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  SUS Lab eLog, Page 26 of 37  Not logged in ELOG logo
ID Date Author Type Category Subject
  665   Thu Jul 11 01:01:30 2013 GiorgosSummarySUSLED connections, Power Boards, and DC magnetic field boards

LED connections

We use LEDs to indicate whether our power boards work. For one power board, we need two LEDs, one for the positive and one for the negative voltage. For boxes that contain more than one power board, we will still use onlyy one pair of LEDs, since we only care to test whether our power supply works. We have six LEDs and we will use 2 for the HE sensors board, 2 for the coil-actuation box and 2 for the strain gauge box. Today, we made the connections for our LEDs; on our power boards, there are two letters: A and K. A is for the signal wires (positive/red or negative/black) and K is for the ground wires.

Power Boards

The missing components for our power boards have arrived, so we finished our power board circuits, drilled the holes on the box panels and screwed the power boards. We also created more power lines, such that we have enough for the 7 coil-actuator and 3 DC offset boards, as well as the HE sensor boards. We prepared power lines for all HE sensor boards and 6 coil-actuator ones, but ran out of components; we still need to create power lines for one coil-actuator board and 3 DC offset ones.

DC magnetic field offset

For the boards that will provide tuning of the magnetic field, we only use a current booster circuit (configuration with a current buffer in a feedback loop with a low-noise op-amp). We built all three boards.






  664   Tue Jul 9 18:51:42 2013 BenDailyProgressCrackleCreating a Script to minimize correlation from

 Minimizing the coherence of the intensity noise signal.

We need to use the difference between the the output voltages of the PDs at the symmetric and asymmetric michelson ports. This lets us eliminate intensity noise while not messing with the displacement data. The gains of these two signals (Vas & Vsy) may not be quite the same though, so a small level of coherence may still exist. I created a MATLAB script which takes in Vas and Vsy signals, and find the coefficient by which we should multiply one of the input voltages in order to minimize the coherence.

To do this, I first created a simple function (integralCoherence.m) to find the integral of the coherence for a given Coefficient and given Vas and Vsy inputs. The coherence is found using MATLAB's mscohere function.

Next, I created a function which uses Newton's root-finding method to detect which coefficient value would give integralCoherence the smallest value possible. In other words, this function (rootFinder.m) minimizes integralCoherence(Coefficient). To do this, I seed the function with a coefficient of 1, then ask MATLAB to find the first and second derivatives of integralCoherence at 1. The first divided by the second derivative is a quantity I call the "jump." The best coefficient must lie somewhere in the direction of the jump according to Newton's method. We add the jump to the original coefficient and start over. The process iterates until the jump size is less than 1e-6, which means that we are converging on a best value for the coefficient. That coefficient is returned.

Finally, I made a test script which loops over several possible offset values for the Vas signal (relative to the Vsy signal), and returns the best coefficient to minimize the hypothetical intensity noise in each case. I have included a plot of the percent error of each output coefficient with respect to the input value. Although we are seeing some trends in the error, this does not seem to be too much of an issue, as the relative deviation is generally well below 1e-3.

Tomorrow, I will create a function that takes real-life Vas and Vsy values as input and outputs the best coefficient.

Attachment 1: RelativeDeviation.pdf
  663   Tue Jul 9 18:13:29 2013 GiorgosSummarySUSStrain Gauge Voltage Offset

Strain Gauge Boards

Our conditioning boards did not have a low-pass filter. That is a problem, since these circuits were designed to amplify a DC voltage offset, but the op-amp cannot provide that gain at very high frequencies. We introduced a capacitor to create a low-pass filter and made sure the cut-off frequency of our setup was lower than the one of the op-amp: f= 1/(RC*2pi). For our R=24kΩ, we chose C=0.1μF. So, we built a low-pass filter for our 6 strain gauge boards and then measured the DC voltage offset. Our digital voltage meter can read up to 200mV, so we adjusted the one adjustable resistor to get the offset voltage as low as a few mV. As we slightly pussed on the strain gauge sensors, the voltage increased indicating that our circuits work fine.

