Today I designed a better circuit to measure the TEC's response with the oscilloscope. It is called a bridge circuit, and allows for the output voltage to be centered around 0 instead. This type of circuit is often used for different sensors, and seems to fit our purposes well here. The schematic is attached here.
After I built this circuit (modified the circuit I was previously using), I tested it with the TEC to see how the PID gain calibration looked. This took awhile to get a signal, because it seems like the oscilloscope I was using had some problems. I took data of heating and cooling shown below (didn't bother converting to temperature since we're mostly interested in how the temperature or voltage settles right now).
A lot of the data I tried to take today had the same sort of oscillations as for the cooling data shown above (about 0.04 Hz). However, I didn't see such oscillations when I hooked the circuit up to a multimeter and monitored the voltage changes over time. In fact, the voltmeter suggested that the voltage stabilized much more quickly. I'm going to look at this again tomorrow to see if I can figure out the cause of these oscillations, and perhaps tune the PID gain on the TEC better now that I can see how the temperature settles much more easily and quantitatively.
Today, I also finalized the Solidworks drawings for the insulator that will be used to thermally isolate the laser diode from the rest of the setup, as well as the heat sink that will be in contact with the Peltier element. These files are on the SVN, and I will try to go to the machine shop with these soon. I should have done this earlier.
I will be presenting my project at the end of August, so Tara wants me to put together a talk so we can rehearse next week. I am going to start doing this in my free time.
I spent awhile reading about PID controllers in order to understand how to tune the TEC. P represents proportional gains, and deals with the present error from the set value. I represents integral gains, and deals with past errors. D represents derivative values, and uses the current data to predict future errors. They each affect how the TEC overshoots/oscillates about the correct temperature in different ways. I figured out that the oscillations that I saw yesterday in the heating and cooling data were due to improper tuning of the PID gain. I decreased the integral gain and it seemed to fix the problem.
I also discovered that the oscilloscope was on the wrong setting, with 10x attenuation. I noticed this when converting the data from output voltage to temperature. I changed the settings to 1x attenuation and took data for heating and cooling, shown below. There only seems to be one slight overshoot when changing the temperature by about 1 degree, which is entirely reasonable. The correct temperature settles after about 1 minute.
While these measurements were useful in tuning the PID gain so that the temperature settles quickly, there was a discrepancy in the measured resistance across the thermistor and the resistance calculated from the measurement of Vout. Using the TEC controller, I brought the resistance of the feedback thermistor to 10k, but this resulted in a Vout that predicted a thermistor resistance of 9.91k (0.2 degrees K difference). In order to zero Vout, I had to bring the thermistor resistance down to 9.892k. I'm trying to think of a way to calibrate this difference, but I'm not sure which thermistor is reading more accurately right now. I'm going to read more about using thermistors as temperature sensors to see if there is anything I can try to do for this.
I'm also still trying to think if there's a way to adjust the P, I, and D controls so that I can actually go back to previous values. The controls are unlabeled on the TEC controller we have, so they cannot be accurately returned to specific settings. It seems well calibrated for the moment, though.
Tara would like me to present at the SURF Seminar Day in August (either on the LIGO field trip to the Livingston Observatory or at Caltech), so I spent yesterday and today putting together my presentation and trying to organize the work I have done/plan out what to say. The entire presentation will have to be focused on the noise calculations and design, since we are still waiting on parts to arrive (namely, the collimating lens so we can focus the beam to make a free running noise measurement). The presentation for right now is on the SVN: https://nodus.ligo.caltech.edu:30889/svn/trunk/ecdl/documents
Made some modifications to the Solidworks design. All of these have been changed on the SVN.
Tomorrow morning I will go to the machine shop to get the base plate and left plate modified, and get them to machine a heat sink and plastic insulator.
Today I got the newly machined parts. I put together the TEC element and stuff again and will calibrate the next time I get a chance.
Erica and I practiced our presentations in front of Tara. I got a lot of feedback and I'm going to edit my presentation in my free time outside of lab. It was also useful to see someone else's work to get an idea of how to present.
I'm working on putting together a Michelson interferometer to measure the laser diode free running noise. I don't have the actual collimating lens, so I'm using a f=5mm lens from a fiber optic. I have mirrors and I borrowed a beam splitter from the GYRO experiment. Picture below. I'm working on getting the beams to combine by adjusting the mirrors. Will continue doing this tomorrow.
