coil output port.
• Signal was taken out using clips from V2+, V2- and GND.
• Signal was first sent to SR560 in A-B mode with a gain of 10 in battery mode.
• The board along with SR560s were covered in aluminum foil to reduce 60Hz harmonics.
• The output from 50Ohm port was sent to SR785 and read with floating ground.
• Instrument noise was measured by shorting the clips with each other.
• Transfer function of measurement setup was measured and divided by final output.
• Measurements were taken with file D1100687_SP.yml and D1100687_SP_LF.yml.
• D1100117_S2100027_Voltage_Noise_Spectrum.pdf and D1100117_S2100028_Voltage_Noise_Spectrum.pdf shows the measured voltage noise spectrum at the output monitors when loaded with 36 Ohms.
• D1100117_S2100027_Current_Noise_Spectrum.pdf and D1100117_S2100028_Current_Noise_Spectrum.pdf shows the esitmate current noise through the coil calculated by dividing the measured voltage noise by 2436 Ohms.

I did the recommended modifications on of the boards with serial number S2100028. These included:

• R13, R27: 160 -> 75
• C11, C21: 470 nF -> 68nF
• C19: 4.7 uF -> 470 nF
• R15: 3.23 kOhm -> 1.82 kOhm

I took transfer function measurements with same method as in 40m/15774 and I'm presenting it here to ensure the modifications are correct and if I should proceed to the next board as well. I didn't have the data used to make plots in here but I think the poles and zeros have landed in the right spot. I'll wait for comments until tomorrow to proceed with changes in the other board as well. I'll do noise measurements tomorrow.

I fitted zeros and poles in the measured transfer function of D1100687 S2100027 and got zeros at 130 Hz and 234 Hz and poles at 10Hz and 2845 Hz. These values are different from the aimed values in this doc, particularly the 234Hz zero which was aimed at 530 Hz in the doc.

I also took the noise measurement using the same method as described in 40m/15780. The noise in Acquisition mode seems to have gone up in 10 Hz - 500 Hz region compared to the measurement in 40m/15780 before the modifications.

All channels are consistent with each other.

Edit Mon Feb 1 12:24:14 2021:
Added zero model prediction after the changes. The measurements match with the predictions.

Edit Wed Feb 3 16:46:59 2021:

Added zero modeled noise in the noise spectrum curves. The acquisition mode curves are in agreement with the model. The noise in Run mode is weirdly lower than predicted by zero.

I found a white paper  from Acromag which discusses how to read differential signal using Acromag units. The document categorically says that differential signals are always supposed to be transmitted in three wires. I provides the two options of either using the RTN to connect to the signal ground (as done in Attachment 3) or locally place 10k-100k resistors between return and IN+ and IN- both (Attachment 2).

I have provided possible scenarios for these.

Using two wires to carry differential signal (Attachment 1):

• I assume this is our preferential way to connect.
• We can also assume all single-ended inputs as differential and do a signal condition agnostic wiring.
• Attachment 3 show what were the results for different values of resistors when a 2Hz 0.5V amplitude signal from SR785 which as converted to differential signal using D1900068 was measured by acromag.
• The connection to RTN is symmetrical for both inputs.

Using three wires to carry differential signal (Attachment 2):

• This is recommended method by the document in which it asks to carry the GND from signal source and connect it to RTN.
• If we use this, we'll have to be very cautious on what GND has been shorted through the acromag RTN terminals.
• This would probably create a lot of opportunities for ground loops to form.

Using an acromag card without making any connection with RTN is basically not allowed as per this document.

I have made the modifications on the other board D1100687 S2100028 as well. The measurements were taken as mentioned in 40m/15784. All conclusions remain the same as 40m/15784. The attached zip file contains all measurement data, before and after the modifications.

Edit Wed Feb 3 16:44:51 2021 :

Added zero modeled noise in the noise spectrum curves. The acquisition mode curves are in agreement with the model. The noise in Run mode is weirdly lower than predicted by zero.

I have made modifications recommended in this doc. The changes made are:

• R24: 19.6k to 4.99k Ohms
• R20: 19.6k to 4.99k Ohms
• R23: 787 to 499 Ohms
• Removed C16.

I took transfer function measurements, fitted them with zeros and poles and plotted it against the zero model of the circuit. The zeros and poles we intended to shift are matching well with 3Hz zero and 30 Hz pole. The later pole at 1500 Hz is at a higher value from what is predicted by zero.

I also took noise measurements and they are in good agreement with the noise predicted by zero.

Edit Wed Feb 10 15:14:13 2021 :

THE NOISE MEASUREMENT WAS WRONG HERE. SEE 40m/15799.

As suggested, I wrapped the satellite amplifier box D10028128 S2100029 in blanket and foam and took very low frequency spectrum starting from 32 mHz to 3 Hz. The results are attached along with stiched high frequency measurements from 40m/15793.

Very Low Frequency Spectrum Measurement

• D1002818 S2100029 box was powered and covered in a foam blanket.
• Additionally, it was covered from all sides with foam to reduce wind and temperature effects on it.
• The rear panel DB25 connector was connected to a breakout board where pins od PDA input and GND were shorted, shorting the transimpedance circuit input.
• The output was read from PDMon DB9 output at front panel which was converted to 4 BNC channels using breakout board.
• Two channel noise was measured at once using D1002818_SP.yml parameter file.
• Instrument noise at all the used input ranges were measured separately by shorting the input of the BNC cables.

Edit Wed Feb 10 15:14:13 2021 :

THIS MEASUREMENT WAS WRONG. SEE 40m/15799.