Panels for Coil Actuators & Hall-Effect sensors' power boards

At the end of the day, I marked the holes for 5 power boards on the panels of our coil (3 power boards) and Hall-effect sensors' box (2 power boards).

  662   Tue Jul 9 01:24:56 2013 GiorgosSummarySUSHE sensors test, arrangement, and offset & Strain Gauge arrangement

Today, we first talked about the connection of the HE conditioning boards to the HE sensors and the arrangement of the wires on the connector. There are 7 HE sensors named after their position (e.g. W=West). Starting from the right, the first four pins denote the sensors that lie above the plate.The bottom row is the bottom part of the connector to the HE signal. X denotes the pins not used and the last three pin places on the left are for the sensors imbedded in the coils, which are though--for the time being--not used.




We tested the transfer functions (TF) of our 7 HE sensor conditioning boards. Six of them had identical TF, same as the ones we expected and one of them (S1) had a similar TF, but a totally different phase. We extracted them to a floppy disk and inserted them to a computer, where we created files that contain the data of the TF plots. Tomorrow, we need to plot the data in Mathematica. We also measured the offset for our Hall-effect sensors on the oscilloscope. We used Vin=0 to measure the actual offset and then adjust R2 to null it. Here are the recorded offsets:

AC1:2.37V, AC2: not working, AC3:2.34V, S1:2.5V, S2:2.5V, N:2.5V, W:2.46V.

We also looked at the connector for the Strain Gauge (S.G.) and DC motors (M). We have six connections for each. We named our S.G. boards, depending on the location of the corresponding--in our setup--strain gauge. IMoving from the right to the left, the strain gauge sensors correspond to: TS (top south), TW, TE, BN (bottom north), BS, BE. We found a problem with the BE op-amp; it must be broken. We tested the output signal of some boards and we did not find a steady DC amplified voltage we expected; we thought of introducing a low-pass filter (since DC signals have ideally a 0Hz frequency)before the signal reaches the strain gauge op-amp.


Tomorrow, we will measured the TF of the Strain Gauge boards to see what is wrong. We will also insert a low-pass filter with a cut-off frequency around 10Hz.

Attachment 2: Strain_Gauge_&_Motors_Connections.png
  661   Wed Jul 3 22:56:53 2013 GiorgosDailyProgressSUSADC/DAC controllers, Power Boards

Today, we completed the power boards and tested to make sure they work. We had a problem with the LM2991 negative voltage regulator; its tab at the back is connected to the input voltage and should therefore not touch our panel, because it is in this way grounded. We will fix that problem by placing spacors below the power boards.

We also built the connections for our DAC and ADC controllers and designed our DAC & ADC panels so that we can order a company to drill holes for our wires.

To this point, we have to finish the circuits for some of our Hall-effect sensors boards (our missing components arrived) and test all of our circuits to make sure they work as desired. After that, we will move to the second stage of this summer research, taking measurements, debugging problems, improving computer processing, and ultimately successfully isolating the levitated plate up to very low frequencies (starting from 5Hz and hopefully going down to 0.01Hz)

  660   Wed Jul 3 00:01:51 2013 GiorgosDailyProgressSUSDAC and ADC controllers

On Monday and Tuesday, we talked about the ADC & DAC controller and the time signal input from the function generator.

We also designed the boxes, drilled the holes and wired up both the ADC and DAC controller.

In the meantime, we received the components that were missing from the power boards (precise resistors). We started wiring more power boards and we will finish them tomorrow morning.

  659   Mon Jul 1 17:17:35 2013 nicolasLab InfrastructureVacuumSome vacuum pumps that we saw in the Ogin lab

We went into the Ogin  future Crackle Lab and found a few things that looked like vacuum pumps. There are two roughing pumps (one has the words LEAKS written on it), and most of a turbo pump. They all look like replacement parts for the presumably working pump setup that is hooked up the the thermal noise chamber in that room.