Today Tara and I worked on getting a noise measurement for the bare laser diode using a Michelson interferometer with different arm lengths. The setup is attached. However, at a differential arm length of 20 cm we were unable to see interference because it was too difficult to focus the beams. Tara suggested I use a symmetric Michelson interferometer to see if I can get interference, since the noise levels might be too high for such a large arm length. I then tried much smaller differential arm lengths and I was able to get interference at 1 cm and 5 cm.
I took background measurements of the noise from the SR785 (about 20 nV/rtHz) and from the blocked photodiode (electrical noise, about 50 nV/rtHz). Since these were both small, we can be confident that the measurements we took are mostly the noise from the frequency of the laser diode.
The results from the 1 cm and 5 cm measurements are attached. We seem to have noise levels close to what we predicted (1 MHz/rtHz), which seems odd since there will be extra noise from mechanical components, temperature fluctuations, and a worse current driver than we planned to use. In addition, this doesn't explain why we weren't able to get interference at a differential arm length of 20 cm. The 5 cm measurements have even lower noise levels for some reason. I'm not sure if I'm doing something wrong with factoring in the gain, so I'm going to check my math. Gain still confuses me a little since there's a different gain on each machine I used. Overall, the measurements seem suspiciously low noise.
I'm going to check these calculations again this weekend to make sure I didn't mess up. I will also revise my presentation so that I will be ready to present on the LLO SURF field trip.
Newest version on the SVN with the latest data and Tara's commentary from my practice presentation. I'll probably end up working on this while in Louisiana if I hear back from Tara about whether I did something wrong with the noise measurements.
Today I tried to calibrate the PID gain for the TEC controller. I noticed some connections needed repairing to I resoldered them, and checked every single connection.
However, the TEC controller still couldn't turn the Peltier element on, citing a "OPEN" problem (I believe according to the manual this means that something about the TEC connections are wrong). I checked these several times with my past notes and the instruction manual, but could not fix the problem. Then I tried cleaning the silicone thermal paste off of the Peltier element and was able to briefly make the Peltier element turn on. As soon as I tried reinstalling this in the ECDL setup, it stopped working. I was able to get the element working again briefly, but it was never stable (would stop working after a minute). I believe that I can use isopropyl alcohol without damaging any parts, but I want to do more reading online before I try this so that I am sure. It seems that trying to wipe the silicone paste out is insufficient, as I spent awhile trying this to recreate my results.
Today Tara showed me where to find isopropanol in order to clean the Peltier element. After cleaning it carefully, the TEC controller worked fine again! I am going to avoid using the silicone thermal paste for now in order to avoid this same problem, but if it becomes necessary I will add small amounts very carefully. I'm not sure how safe it is to clean the Peltier element often. The thermistor is being held onto the diode mount with aluminum tape.
I worked on tuning the PID gain on the TEC controller. It seems a lot less stable than before, having a hard time settling on one temperature. Perhaps this is because I am not using the silicone thermal paste. I want to continue tweaking these settings, although I have them at something reasonably workable right now. It takes a minute or two for temperatures to settle, but they seem stable once a temperature has been reached.
I cleared off a shelf in the ATF lab to keep my things. The collimating lens and lens adapter arrived, and Tara and I had to search for awhile to figure out where he put it (since they arrived while I was away). I put the lens into the lens adapter, and put this into the lens mount. Immediately, I noticed 2 problems which need to be fixed immediately:
Tara mentioned that the TEC may not work as well without some of the silicone thermal paste so I added some and returned the PID gains. Sure enough, this helped the temperature stabilize (and now I know if it stops working to clean out the Peltier element with isopropanol).
I emailed DMass about finding a PCB for building the low noise current controller. (I was supposed to do this last week but it slipped my mind)
I moved the laser diode and socket to the actual laser diode mount (from the Michelson setup used in August). Since the laser diode extrudes, we do not have the problem I mentioned last time with the base plate since the hole for mounting the collimating lens is now close enough that we should have enough adjustability to focus the beam. I searched the ATF lab and found a piece of metal about 3mm high, which will fix the difference in height of the diode mount and the collimating lens mount. However, this piece needs to be trimmed down, which I will try to discuss with Tara. Not sure if we have those capabilities here or if I need to take it to the machine shop?