I measured the output DC voltage of the satellite amplifier box at PDMon port when the PDA input was shorted and got following offsets:
 

However, I think I'm making a mistake while measuring this offset as well as all the noise measurements of this satellite amplifier box so far. Since it is a current input, transimpedance circuit, the noise of the circuit should be measured with open input, not closed. Infact, by shorting the PDA input, I'm giving DC path to input bias current of AD833 transimpedance amplifier to create this huge DC offset. This won't be the case when a photodiode is connected at the input which is a capacitor and hence no DC path is allowed. So my issue of offset was bogus and past two noise measurements in 40m/15797 and 40m/15793 are wrong.

Here is a proper measurement for PD transimpedance amplifier circuit in the Satellite amplifier box D1002818 S2100029. The input from rear DB25 connector was left open and measurement was taken with AC coupling with correction by the AC coupling transfer function (Zero at 0, pole at 160 mHz). I have calculated the input referred displacement noise by calculating the conversion factor of OSEM in A/m. From 40m/12470, old conversion factor of OSEM to output of sat amplifier was 1.6 V/mm. then, the transimpedance was 39.2 kOhm, so that must mean a conversion factor of 1.6e3/39.2 A/m. This I scaled with increased drive current by factor of 35/25 as mentioned in this document. The final conversion factor turned out to be around 57 mA / m. If someone finds error in this, please let me know.

There is excess noise in the low-frequency region below 5-6 Hz. If people think I should make a measurement of amplified noise to go further away from the instrument noise floor, let me know.

I understand this mst be frustrating for you. But we did not change these settings, knowingly atleast. We have documented all the things we did there. The only thing I can think of which could possibly change any of those channels are the scripts that we ran that are mentioned and the burt restore that we did on all channels (which wasn't really necessary). We promise to be more vigilant of changes that occur when we are present in future.

No we didn't fix the issue. We'll post some screenshots tomorrow. From "sitemap>Shutter>PSL" we meant in Shutter medm window, we clicked on the PSL close button. As pointed later, it switches C1:AUX-PSL_ShutterRqst while the PSL shutter switch on Lock MC medm screen switches C1:PSL-PSL_ShutterRqst. We were not sure if this was intentional, so we didn't change anything.

allegra had fresh Debian 10 installed on it already. I installed cds-workstation packages (with the help of Erik von Reis). I checked that command line caget, caput etc were working. I'll see if medm and other things are working next time we visit the lab.

Here, I present the same input matrices now normalized row by row to have same norm as current matrices rows. These now I plotted better than last time. Other comments same as 15902. Please let us know what you think.

Thu Mar 18 09:11:10 2021 :

Note: The comparison of butterfly dof in the two cases is bit bogus. The reason is that we know what the butterfly vector is in sensing matrix (N_osems x (N_dof +1)) and that is the last column being (1, -1, 1, -1, 0) corresponding to (UL, UR, LR, LL, Side). However, the matrix we multiply with the OSEM data is the inverse of this matrix (which becomes the input matrix) which has dimensions ((N_dof + 1) x N_osem) and has the last row corresponding to the butterfly dof. This row was not stored for old calculation of the input matrix (which is currently in use) and can not be recovered (mathematically not possible) with the existing 5x4 part of that input matrix. We just added (1, -1, 1, -1, 0) row in the bottom of this matrix (as was done in the matlab codes) but that is wrong and hence the butterfly vector looks so bad for the existing input matrix.

Proposal: We should store the last row of generated input matrix somewhere for such calculations. Ideally, another row in the epics channels for the input matrix would be the best place to store them but I guess that would be too destructive to implement. Other options are to store this 5 number information in wiki or just elogs. For this post, the buttefly row for the generated input matrix is present in the attached files (for future references).

Rana asked us to write out here the new MC3 input matrix we have calculated along with the old one. The new matrix is not working out for us as it can't keep the suspension loops stable.

Matrices:

Note that the new matrix has been made so that the norm of each row is the same as the norm of the corresponding row in the old (current) input matrix.

Peak finding results:

Note: The highest peak on SIDE OSEM sensor free swinging data as well as the DOF basis data created using existing input matrix, comes at 0.978 Hz. Ideally, this should be 1 Hz and in MC1 and MC2, we see the resonance on SIDE DOF to show near 0.99 Hz. If you look closely, there is a small peak present near 1 Hz actually, but it is too small to be the SIDE DOF eigenfrequency. And if it is indeed that, then which of the other 4 peaks is not the DOF we are interested in?

On possiblity is that the POS eigenfrequency which is supposed to be around 0.97 Hz is split off in two peaks due to some sideways vibration and hence these peaks get strongly coupled to SIDE OSEM as well.

P.S. I think something is wrong and out limited experience is not enough to pinpoint it. I can show up more data or plots if required to understand this issue. Let us know what you all think.

A longer measurement is set to trigger at 5:00 tomorrow on April 2nd, 2021. This measurement will run for 35 iterations with an excitation duration of 120s and bandwidth for CSD measurement set to 0.1 Hz. The script is set to trigger in a tmux session named 'cB' on pianosa.

Last run gave similar results as the quick run we did earlier. The code has been unable to strike out couplings with POS. We found the bug which is causing this. This was because the sampling rate of MC_F channel is different from the test-point channels used for PIT and YAW. Even though we were aware of it, we made an error in handling it while calculating CSD. Due to this, CSD calculation with POS data was performed by the code with zero padding which made it think that no PIT/YAW <-> POS coupling exist. Hence our code was only able to fix PIT <-> YAW couplings.

We'll need to do another run with this bug fixed. I'll update this post with details of the new measurement.

To test if our method is working at all, we went for the simpler case of just uncoupling PIT and YAW. This is also because the sensor used for these two degrees of freedom is similar (the MC Trans WFS).