  658   Mon Jul 1 08:46:47 2013 Edward TaylorDailyProgress Mechanical Loss of Silicon Flexures (With Comsol Models)

I include comsol models of 6 different eigenfrequencies for a certain silicon flexure. In addition,  the expected graphs of the thermoelastic noise and phonon-phonon loss are also presented for the various mode shapes.

Attachment 1: CantileverFreqandNoise.pdf
CantileverFreqandNoise.pdf CantileverFreqandNoise.pdf
  657   Mon Jul 1 08:08:22 2013 Edward TaylorLab Infrastructure IR Lab Cryostat Photos
CryoSUS / IR labs Cryostat
  656   Fri Jun 28 16:54:02 2013 GiorgosDailyProgressSUSLow-pass filtering, Current booster, and Design of the Box Panels

Today, we determined our low-pas filter circuits and its components. To achieve our cut-off filtering, we need to change the values of our zeroes and poles of the transfer function. Specifically, we saw that only if P1<Z0<P0 can we achieve low-pass filtering.Low-pass_filter_for_dewhitening.png

It is essential that are cut-off frequencies are very close to the values we used in our high-pass circuits, so that we achieve perfect de-whitening, especially for the two lowest cut-off values; we computed the values of our resistors and capacitors that yield such transfer functions and started wiring our 7 actuator PCB boards. However, the low-pass circuits cannot source a lot of current and so our PCB boards, apart from the low-pass filter, also include a current-booster circuit, which consists of the high-noise current buffer that is in a feedback loop with a low-noise op-amp. Later, we designed the box panels where we will place our 7 PCB actuator boards, along with 3 DC control boards, and 3 power boards.

  655   Fri Jun 28 15:08:25 2013 BenDailyProgressCrackleFirst Data Analysis for Frequency Noise of Asymmetric Michelson

 I converted the raw Volts/rootHz data retrieved from the asymmetric Michelson experiment to frequency noise using the proportionality constant derived in Eric's June 19 Post. I then plotted frequency noise as a function of frequency on a log log plot. The noise for the asymmetric setup is shown in red (it is the noisiest). The other three data sets can be thought of as components of the total signal. The symmetric setup (in green) theoretically give us the noise for everything besides frequency noise from the asymmetry. The red line was found by taking the spectrum of the laser shining directly into the detector (the intensity noise component), and cyan curve is from when the laser was turned off.

Attachment 1: NoiseData.pdf
  654   Fri Jun 28 13:04:27 2013 Edward TaylorLab Infrastructure Experimental Cavity Dimensions

The bottom of the cryostat contains a chamber where the main components of the experiment will be contained. This space is cylindrical in nature with heighth = 5.6cm and diameter = 13cm. In addition, the cylinder has two inner lips which create an inner diamter = 11.4cm. Furthermore, hole spacing for the screws that will attach the apparatus to the cryostat is approximately 1cm.


Also, we are working with a square window through which a laser will pass. The opening is an embedded circle with diamter 2.7cm and the square itself having length = 5.5cm.

  653   Fri Jun 28 11:47:03 2013 BenDailyProgressCrackleMeasuring Spectrum of the Asymmetric Michelson

 Yesterday was very productive. I began by continuing to align the two output beams horizontally of the asymmetric Michelson. Unfortunately, it wasn't working - I was limited greatly in my range of motion by the fact that the long arm had such a narrow window in which it could both come back in perpendicularly, and still not be reflected straight back into the laser (which would be bad for the laser). Eric suggested that instead, I try to use the laser he had been using for his crackle experiment which travels through a long, fiber optic cable. This gave us two benefits. First, the fiber optic cable is supposed to act as a collimator, so our lens would be unnecessary. The spot size would automatically be the same everywhere along the beam, including at the detector. Also, the tiny radius of the fiber optic "groove" meant that there was basically no chance of shining the laser directly back into the tube and into the laser. This allowed me to align the beam out and back along the same path, which was less challenging than before. After some fiddling, it worked! We could see fringing effects clearly. (slightly hard to see in this photo)