I wasn't able to find Tara today but I need to talk to him about:
Finally, I'm still working on editing my SURF paper. I'm new to LaTex so it's taking me awhile to perform the edits Rana suggested.
Today, I met with Tara and discussed the delay line, which will be used to tune the wavelength of the bare laser diode and measure the free running noise of the ECDL. I will write up notes of what we talked about and how the delay line will work and post these soon, along with a list of items I will need.
Collecting materials for delay line: I will be using an RF photodiode and the series of lenses from the Gyro setup, which Evan is not using right now. I'm in the process of disassembling and reassembling this setup on the table that I'm using. Evan said the mode matching was already messed up, so I will be working on focusing the beam. I will be using NPRO light from the CTN lab via a fiber cable.
Tara helped me modify the piece I found last time, which goes under the collimating lens mount to fix the optical height. I learned how to tap a hole in the metal. I moved the ECDL setup and got the current driver back up and running, and was able to focus the beam using the collimating lens we purchased. The setup so far is attached.
Tara and I decided on a modification to make to the grating mount which will allow for us to make vertical tilt adjustments (we will have 2 holes with adjustment screws, not one). I am going to draw this in Solidworks so that I can get it machined tomorrow at the Caltech machine shop.
I got the modified grating design into the Caltech machine shop; this part should be done by tomorrow. We decided to use 2 vertically placed 1/4-80 holes which will have adjustment screws. This will allow for tilt adjustment.
I found a mixer and a splitter in the CTN lab plus the appropriate adapters to use. I'm still working out how the cable length difference will affect the sensitivity of our measurement.
I have the PD removed from the Gyro table along with the lenses that were used to focus the beam to go into the PD. This was more difficult than I expected to remove these pieces since I'm short and didn't want to disturb the other setup. We are still need several things:
I'll go pick up the modified grating mount from the Caltech machine shop tomorrow so that I can wash it tomorrow afternoon (I don't have much time tomorrow) and do more on Wednesday.
Note: last week I picked up the modified diffraction grating mount. I forgot to bring it in today but I'll put it back in the ATF lab on Thursday.
I've spent the last week reading a few papers Tara sent me about mode matching/old elog entries by various people. I couldn't find Tara around the lab today, so I'll try and talk to him this week to figure out exactly what I'm doing. I'm still a bit confused about how to do the setup, although I've been starting to sketch what I'm planning on doing. I'm also messing around with a few mode matching programs to help me plan my setup.
I sent up the power supply for the PD and confirmed it works. I'm going to try to talk to Tara tomorrow or Friday so I know what I need to do in the next week. My midterms are starting so I may have a hard time being around the lab much until afterwards.
I increased the power to the 14.75MHz EOM from 0.5 dBm to 3.5 dBm. This was done by changing the attenuator from -6dBm to -3dBm.
As a quick test to see improvement in electronic noise level, I increased the detection gain. The modulation depth was changed from 0.14 to 0.2 rad (following the EOM's paramters in psl:855). I could not see any improvement yet. So I'll try to damp the mechanical peaks in the signal so that flat noise level can be measured better.
The slope of error signal for RCAV and ACAV are 44.25/57.62 kHz/V
I changed the attenuator to -1dBm. From a quick measurement, there's no improvement in the beat signal at high frequency. Any improvement in the band we might be able to see is dominated by acoustic and PLL. [more details later]
I got an EOM driver from Rich Abbott, I'm checking if this thing works well or not.
The EOM driver is just an amplified resonant circuit with +/- 18V input. With the driver connected to a broadband (BB) EOM, we can use the combination to add sideband to the laser. This is better than a resonant EOM because we can pick a range of frequencies, instead of having a fixed one from the manufacturer.
I checked the TF between input and the RF mon, the resonant peak can be moved between 20.7 MHz and 24.5 MHz by adjusting an inductor on the board. Since the RFPD for the PMC is 21.5 MHz, I'll use it to check the modulation index of a BB EOM equipped with this board.
The plan is
Once I verify this I will check the frequencies for refcavs and pmcs, so that I can decide the value of L and C on the board.