We saw a successful decrease in cross-coupling between PIT and YAW over the first 50 iterations that we tried. Here are some results:

Final output matrix:

Plots:

• Attachment 1 shows distance of sensing matrix from identity as iterations go.
• Attachment 2 shows the off-diagonal elements of sensing matrix as the iterations increase.
  • It is worth noting that PIT -> YAW coupling was the main element that was reduced successfully while the YAW -> PIT was reducing but much more slowly.
  • Most of the remaining cross coupling in the end was from YAW -> PIT.
• Attachment 3 shows first 10 oscillations in the time series data during excitation of some of the iterations.
• Attachment 4 shows the cross spectral density of the sensed data during excitation with each other. This has been normalized by reference PSD data (taken with no excitation) of the sensed DOFs involved in the CSD calculation.
• Attachment 5 shows the TF estimate made by normalizing CSD data column wise by the diagonal elements. The excitation frequency point in these plots become the Sensing matrix in the calculation.
  • One can notice how the PIT -> YAW element is going down in these plots.
  • Even though we are using only the real value of the sensing matrix, the imaginary values are also going down.

Next, tried uncoupling POS and PIT:

• Next, we tried to uncouple POS and PIT. We expect them to be more coupled than with YAW.
• At the time of writing this post, 15 iterations of this attempt have been completed and it is not looking good .
• The distance of the sensing matrix from identity is growing at an accelerated rate.
• The POS output matrix column seems to be trying to go towards the negative of PIT output matrix column! Why? We don't know.
• We have seen in the past that once POS transforms into PIT or YAW, it just makes the output matrix worse as no feedback actually goes into the POS column. Eventually, the IMC will cease to remain locked.
• So, I'm cancelling this attempt for now. Will consider more alternatives later.

I'm not sure I understand what F2A is? I couldn't find a description of this filter anywhere and don't remember if you have already explained it. Can you describe what is needed to be done again, please? We would keep SUS state space model and seismic transfer functions calculation ready meanwhile.

Sorry about that. It must be me. I'll make sure it doesn't happen again. I was careless to not check back, no further explanation.

• I have confirmed that the old two monitors' backlighting is not working. One can see the impression of the display without any brightness on them. Both old monitors are on the shelf behind.
• Today we got a monitor and mouse from Mike. I had to change /etc/default/grub GRUB_GFXMODE to 1920x1200@30 on allegra for it to work with the(any) monitor.
• Allegra is Debian 10 with latest cds-workstation installed on it. It is a good test station to migrate our existing scripts to start using updated cds-workstation configuration.

That makes sense. I assumed that IFO-STATE is configured as you have proposed it to be configured. This could be implemented in later.

Plan of action

• Get the transfer functions of the suspension plant from actuated DOF to sensed DOF. We'll verify Bhavini's state-space model and get these transfer functions. Use the model TFs, not measured.
• For each of POS->POS, PIT->PIT, and YAW->YAW, we'll get the resonant frequency and Q of the resonance from these models. No, forget about the Q.
• We can correct the resonant frequencies from the measured ones in our free swinging data.
• Now, we'll repeat the following for each column of output matrix filters (inspired from scripts/SUS/F2Pcalc.py, but not fully understood how/why):
  • Select col (eg. POS)
  • Set f₀ to the resonant frequency.
  • Calculate where G_UL is the corrected DC gain we got after output matrix optimization earlier. (Not sure how, why?). No, use the SS model.
  • Calculate f_UR, f_LL, and f_LR like above.
  • Set   (This just seems like a way of keeping some approximately low Q, ideally we should keep this same to what we got above but that might cause saturation issues like Rana mentioned in the meeting)
  • Then, set the following filter in the output matrix element for UL:
                         
    which is in zpk form equivalent to:
                         
  • Repeat the above for UR, LL, LR.
  • Note that this filter function takes values G_UL at DC and at high frequencies while it would dip at the resonant frequency for POS with depth and narrowness directly proportional to Q_UL. No, the DC gain is different from the AC gain.
• However, the F2P filter plots we found in several places on elog look a bit different. Like here: 40m/4719. One important difference is that the filter magnitude always become 1 after the resonance at higher frequencies. Yes, this is  what we want, since you already did the balancing at high frequencies.
• A preliminary plot of the above calculation for the 1,1 output matrix filter bank (POS -> UL) is attached in Attachment 1.

Discussion:

• We can make 12 such filters for the 12 numbers we got for the optimized output matrix. Is that the aim or should we do it only for the POS column as has been done in past?
• We are not sure how the choice of Q is made in setting the above filter function. We'll think more about it to understand this.
• We are also not sure how the choice of f_UL is made above. It looks like depending on the correction gain, we want to slide the zero positions with respect to the pole positions which are fixed at the resonant frequency as expected. This seems to have some complex explanation.
• Please let us know if we are planning this right before we dive into these calculations/script writing. Thanks.

Edit Thu Apr 15 08:32:58 2021 :

Comments are from Rana.

Corrected the plot in the attachment. It shows the correct behavior at high frequencies now.

Here' s aquick update before we leave for lunch. We have managed to calculate some filter that would go on the POS column in MC2 output matrix filter banks aka F2A aka F2P filters. In the afternoon if we can come and work on the IMC, we'll try to load them on the output matrix. We have never done that so it might take some time for us to understand on how to do that. Attached is the bode plot for these proposed filters. Let us know if you have any comments.

Following up from morning's work, I balanced the coils at DC as well. Attachment 1 is screenshot of striptool in which blue and red traces show ASCYAW and ASCPIT outputs when C1:SUS-MC2_LSC_OFFSET was switched by 500 counts. We see very slight disturbance but no real DC offset shown on PIT and YAW due to position step. This data was taken while nominal F2A filter calculated to balance coils at DC was uploaded

I have uploaded the filters on filter banks 7-10 where FM7 is the nominal filter with Q close to 1 and 8-10 are filters with Q 3, 7 and 10 respectively. The transfer function of these filters can be seen in Attachment 2. Note, that the high frequency gain drops a lot when higher Q filters are used.