After replacing some of the mirror mounts with more sturdy "Polaris" mounts, we were ready to take some data. Eric positioned the beam so that we were sitting right on a fringe (the PD output voltage would wander around with slight shakes of the table or loud noises), and he let the spectrum analyzer take its data. There was a lot of unexpected noise in the signal. We wondered if this was acoustic noise, so I covered the setup with a large blue plastic box (with a hole cut out for the long arm). Suddenly low frequency noise (less than 10Hz or so) dropped significantly, so we decided to use the box for our measurements. Eric took a noise spectrum both for the asymmetrical setup, and for a symmetrical setup with both arms just about 8.8 cm in length.


Here is an overhead view of the asymmetric setup. The source is at the bottom, and the two grey mounts are the end mirrors of the arms. The detector is on the right, and the other two mirrors are used to bring the beam to the correct vertical level. The small iris in the bottom right is irrelevant.


Here is the long arm of the Michelson which passes outside of the blue box in the asymmetric setup.


 Additionally, yesterday I learned how to plot the noise spectrum density using MATLAB's pwelch function. I also learned to make good-looking plots. Here is the PSD for 4 random signals that Eric came up with.


  652   Fri Jun 28 00:30:46 2013 GiorgosDailyProgressSUSHall-effect sensors circuits

Today, we analyzed the transfer function of our circuit:


 Specifically, Vout/Vin= -((R_1 〖+R〗_2)R_3)/(R_1 R_2 ) {((s+Z_o)P_1)/((s+P_o)(s+P_1))} where Z0, P0, and Pare important parameteres of our transfer function.

Were we to graph it against frequency:Transfer_function_of_Hall-effect_sensors_circuits.png

So, we determined the values of R1, R2, R3, C1, and C2 so that Zo occurs at 0.1Hz (close to the estimated natural frequency of the levitated plate), Po=50Hz and P1=200Hz Specifically, R1=R313kΩ, R2=1.5kΩ, C1=2.2μF, and C2=61nF. We did not have a 61nF capacitor, so instead we used a 47nF one (slightly changing our P1 point).

Our Hall-effect sensors give a constant 2.5V when no magnetic field is present. Therefore, we need to include an offset of 2.5V. We will achieve that with a voltage divider with R1=13kΩ, R2=4.3kΩ, R3=R4=2.2kΩ.

In the afternoon, I wired up 7 PCB boards for the Hall-effect sensors circuits. These include the -2.5 offset and the high-pass filter. We used the spectrum analyser to see whether the transfer function of our circuits are as predicted; the experimental and theoretical data agreed.

  650   Wed Jun 26 22:58:44 2013 GiorgosDailyProgressSUSHall-effect sensors and transfer functions

Today, I wired up the power PCB boards for the DC motors and then talked and read about Hall-effect sensors. The box that will hold the sensors is too small, so we spent some time figuring a way to combine two boxes into one to fit all of our components. Then, we spent a lot of time talking about transfer functions, Laplace transforms and how they help us disentangle linear, time-dependent equations from their time-relationship and yield convenient equations that describe the behavior of a system at different frequencies.

We want to apply a high-pass filter to "remove" the low-frequency noise from the signal in our Hall-effect sensors and achieve low frequency seismic noise isolation. So, we used the following high-pass filter circuit and found its transfer function Vout/Vin. It is my homework to find the values for R1, R2, R3, C1, and C2, so that our transfer function gives large output at high frequencies (above 50Hz) and small output at low ones (below 5Hz).


  649   Wed Jun 26 18:02:47 2013 BenDailyProgressCrackleAsymmetric Michelson: Still No Interference

 Today I only had a little bit of time to work in the lab. With the current set up, A La Mode indicated that there was an offset between the beams from the short and long arms as can be seen in this graphic.