The EOM driver is working. For the same modulation depth, it can drive a broadband EOM using less power.
==measurement and result==
I used PMC setup to test this EOM driver because its frequency range is only ~21 - 24 MHz, and the sideband for locking PMC is 21.5 MHz. So what I did:
From the schematic, the board is supposed to have 30V output from 0.15 V output (x200). In dBm that will be 20log(200)~ 23dBm. So It is roughly ok.
fig1: BB EOM with the driver. One of the unused output is for output mon.
fig2: Error signal from scanning the laser, with BB eom and the driver, measured at mixer out.
The board is definitely working and will benefit us, definitely for locking PMC. If we use a marconi to drive a BB EOM, the max output is 13dBm. The power is halved (one to the EOM, one to the mixer). That means ~ 10dBm to the EOM (we will probably split more some where for RFAM pickup, but we can do that on the line that goes to the mixer), so assuming we have ~ 10dBm for the EOM. With the board it will be ~ 10+18.5 dBm = 28.5dBm (~6V) . It should give modulation depth of 0.09, see psl:745, This might not be enough for locking the refcav( see,psl:929. where we have beta of 0.18), but we can add another RF amplifer, or use the board for PMC servo . I'll check what are the appropriate modulation depth for locking PMC and refcav.
We modified the EOM driver, so that the resonant frequency is now~ 14.75MHz. The full test will be done later.
As mentioned in PSL:1311, the resonant frequency on the EOM driver was not at 14.75MHz. Evan and I discussed about how to modify it and decided tof change L4 from 1.4uH to 3 uH, see the schematic here.
above, the driver after the inductor was replaced. The new one has a shield to reduce any magnetic field leakage. The legs are not fit with the footprint on the PCB, so I had to solder it to another wire to reach the footprint.
above: the TF of the driver measured between the drive and the mon output. Red trace shows the TF before the modification. Yellow trace shows the TF after the modification, notice the peak is at 14.75MHz, the Q is about the same.
Yesterday I think I narrowed down the source of the 2 kHz frequency noise hump: it is voltage noise from the TTFSS being injected into the broadband EOM.
With the north cavity unlocked (and the TTFSS set to "test"), I monitored the (undemodulated) RAM using the auxiliary 1811 and the HP4395A. There were clear, broad 600 Hz humps on either side of the 14.75 MHz carrier. It disappeared when I unplugged the drive to the broadband EOM.
Then I looked at various test points on the TTFSS HV board with the SR785. On the COM → EOM path, the TF shaping takes the COM noise and produces (what I think is) the same 600 Hz bump, which is then sent to the EOM. In the beat, the bump appears at 2 kHz because of the north TTFSS boost; with the boost off, it reverts to 600 Hz.
This is the case on both TTFSS boards, but it only leaked into the beat on the north cavity. So I suspected it was an issue with how the EOMs are aligned on the north path. On north, the BB EOM was immediately followed by the resonant PDH EOM; on south, between the BB EOM and PDH EOM there is a PMC, an FI, and some other optics.
I moved the resonant EOM so that it follows the EOAM. After the post-EOAM PBS, I did the following:
Then I redid the mode-matching into the north cavity and measured the beat. I kept it locked for about 90 minutes and didn't see the 2 kHz hump appear, so I'm guessing this solved the issue.
On saturday a qualitative effect of the modulation produced by the EOM located in the PDH-north loop has been checked.
The goal was to have a look at the error signal of the PDH-north while the laser PZT was scanning frequecies around the two s-p TEM00 resonances. Because a that time I did not find the right error-signal connections on the FSS board (next elog will clarify where it is) I have demodulated the signal with an external mixer (and with a low pass filter) and monitored it. The picture shows the error-signal that we have with this setup:
Before the installation of the AEOM in the South cavity I wanted to have look to the beam profile along the paths. EOMs provokes distortion of the beam shape which may affect our mode-matching. It is important to keep the beam very small (200-500um diameter).
I think they are ok in the North path, a bit less good for the south path. Anyway I am going to use the beam as it is for the AEOM in the South path, replacing the EOM 21MHz used for the PMC with the AEOM that will be used for the ISS.