These filters are designed such that the total DC gain after the application of coil outputs gains for high frequency balancing (as done in morning 16054) balances the coils at DC.

Since I had access to the complete output matrix that balances the coils to less than 1% cross coupling at high frequencies from 16009, I also did a quick test of DC coil balancing with this kind of high frequency balancing. In this case, I uploaded another set of filters which were made at Q close to 1 and gain such that effective DC gain matrix becomes what I got by balancing in the above case. This set of filter also worked as good as the above filters. This completes the proof that we can also use complete matrix for high frequency coil balancing which can be calculated by a script in 20min and works with DC coil balancing as well. In my opinion, this method is more clear and much faster than toggling values in coil output gains where we have only 4 values to optimize 6 cross-coupling parameters. But don't worry, I'm not wasting time on this and will abandon this effort for now, to be taken up in future.

Next up:

• Tomorrow, we'll finish DC balancing for MC1 and MC3 with the method I practiced today. This should not take much time and should be completed before the meeting.
• I'll also, calculate and upload the F2A filters for MC1 and MC3.
• Next, we'll optimize gains in the suspension damping loops by doing step response test (with TRAMP = 0s). We'll look for decaying response (at MC_F, and WFS sensors) with a few oscillations for each step in POS, PIT, and YAW.

Edit Tue Apr 20 21:25:46 2021 :

Corrected the calculation of filters in case of Q different than . There was a bug in the code which I overlooked. I'll correct the filter bank modules tomorrow.

Edit Wed Apr 21 11:06:42 2021 :

I have uploaded the corrected foton filters. Please see attachment 3 for the transfer functions calculated by foton. They match the filters we intended to upload. Only after uploading and closing the foton filter, I realized that the X=7 filter plot (bottom left in attachment 3) does not have dB units on y-axis. It is plotted in linear y-scale (this plot in foton is for phase by default to I guess I forgot to change the scaling when repurposing it for my plot).

[Anchal, Koji]

Removed the top sheet

• Opened first from the door side so that any dust would spill outside.
• Then rolled the sheet inward to meet in the middle.
• Repeated this twice for the 2 HEPA filters.

Removed the sheets on the table

• Lifted sheet up making sure the top side face outside always.
• Rolled it sideways halfway through.
• Cut down the sheet vertically.
• Slided the doors to the other side and rolled the remaining half.
• On the door side, the sheets above the ALS optics were simply lifted off.

Restarting PSL

• Turned on the HEPAs at the max speed
• Switched on laser to jsut above the threshold
  • Before the 1st eom, power was 20mW 
  • After the EOM/AOM, 18mW. So about 90% transmission through all polarizing optics.
  • We saw the resonances of the PMC but could not lock it even with highest gain available (30 dB).
• Increased the input power to PMC to 100mW
  • Locked the PMC at 30dB gain
  • The transmitted power was ~50-60 mW. (Had to use power meter suspended by hand only.
  • The right before the IMC (after the 2nd EOM) 48mW. So none of the alignment was lost.
• Opened the PSL shutter.
• We were able to see IMC reflection signal.
• We were also able to see IMC catching lock as the servo was left ON earlier.
• Switched off the servo.
• Decided to increase the power while watching PMC Trans/Refl and IMC REFL
• Injection diode current to innolight was increased slowly to 2.10A. Saw a mod hopping region aroun 1.8A.
• We recovered the PMC Trans >0.7 V.
• PZT was near the edge, so moved by one FSR.
• The PMC refelction signal is still shown in red at around 48 mV.

Back to control room

• IMC was locked almost immediately by manually finding the lock while keeping IMC WFS off to preserve the offsets from yesterday.
• Then switch on IMC WFS. Working good.
• Then unlocked the servo and switched on IMC Autolocker. Lock was caught immediately.

Decided to start locking the arms

• The arm transmissions were flashing but at 0.2~0.3 level.
• Decided to adjust TT1 and TT2 Pitch and Yaw to align the light going into the arms.
• This made TRY ~0.6 / TRX ~0.8 at the peak of the flashing
• Locked the arms. (By switching on C1:LSC-MODE_SELECT which engages all servos).
• Used ASS to align Yarm then align Xarm. Procedure:
  • Sitemap > ASC > c1ass
  • Open striptool to look at progress. ! Scripts YARM > striptool.
  • Switch on ASS. ! More Scripts > ON
  • Wait for the TRY to reach to around 0.97.
  • Freeze the outputs. ! Scripts > Freeze Outputs.
  • Offload the offsets to preserve the output. ! More Scripts > OFFLOAD OFFSETS.
  • Switch off ASS. ! More Scripts > OFF
  • Repeted this for XARM.
• At the end, both XARM and YARM were locked with TRX ~ 0.97 and TRY ~ 0.96.

Yes, during the AC balancing, POS column was set to all 1. This table shows the final values after all the steps. The first 3 columns are DC balancing results when output matrix was changed. While the last column is for AC balancing. During AC balancing, the output matrix was kept to ideal position as you suggested.

Koji mentioned that the mode of the laser is different for lower diode currents. So that might be the reason why we got less transmission at the low input power but more afterward.

The mix up was my fault I think. I restored the channels manually instead of using burt restore. Your message suggests that we can set burt to start noticing channel changes at home point and create a .req file that can be used to restore later. We'll try to learn how to do that. Right now, we only know how to burt restore using the existing snapshots from the autoburt directory, but they touch more things than we work on, I think. Or can we just always burt restore it to morning time? If yes, what snapshot files should we use?