The pictures show the beam profile with the measurement done and with some ABCD matrix simulation for North and South path. They should come with an optical layout which I will make as soon as I will get OMNIGRAFFLE. I use inkscape but I will avoid that in order to be compatible with Rana and Aidan.
The AEOM has been installed in the South path replacing the EOM 21MHz used for the PMC. There is a high noise that I clearly see at the photodiode in transmission.
When I have placed the AEOM in the path I have decided to take the alignment of the previous EOM as reference. Not ideal because the reference should be the incoming beam. The beam is not parallel to the table and it was decided to be as less as possible invasive. The mode matching and the alignment gave at that time 20% of visibility (at each polarization). After the installation parameters where unchanged. Later I have improved the alignment bringing the visibility at 30% for both the polarizations. After that, when everything was in place I have easily locked the cavity but the power in transmition was showing a very high noise. I have spent all the day trying to twick the alignment because and servo loop gain, but we need to solve this before going further. My back does not allow me to proceed for today.
I have also noted that the South Laser which is labeed 2W laser has the lambda/4 and the lambda/2 rotated in a way that at the output of FI we had few mm. I am not sure if damping the power at the FI is a good thing.
Just a quick noted about some resudual beam spots I noticed after the second PBS in the south path (after the AEOM).
When I went to minimise transmitted power through the second PBS with a power meter two spots were present and most obvious. I didn't really understand where these would come from. Checking further up the path, these features are not present before the AEOM and PBS. I swapped out the PBS, then the waveplates: no change. I then compleatly removed the AEOM from the path (but not the PBS) and the spots were removed. The beam looked well centered (by eye) and had good cleanance from the edges of the aperature.
Antonio thought it might be some alignment issues with the modulator and did some precition alignment. His method was to look at power at the output and maximise (we measured power of 296 mW and out of 280.7 mW which gave an loss of ~5%). He also fixed the CCD viewer position and compaired the computed center of the beam with the AEOM removed and then installed. This seemed to improve the non-polarisation filtered beam throughput and improve the shape of the beam.
The optimal operating point for the AEOM should be at 50% throughput through the PBS so the remianing junk spots should be small. Still not sure what effect is producing the residual remaining spots were. This is something we should keep in mind.
It appears that the EOM driver in the north path is not amplifying RF signals as we might expect. This is used to drive the broadband Newfocus 4004 EOM at 14.75 MHz. On Friday I had disconnected it from its power supply to re solder a banana plug. I don't have a record of the voltage used so I have used the specified ±18 V in its schematic (see PSL:1090). It was not this value before, I think I remember ~14.5 V
I have now managed to reoptimize the alignment into the north cavity but was unable to lock. It appears that there is no error signal being generated. I checked the RF coming out of the amplfier with it powered up, it was -28 dBm (with 50Ω load of the Aglient 4395A) the 14.75 MHz RF signal going into the amplifier was -14 dBm. It is evident from Tara's posts on testing and returning the EOM driver (see PSL:1092 and others in that sequence) that gain should be on the order of 18.5 dB to 23 dB. Something is not right.
When I power down the EOM driver all the RF goes, so it is actively doing something. I hooked the input and output to the Aglient and attempted a transfer function (attached below). Its not obvious that there is any resonant amplification. Not sure what is going on here. It is clear that that it seems to be attenuating signal going through it and not helpfully amplifying.
I measured the transfer function with the Aglient, strait through from input to output, I realize now that this isn't quite right because the driver is designed to be loaded with the EOM at its output to work properly.
I did this again with the EOM driver connected directly to the Newport 4004 and looking at the response through the monitor port. There is kind of a a peak but nothing like what Tara had here PSL:1317
I realize now, that the EOM driver needs to be connected to the EOM to operate correctly. However, its still no clear why the transfer function has changed shape and what is leading to no error signal being produced at the monitor port of the FSS.
Although I haven't done anything more in determining why the shape of the EOM driver transfer function doesn't quite match what Tara initially measured, it appears that we are not getting an error signal for the north path. Its possible that the units are/aren't logarithm in the previous post compared to what Tara measured. All this aside, it seems to peak at about the right frequency (14.75 MHz) and have the right gain (as measured at the monitor).
I just noticed that the power supply lines going into the unit are bare (picture). Its possible that when I was trying to obtain an error signal on the oscilloscope, they were just shorting. This might explain some intermittence in the past. I will heat shrink these later in the day. With them off the table I now see a decent error signal in line with previous checks of its Vpp.