I have uploaded all the new settings mentioned in 16066 and 16072. The settings were uploaded through a single script present at anchal/20210428_IMC_Tuned_Suspension/uploadNewConfigIMC.py. The settings can be reverted back to old settings through anchal/20210428_IMC_Tuned_Suspension/restoreOldConfigIMC.py. Both these scripts can be run only through python3 in donatella or allegra.

GPSTIME of new settings: 1303690144

New settings include:

• New input matrices for MC1 and MC2.
• New Output coil gains for AC balancing on all three optics.
• Switching ON the FM8 filter modulae (Q=3 F2A filter) in POS column on output matrix of all optics.

We'll wait and watch the performance through summary pages and check back the performance on Monday.

In 16087 we mentioned that we were unable to do a step response test for WFS loop to get an estimate of their UGF. The primary issue there was that we were not putting the step at the right place. It should go into the actuator directly, in this case, on C1:SUS-MC2_PIT_COMM and C1:SUS-MC2_YAW_COMM. These channels directly set an offset in the control loop and we can see how the error signals first jump up and then decay back to zero. The 'half-time' of this decay would be the inverse of the estimated UGF of the loop. For this test, the overall WFS loops gain,  C1:IOO-WFS_GAIN was set to full value 1. This test is performed in the changed settings uploaded in 16091.

I did this test twice, once giving a step in PIT and once in YAW.

Attachment 1 is the striptool screenshot for when PIT was given a step up and then step down by 0.01.

• Here we can see that the half-time is roughly 10s for TRANS_PIT and WFS1_PIT corresponding to roughly 0.1 Hz UGF.
• Note that WFS2 channels were not disturbed significantly.
• You can also notice that third most significant disturbance was to TRANS_YAW actually followed by WF1 YAW.

Attachment 2 is the striptool screenshot when YAW was given a step up and down by 0.01. Note the difference in x-scale in this plot.

• Here, TRANS YAW got there greatest hit and it took it around 2 minutes to decay to half value. This gives UGF estimate of about 10 mHz!
• Then, weirdly, TRANS PIT first went slowly up for about a minutes and then slowly came dome in a half time of 2 minutes again. Why was PIT signal so much disturbed by the YAW offset in the first place?
• Next, WFS1 YAW can be seen decaying relatively fast with half-life of about 20s or so.
• Nothing else was disturbed much.

• So maybe we never needed to reduce WFS gain in our measurement in 16089 as the UGF everywhere were already very low.
• What other interesting things can we infer from this?
• Should I sometime repeat this test with steps given to MC1 or MC3 optics?

Tried locking the arms

• Ran IFO > Configure > ! (YARM) > Restore YARM. Nothing happened.
• Trying to align through tip-tilt:
  • Previous values: TT1: PIT: -1.7845, YAW: -0.2775; TT2: PIT: -1.3376, YAW: -0.0436
  • Couldn't get flashing of light in the arms at all.
  • Restored values to previous values.
• Noticed that ITMY OPLEV YAWW Error has gone very high overnight while other oplevs remained the same.
• Trying to change the C1:SUS-ITMY_YAW_OFFSET to bring this oplev yaw error back to near zero.
• Changed C1:SUS-ITMY_YAW_OFFSET from -34 to 50. OPLEV_YEROR reduced to around -23 from -70.
• Same thing with BS PIT. OPLEV_PERROR is highlighted in red at -52.
• Changing C1:SUS-BS_PIT_OFFSET from 55 to 30. This brought OPLEV_PERROR to -15 from -52.
• Trying to align PRM by changing C1:SUS-PRM_PIT_OFFSET and C1:SUS-PRM_YAW_OFFSET.
• Inital values: C1:SUS-PRM_PIT_OFFSET: -20 , C1:SUS-PRM_YAW_OFFSET: 39.

Did the WFS step response test on IMC in between while waiting for help. See 16094.

Back to trying arm locking

• Tried IFO > Configure > ! (XARM) > Restore YARM. Nothing happened.
• Tried IFO > Configure > ! (YARM) > Restore YARM. Nothing happened again.
• Tried Movie Capture of AS screen from VIDEO > Movie Capture > AS. But the script failed due to module not present error.

PMC got unlocked

• Infront of me, PMC got unlocked. I did not go to PMC locking the screen at all since morning.
• I opened the C1PSL_PMC screen. The PSL Autolocker blinker is not blinking but the switch is set to Enable. 
• I do not see any automatic effort happening for regaining lock at PMC.
• I'll try it manually. I was able to get the PMC locked again. C1:PSL-PMC_PMCTRANSPD is showing 0.761 V and C1:PSL-PMC_RFPDDC is showing 0.053 V.
• Now IMC auto locker seems to be trying to get the lock acquired.
• It acquired a lock a few times but struggled to keep it on. I reduced C1:IOO-WFS_GAIN to 0 and then the lock could stay on. Seemed like the accumulated offsets were not good.
• So I cleared the history on WFS1, TRANS and WFS2 filter banks and then ramped the WFS overall gain (C1:IOO-WFS_GAIN) back to 1 and now IMC seems to stay locked in a stable configuration.
• However, I still don't know what caused the PMC to get unlocked in the first place. Did my repeated arm locking attempts did something to the main laser frequency?

Back to trying arm locking

• Tried IFO > Configure > ! (YARM) > Restore YARM again. Nothing happened again.

I double checked today and the F2A filters in the output matrices of MC1, MC2 and MC3 in the POS column are ON. I do not get what SDF means? Did we need to add these filters elsewhere?

t Both arms were locked simply by using IFO > Configure > ! (YARM) > Restore YARM. I had to use ASS to improve the TRX/TRY to ~0.95.

I measured C1:LSC-XARM_IN1_DQ and C1:LSC-YARM_IN1_DQ while injecting band limited noise in C1:IOO-WFS1_PIT_EXC using uniform noise with amplitude 1000 along with filter defined by string: cheby1("BandPass",4,1,80,100). I calibrated the control arms signals by 2.44 nm/cts calibration factor directly picked up from 13984.

For the duration of this test, all LIMIT switches in the WFS loops were switched OFF.

I do not see any affect on the arm control signal power spectrums with or without the noise injection. Attachement 1 shows the PSD along with PSD of the injection site IN2 signal. I must be doing something wrong, so would like feedback before I go further.

With the input matrix, coil ouput gains and F2A filters loaded as in 16091, I tested the suspension loops' step response to offsets in LSC, ASCPIT and ASCYAW channels, before and after applying the "new damping gains" mentioned in 16066 and 16072. If these look better, we should upload the new (higher) damping gains as well. This was not done in 16091.

Note that in the plots, I have added offsets in the different channels to plot them together, hence the units are "au".

We repeated the same test with IMC unlocked. We had found these gains when IMC was unlocked and their characterization needs to be done with no light in the cavity. attached are the results. Everything else is same as before.

Edit Tue May 4 14:43:48 2021 :

• Adding zoomed in plots to show first 25s after the step.
• MC1:
  • Our improvements by new gains are only modest.
  • This optic needs a more careful coil balancing first.
  • Still the ring time is reduced to about 5s for all step responses as opposed to 10s at old gains.
• MC2:
  • The first page of MC2 might be bit misleading. We have not changed the damping gain for SUSPOS channel, so the longer ringing is probably just an artifact of somthing else. We didn't retake data.
  • In PIT and YAW where we increased the gain by a factor of 3, we see a reduction in ringing lifetime by about half.
• MC3:
  • We saw the most optimistic improvement on this optic.
  • The gains were unusually low in this optic, not sure why.
  • By increasing SUSPOS gain from 200 to 500, we saw a reduction of ringing halftime from 7-8s to about 2s. Improvements are seen in other DOFs as well.
  • You can notice rightaway that YAW of MC3 keeps oscillating near resonance (about 1 Hz). Maybe more careful feedback shaping is required here.
  • In SUSPIT, we increased gain from 12 to 35 and saw a good reduction in both ringing time and initial amplitude of ringing.
  • In SUSYAW, we only increased the gain to 12 from 8, which still helped a lot in reducing big ringing step response to below 5s from about 12s.

Overall, I would recommend setting the new gains in the suspension loops as well to observe long term effects too.

For past few days, a weird sound of decaying gas leakage comes in the 40m control room from the south west corner of ceiling. Attached is an audio capture. This comes about every 10 min or so. 

We have uploaded the new damping gains on all the suspensions of IMC. This completes changing all the configuration to as mentioned in 16066 and 16072. The old setting can be restored by running python3 /users/anchal/20210505_IMC_Tuned_SUS_with_Gains/restoreOldConfigIMC.py from allegra or donatella.

GPSTIME: 1304265872

Here's my first attempt at doing angular actuation calibration for IMC mirrors using the method descibed in /users/OLD/kakeru/oplev_calibration/oplev.pdf by Kakeru Takahashi. The key is to see how much is the cavity mode misaligned from the input mode of beam as the mirrors are moved along PIT or YAW.

There two possible kinds of mismatch:

• Parallel displacement of cavity mode axis:
  • In this kind of mismatch, the cavity mode is simply away from input mode by some distance .
  • This results in transmitted power reduction by the gaussian factor of where is the beam waist of input mode (or nominal waist of cavity).
  • For some mismatch, we can approximate this to
                                                                            
• Angular mismatch of cavity mode axis:
  • The cavity mode axis could be tilted with respect to input mode by some angle .
  • This results in transmitted power reduction by the gaussian factor of   where is the beam divergence angle of input mode (or nominal waist of cavity) given by .
  • or some mismatch, we can approximate this to
                                                                             

Kakeru's document goes through cases for linear cavities. For IMC, the mode mismatches are bit different. Here's my take on them:

MC2:

• MC2 is the easiest case in IMC as it is similar to the end mirror for linear cavity with plane input mirror (the case of which is already studies in sec 0.3.2 in Kaker's document).
• PIT:
  • When MC2 PIT is changed, the cavity mode simple shifts upwards (or downwards) to the point where the normal from MC2 is horizontal.
  • Since, MC1 and MC3 are plane mirrors, they support this mode just with a different beam spot position, shifted up by .
  • So the mismatch is simple of the first kind. In my calculations however, I counted the two beams on MC1 and MC3 separately, so the factor is twice as much.
  • Calling the coefficient to square of angular change , we get:
                                  
  • Here, R is radius of curvature of MC1/3 taken as 21.21m and L is the cavity half-length of IMC taken as 13.545417m.
• YAW:
  • For YAW, the case is bit more complicated. Similar to PIT, there will be a horizontal shift of the cavity mode by .
  • But since the MC1 and MC3 mirrors will be fixed, the angle of the two beams from MC1 and MC3 to MC2 will have to shift by .
  • So the overall coefficient would be:
                                  
  • The factor of 4 in denominator of seconf term on RHS above comes because only half og angular actuation is felt per arm. The factor of 2 in numerator for for the 2 arms.