I'll mark this mystery down as solved.
For the current phase modulation setup for the FSS loops we are using one resonant EOM at 14.75 MHz and one BB EOM driven by one of these boards: DCC-D1200794 which has been modified to be resonant at 14.75 MHz. There is a RF resonant stage and a buffered monitor. It is not clear to me how the transistor works in this circuit, doesn't it clip the +ve voltages?
Tara sourced this board from Rich Abbott (see posts in this thread PSL:1090) and I don't have any more. I also don't have experience building RF stuff so no sure design aspects of PCBs for this. All the files are on DCC maybe we can order new PCBs? Or do we want a different design?
Update (Wed Jan 4 12:25:27 2017): added picture.
The DC Power Supply in the EE shop had a broken negative banana plug terminal, so I replaced it.
Next is to take a transfer function of the EOM driver with -/+18 Volts.
The first diagram shows the set-up without the Bias Tee. Here we will confirm the monitoring out channel is functional and the dummy EOM acts as expected. After we will add the Bias Tee as shown in the second diagram. Using the monitoring channel again we can then see the effect of adding the Bias Tee to the output of the amplifier circuit, but before the dummy EOM.
ws1 is unable to read and write on some EPICS channels while I can see these channels in fb4 or acromag1. These channels are:
I'm not sure what is causing this. I have rebooted cromag1 several times but this problem persists. Interestingly, there are a lot of channels which are getting updated on medm screens so the origin of the problem is probably localized to a single .db file. But everything looks fine to me, at least after first few debugging trials.
Well, because of this, it is impossible almost impossible to even manually tune the beatnote frequency to the required point. I'll first fix this because it seems like an error which shouldn't be ignored. Suggestions are welcome as I am new to EPICS-Modbus-upstart-docker things and might be missing something silly.
Three newly repaired SR560s have arrived. I put two in EE lab, and one in PSL.
There is a 35 MHz pick up from cables to the crate. Right now there are ACAV and RCAV_RCTRANSPD that cause the pick up.
When I unplug the cable to the crate and just measure directly at the PD output, the signal is fine.
I switched the RFPD between RCAV and ACAV, now the gain for FSS loop is set to 26 dB, but the beat signal does not change.
From previous elog entry, RCAV's RFPD does not have a peak at 35.5 MHz, and ACAV's RFPD has a peak ~36 MHz. And the FSS loop did not have enough gain,
so I switched RFPDs between both cavities. Attached pictures below show the error signal from RFPD when the cavities are scanned, before and after I switched them.
The power into the cavities are ~ 1mW for both cavities.
The phase is adjusted to produce the best error signal.
Phase adj were 4.16 then changed to 5.67 V
I also inverted the phase on PDH box for ACAV.
FSS gain can be reduced from 30(max) to 26 dB.
Then I measured the beat noise, with 100 kHz input range, gain 5. There are no change compared to before.
Last week I accidentally changed the polarization of the beam to the 35.5 MHz EOM. So I optimize it again to minimize an RFAM effect.
I used the signal of the transmitted beam behind RCAV on the PD for beat signal. Since the cavity pole is around 37 MHz, I should be able to see the
signal at 35.5 MHz easily. I connected the RF signal from the PD to a spectrum analyzer and adjusted the 1/2 wave plate to minimize the peak at 35.5 MHz.
However I also notice two peaks at 35.29 and 35.71 MHz (35.5 +/- 0.21 MHz) which are approximately the same size as the 35.5 MHz peak.
I'm not sure where they come from.
got some 2mm InGaAs photodiodes from Peter. So we can go ahead and replace the dirty one from the ACAV RFPD tomorrow and re-tune it to 35.5MHz.
I also got the Jenne laser and its power supply from 40m for TF measurement. They are on the PSL table.
we replaced the photodiode in the RFPF fro the reference cavity (RCAV). The old one looked like shit. Below pictures of the old and new photodiode.
old photodiode - picture 1
old photodiode - closeup view
The ACAV RFPD stopped working this afternoon. It had high current consumption on the +15V supply, causing the supply to drop down to 2.3V.