MC1/3:

• First, let's establish that the case of MC1 and MC3 is same as the cavity mode must change identically when the two mirrors are moved similarly.
• YAW:
  • By tilting MC1 by , we increase the YAW angle between MC1 and MC3 by .
  • Beam spot on both MC1 and MC3 moves by .
  • The beam angles on both arms get shifted by .
  • So the overall coefficient would be:
                                  
  • Note, this coefficient is same as MC2, so it si equivalent to moving teh MC2 by same angle in YAW.
• PIT:
  • I'm not very sure of my caluculation here (hence presented last).
  • Changing PIT on MC1, should change the beam spot on MC2 but not on MC3. Only the angle of MC3-MC2 arm should deflect by .
  • While on MC1, the beam spot must change by and the MC1-MC2 arm should deflect by .
  • So the overall coefficient would be:
                                  

Test procedure:

• We first clicked on MC WFS Relief (on C1:IOO-WFS_MASTER) to reduce the large offsets accumulated on WFS outputs. This script took 10 minutes and reduced the offsets to single digits and IMC remained locked throughout the process.
• Then we switched off the WFS to freeze the outputs.
• We moved the MC#_PIT/YAW_OFFSET up and down and measured the C1:IOO-MC_TRANS_SUMFILT_OUT channel as an indicater of IMC mode matching.
• Attachement 1 are the 6 measurements and there fits to a parabola. Fitting code and plots are thanks to Paco.
• We got the curvature of parabolas from these fits in units of 1/cts^2.
• The coefficients calculated above are in units of 1/rad^2.
• We got the angular actuation calibration from these offsets to physical angular dispalcement in units of rad/cts by .
• AC calibration:
  • I parked the offset to some value to get to the side of parabola. I was trying to reduce transmission from about 14000 cts to 10000-12000 cts in each case.
  • Sent excitation using MC#_ASCPIT/YAW_EXC using awg at 77 Hz and 10000 cts.
  • Measured the cts on transmission channel at 77 Hz. Divided it by 2 and by the dc offset provided. And divided by the amplitude of cts set in excitation. This gives analogous to above DC case.
  • Then angular actuation calibration at 77 Hz from these offsets to physical angular dispalcement in units of rad/cts by .
• Following are the results:

  Optic Act	Calibration factor at DC [µrad/cts]	Calibration factor at 77 Hz [prad/cts]
  MC1 PIT	7.931+/-0.029	906.99
  MC1 YAW	5.22+/-0.04	382.42
  MC2 PIT	13.53+/-0.08	869.01
  MC2 YAW	14.41+/-0.21	206.67
  MC3 PIT	10.088+/-0.026	331.83
  MC3 YAW	9.75+/-0.05	838.44

   

• Note these values are measured with the new settings in effect from 16120. If these are changed, this measurement will not be valid anymore.
• I believe the small values for MC1 actuation have to do with the fact that coil output gains for MC1 are very weird and small, which limit the actuation strength.
• TAbove the resonance frequencies, they will fall off by 1/f^2 from the DC value. I've confirmed that the above numbers are of correct order of magnitude atleast.
• Please let me know if you can point out any mistakes in the calculations above.

[Anchal Koji]

Koji and I did a few tests with an OSEM emulator on the satellite amplifier box used for MC1 which is housed on 1X4. This sat box unit is S2100029 D1002812 that was recently characterized by me 15803. We found that the differential output driver chip AD8672ARZ U2A section for the UL PD was not working properly and had a fluctuating offset at no input current from the PD. This was the cause of the ordeal of the morning. The chip was replaced with a new one from our stock. The preliminary test with the OSEM emulator showed that the channel has the correct DC value.

In further testing of the board, we found that the channel 8 LED driver was not working properly. Although this channel is never used in our current cable convention, it might be used later in the future. In the quest of debugging the issue there, we replaced AD8672ARZ at U1 on channel 8. This did not solve the issue. So we opened the front panel and as we flipped the board, we found that the solder blob shorted the legs of the transistor Q1 2N3904. This was replaced and the test with the LED out and GND shorted indicated that the channel is now properly providing a constant current of 35mA (5V at the monitor out).

After the debugging, the UL channel became the least noisy among the OSEM channels! Mode cleaner was able to lock and maintain it.

We should redo the MC1 input matrix optimization and the coil balancing afterward as we did everything based on the noisy UL OSEM values.

For future reference, the new settings can be upoaded from a script in the same directory. Run python /users/anchal/20210505_IMC_Tuned_SUS_with_Gains/uploadNewConfigIMC.py from allegra.

I attempted a single arm actuation calibration using IR beatnote (in the directions of soCal idea for DARM calibration)

Measurement and Inferences:

• I sent 4 excitation signals at C1:SUS-ITM_LSC_EXC wit 30cts at 31Hz, 200cts at 197Hz, 600cts at 619Hz and 1000cts at 1069 Hz.
• These were sent simultaneously using compose function in python awg.
• The XARM was locked to mai laser and alignment was optimized with ASS.
• The Xend Green laser was locked to XARM and alignment was optimized.
  • Sidenote: GTRX is now normalized to give 1 at near maximum power.
  • Green lasers can be locked with script instead of toggling.
  • Script can be called from sitemap->ALS->! Toggle Shutters->Lock X Green
  • Script is present at scripts/ALS/lockGreen.py.
• C1:ALS-BEATX_FINE_PHASE_OUT_HZ_DQ was measured for 60s.
• Also, measured C1:LSC-XARM_OUT_DQ and C1:SUS-ITMX_LSC_OUT_DQ.
• Attachment 1 shows the measured beatnote spectrum with excitations on in units of m/rtHz.
• It also shows resdiual displacement contribution PSD of (output referred) XARM_OUT and ITMX_LSC_OUT to the same point in the state space model.
  • Note: that XARM_OUT and ITMX_LSC_OUT (excitation signal) get coherently added in reality and hence the beatnote spectrum at each excitation frequency is lower than both of them.
  • The remaining task is to figure out how to calculate the calibration constant for ITMX actuation from this information.
  • I need more time to understand the mixture of XARM_OUT and ITMX_LSC_OUT in the XARM length node in control loop.
  • Beatnote signal tells us the actual motion of the arm length, not how much ITMX would have actuated if the arm was not locked.
• Attachment 2 has the A,B,C,D matrices for the full state space model used. These were fed to python controls package to get transfer functions from one point to another in this MIMO.
  • Note, that here I used the calibration of XARM_OUT we measured earlies in 16127.
  • On second thought, maybe I should first send excitation in ETMX_LSC_EXC. Then, I can just measure ETMX_LSC_OUT which includes XARM_OUT due to the lock and use that to get calibration of ETMX actuation directly.