I turned out that the logic IC (U8, see schematic) was broken and so the +5V (internally regulated) caused the high current flow which is supplied from the +15V.
I removed U8 entirely as we don't need it. Pin7 of U2 can be left open according to the datasheet in order to enable the device. Diode is now working again.
Already yesterday we made several changes in order to make the RFPDs for both cavities the same. Basically more DC and AC gain.
Attached the modified schematic (only page 1) for ACAV RFPD. Changes are in red.
The TF of 2 35.5MHz RFPD and 21.5 MHz RFPD are measured by the Jenne laser. RCAV's RFPD has the better response, while ACAV's RFPD peak is slightly shift to 37 MHz. But the peak is low Q so we lose only a few dB. PMC's RFPD is now tune to have a resonance peak at 21.5 MHz and a notch at 43 MHz as it should be.
I adjusted the temperature on ACAV and RCAV so that both cavities can be locked simultaneously.
I measured the beat noise and see improvement at low frequency after we fixed the RFPD.
After we reinstalling the RFPDs on the table, we need to wait for the temperature to settle before both cavities can be locked.
(The temperature readout jumps by ~0.05 degree if we work on the table. The cause is not known yet. we tried pulling the cables for T readout,
but this does nothing to the T readout. This problem will be investigated soon.)
I check the beat noise and see some improvements at low frequency . It does not look a lot for now, but I think it might go lower
if I re align the beam to the cavities and the beam for beat measurement. It might drift away a bit during the break, and I haven't checked that yet.
The input power is ~ 1 mW for both cavities.
RCAV gain = 25
ACAV gain= 8.0 on the knob.
I've measured the TF of 500ft of RG58C/U cable to see if the loss is about the same calculated by that piece of software i've found.
I've measured 33.4dB attenuation for the LCOM cable, the calculated value is 32.1dB for some unknown RG58C/U cable.
So i think we should go ahead and calculate the required length and try it.
This is a picture of the 500ft spool we have. As you can see there is only about 1.5inch on that spool, the rest is empty. Height is a few inches.
So regarding the size we can easily have several 100m of cable in a small package
some old designs i've built some years ago using the cheaper DIP versions of the matched BJT pairs from Analog Devices.
Designs are DC-coupled, gain 1000 as they were designed for measuring the noise of photodetectors.
Given values are not 100% what i've used later, only for drawing the schmatics (e.g. gain setting resistors or compensation), but order of magnitude is right.
Eagle-files + Documentation:
We are measuring the noise level of SR560 at low frequency ( ~10mHz) , with the gain 1e4, AC couple, low pass 30 Hz. The data is recorded by C3:PSL-GEN_DAQ14 channel.
The input is terminated with a 50 ohm terminator. The setup of the SR560 under test (S/N 02785) is the same as the one we are using now.
Tomorrow we will measure the noise level of the whole setup (SR560, cables, all mixers and amplifiers included.)
time 11-2-11-3-32-31 (yy-mm-dd-hh-mm-ss)
Disconnect the RFPD signal and terminate with a 50 ohm to measure the noise floor of the cable delay method.
Starting time : 11-2-11-22-35-33 (yy-mm-dd-hh-mm-ss)
The noise floor for cable delay method, and SR560 noise level are measured. The result is plotted below. We are not sure about the
Volt to Hz conversion yet. Once we know the calibration factor, we can decide if we have to replace SR560 with something less noisy or not.
The Red and Blue plots are taken through daq (16 Hz)and FFT, then divided by the gain setup (1e4) and corrected for AC couple.
Green and Purple plots are taken via SR785 for fast measurement.
From the plot, we learn that,
1) SR560 turns out to be the dominating noise source for the current cable delay technique, from dc to 0.2 Hz, since the noise level from SR560 alone
and the whole setup are comparable.
2) The flat part from 200mHz to 2 Hz is probably the noise from analog to digital converter (ADC)
3) SR560 has corner frequency around 100 Hz.
Peaks from 60 Hz and its harmonic become smaller if we disconnect the power cord, and run SR560 with its battery, see fig2.
%[y,f] = myasd('C3:PSL-GEN_DAQ14',981430365,3600*10,1024,16);
%[y16,f] = myasd('C3:PSL-GEN_DAQ16',981430365,3600*3,1024,16);
%[y,f] = myasd('C3:PSL-GEN_DAQ14',981430365,3600*10,1024,16);
%[y16,f] = myasd('C3:PSL-GEN_DAQ16',981498948,3600*3,1024,16);
%correct for 30mHz AC couple on SR560
ycorrect = y .*...
to have real data for comparison i measured the noise of the SR560 for different (high) gain settings.
Will add lower gain settings and line powered measurements later.
test setup: SR560, AC-coupled, low-noise setting, battery powered measured with SR785
datafiles contain raw output noise values, so for input referred noise plz divide by gain given in filename. first col = frequency, second col = noise spectral density
We remeasured the input referred noise(IRN) of the UPDH box, and sensing noise (RFPD + mixer +LO) for ACAV loop.
Sensing noise is ~2 order of magnitude higher than UPDH's IRN.
We do this because we want to check if electronic noise in ACAV loop is limiting our beat signal or not.
Data from yesterday is not good because I did not cover the whole frequency for PSD measurement,
and got the wrong TF for UPDH, and wrong sensing noise. So I redo the measurement and plot the noise
in Vrms/rt Hz instead of converting it to frequency noise for debugging purpose.
To measure sensing noise (RFPD+mixer+LO)
1) block the beam on the RFPD and measure the signal after the mixer (after the low pass filter too, of course.)
the noise is flat and roughly around 1 uV/rtHz.
To measure UPDH's IRN
2) terminate the UPDH with 50 ohm (it is self terminated, so just unplug the input) and measure the output of the PDH,
boost off, gain 1.5 on the knob.
3) measure the TF of the UPDH
Both measurements are not limited by SR785's noise floor which is ~ 5 nV/rtHz flat at 100 Hz and above.
3) divide the UPDH signal from (2) by TF from (3) to get IRN.
The IRN (cyan) looks good. At 100k where the gain is 40 dB and at 10k where the gain is 20 dB,
the IRN is lower than the raw data by 2 and 1 order of magnitude respectively, and the crossing happens at UGF (1kHz),
so everything looks ok so far.
However, the noise from the error point (purple) (same point for sensing noise) when the systems is locked does not match the noise we have now
(data from yesterday.) I'll redo the measurement to see if it's the same or not.
I also notice that the sensing noise alone is already higher than the beat. The slope of the error signal is
cavity's FWHM / Vpk-pk = 108 kHz / 428 mV = 0.25 MHz/V.
So either the measurement for RFPD is wrong, or the slope of the error signal I got is wrong. This will be investigated next.
Just for fun, I match the 120Hz peaks on the RFPD noise and the beat, the calibration factor is 3.78e4 Hz/V. Although I did not take the contribution from other electronics into account, the RFPD noise sits a bit below the current beat. If this were true, we will have to fix the RFPD next.
nice, much better, i only have two little things at the moment:
we should check the mixer signal, the noise level is too high. We should compare it with the other EP signal (of the other cavity). If there is a huge difference between both we should start searching there. We should also check the RF noise level of the RF-PD. Maybe something is broken there.
I think the reason of the high mixer noise is the amplifier for the RFPD signal before the mixer. I remove it for now and the noise seems comparable with that from RCAV's RFPD. The beat measurement between with and without the amplifier is quite similar.
the result will be posted soon.
1)Here is the noise at the error point from ACAV loop during lock.
The mixer signal is really high. The sensitivity range on SR785 was on -50dB. To check that there is no weird coupling
between the signal and the spectrum analyzer I tried changing the range up to - 20 dB, but the noise level is still the same ~ 1uV /rtHz.
2) slope of the error signal,
a) connect the triangular signal from the function gen, 10Vpkpk, 2 Hz to piezo sweep ch on the PDH box, turn the switch on.
b) unplug the mixer signal from the PDH box, connect it on the scope, ch A. connect the triangular signal on ch B.
c) pk-pk value is 424 mV.
I also tried sweeping on the FAST channel. The triangular signal is sent to FAST on the laser, 10Vpkpk 20 Hz
and read out the mixer out signal again. I get the same pk-pk value, 424mV.
Then the width of the linear region is approximately FWHM of the cavity108kHz, (cavity pole x 2), the slope is then
108kHz/424 mV ~ 0.25 MHz/V