I fixed the PSL shutter button on Shutters summary page C1IOO_Mech_Shutter.adl. Now PSL switch changes C1:PSL-PSL_ShutterRqst channel. Earlier it was C1:AUX-PSL_ShutterRqst which doesn't do anything.

Attachment 1 shows the closed loop of Xend Green laser Arm PDH lock loop. Free running laser noise gets injected at laser head after the PZT actuation as . The PDH error signal at output of miser is fed to a gain 1 SR560 used as summing junction here. Used in 'A-B mode', the B port is used for sending in excitation where .

We have access to three ports for measurement, marked at output of mixer, at output of SR560, and at PZT out monitor port in uPDH box. From loop algebra, we get following:

, where is the open loop transfer function of the loop.

So measurement of can be done in following two ways (not a complete set):

1. , if excitation amplitude is large enough such that over all frequencies.
   • In this method however, note that SR785 would be taking ratio of unsuppresed excitation at with suppressed excitation at
   • If the closed loop gain (suppression) is too much, the excitation signal might drop below noise floor of SR785 while measuring .
   • This would then appear as a flat response in the transfer function.
   • This happened with us when we tried to measure this transfer function using this method. Below few hundered Hz, the measurement will become flat at around 40 dB.
   • Increasing the excitation amplitude where suppression is large should ideally work. We even tried to use Auto level reference option in SR785.
   • But the PDH loop gets unlocked as soon as we put exciation above 35 mV at this point in this loop.
2. , if excitation amplitude is large enough such that over all frequencies.
   • In this method, channel 1 (denominator) on SR785 would remain high in amplitude throughout the measurement avoiding the above issue of suppression below noise floor.
   • We can easily measure the feedback transfer funciton with the loop open. Then multiplying the two measurements should give us estimate of open loop transfer function.
   • This is waht we did in 16194. But we still could not increase the excitation amplitude beyond 35 mV at injection point and got a noisy measurement.
   • We checked yesterday coherence of excitation signal with the three measurment points and it was 1 throughout the frequency region of measurement for excitation amplitudes above 20 mV.
   • So as of now, we are not sure why our signal to noise was so poor in lower frequency measurement.

Working in Xend with mask on has become unbearable. It is very hot there and I would really like if we fix this issue.

Today, the Xend Green laser was just unable to hold lock for longer than 10's of seconds. The longest I could see it hold lock was for about 2 minutes. I couldn't find anything obviously wrong with it. Attached are noise spectrums of error and control points. The control point spectrum shows good matching with typical free running laser noise.

Are the few peaks above 10 kHz in error point spectrum worrysome? I need to think more about it in a cooler place to make sure.

I wanted to take a high frequency spectrum of error point to make sure that higher harmonics of 250 kHz modulation frequency are not leaking into the PDH box after demodulation. However, the lock could not be maintained long enough to take this final measurement. I'll try again tomorrow morning. It is generally cooler in the mornings.

This post is just an update on what's happening. I need to work more to get some meaningful inferences about this loop.

I propose we set up a temperature sensor network as described in attachment 1.

Here there are two types of units:

• BASE-GATEWAY
  • Holds the processor to talk to the network through Modbus TCP/IP protocol.
  • This unit itself has a temperature sensor in it. It is powered by a power adaptor or PoE from the switch.
  • Each base unit can have at max 2 extended temperature probes ENV-TEMP.
• ENV-TEMP
  • This is just a temperature probe with no other capabilities.
  • It is powered via PoE from the BASE-GATEWAY unit.

Proposal is

• to put 2 ENV-TEMP sensors (#1 and #2) along the Y-arm at the end and midway. These are powered and read through a BASE-GATEWAY (#A) at the vertex. The BASE-GATEWAY (#A) will serve as temperature sensor for the vertex.
• We put one ENV-TEMP(#3) at the X-end powered and read through by another BASE-GATEWAY (#B) at the midpoint of X-arm.
• Both BASE-GATEWAY are connected by ethernet cables to the network switch. That's it.

These sensors can be configured over network by going to their assigned IP addresses. I'm not sure at the moment how to configure the dB files to get them to write on slow EPICS channels.

We will have an unused port on the BASE-GATEWAY (#B) which can be used to run another temperature sensor, maybe at an important rack, PSL table or somewhere else.

In future, if more sensors are required, there are expansion (network switch like) options for BASE-GATEWAY or daisy-chain options for the probes.

Edit Fri Jun 18 16:28:13 2021 :

See this [wiki page](https://wiki-40m.ligo.caltech.edu/Physical_Environment_Monitoring/Thermometers) for updated plan and final quote.

MC was unable to acquire lock because the WFS offsets were cleared to zero at some point and because of that MC was very misaligned to be able to catch back lock. In such cases, one wants the WFS to start accumulating offsets as soon as minimal lock is attained so that the mode cleaner can be automatically aligned. So I did following that worked:

• Made the C1:IOO-WFS_TRIG_WAIT_TIME (delay in WFS trigger) from 3s to 0s.
• Reduced C1:IOO-WFS_TRIGGER_THRESH_ON (Switchin on threshold) from 5000 to 1000.
• Then as soon as a TEM00 was locked with poor efficiency, the WFS loops started aligning the optics to bring it back to lock.
• After robust lock has been acquired, I restored the two settings I changed above.