Using guess and check, I found that by decreasing the length of the short arm by 5 cm, this could be solved.


We realigned the michelson, and much to our dismay, no fringing appeared at the detector. To rectify this, we tried a few things. First, we replaced the round beam splitter with a cubical one. The round splitter had to be housed in a black mount which meant it was very susceptible to clipping. The cube did not have this problem. After changing this, we still could not see any fringing, but we noticed another problem. Horizontally, the two output beams were not parallel. I began trying to fix this today, and will continue tomorrow.

  648   Tue Jun 25 22:26:03 2013 GiorgosDailyProgressSUSRare Panel Design and Mechanical Set-up

Today, we started designing the box that will hold all of our circuits.

Particularly, we used a software (Front Panel Designer) to design where to drill the holes on the front and rare panels of our box for our wires to pass through; we will order the designs from a company. Our designs had to be very precise for the plugs and connectors on the PCB boards to fit through the holes.


In the afternoon, we drilled holes on the bottom panel of our box for the "amplifying" PCB boards. At the end of the day, we were almost ready with the amplifying boards. Now, we need to order more spacers and screw the PCB boards on the box.


  647   Tue Jun 25 22:11:38 2013 BenDailyProgressCrackleAsymmetric Michelson: Mode Matching and No Interference

 Today Eric and I worked to convert my raw data from yesterday into characteristic beam widths using A La Mode. The first step was to fit the equation a(erf(-(x-m)/2b)+1), (where "erf" is the Error Function) to the data points for each z-displacement. I used MATLAB's "Custom Equation" option in the curve fitting window five times in order to create a fits and find "b" values for each of the 5 z-displacements. I then used the fact that the characteristic width sigma = √2 * b. 

Next I plugged the sigma values into A La Mode, and after a lot of fussing around, we found out that the actual width is 2*sigma, so we had a scaling error. Finally, A La Mode fit a 2-dimensional beam profile to the points. The MATLAB output is below with the waist indicated.


With this done, I carefully measured the path length of the laser in both arms. By imagining the whole michelson as a strait path, I could plug in the position of the source and the target (the detector) into A La Mode, and I could also determine the z-displacement necessary in order to place a lens on the long arm. We then asked A La Mode to choose a position and focal length for the lens that would force the spot size of the beam from the long arm to match the spot size from the short arm at the detector. Unfortunately, A La Mode couldn't do it. We were not quite positive whether this reflects the physical impossibility of the problem, or just a computing problem, but we decided to turn to other means. 

We placed a lens that was in the ballpark of what should work 55cm from the end of the long arm (Eric calculated this), and indeed, the spot sizes were visually identical. Much to our frustration though, no fringing could be seen. The two red dots could be placed on top of each other without any fuzziness or black lines. Perhaps this was because the way I had created the michelson, the angle between the arms was nowhere near 45. We worked to "straighten out" the interferometer in order to remedy this, and I remeasured the distances. Eric also realized that the two output beams (from the long and short arms) were not parallel (vertically). Tomorrow I will work to set up another degree of freedom using another mirror so that this problem can be eliminated.

  646   Tue Jun 25 19:58:38 2013 ericqDailyProgressCrackleWhitening, etc.

Entirely too long since my last elog, mostly due to frustration with the fiber feedthrough. The lastest effort was vacuum epoxy-ing fibers on both sides of the sleeve, but the chamber never went below 10 Torr 

 My only hope for this not being the feedthrough's fault is that I'm reusing a copper gasket for the small CF flange that it's on, so the groove may be leaking. Going to replace it and try again. In the meantime, I've been in contact with OZ optics to make sure one of their feedthroughs (as detailed here) is up to scratch, but they're slow to reply.

I also redid all of the signal routing into the vacuum chamber, since the old feedthrough was no longer modifiable for things like the picomotors. Spent a little while chasing ground loops, but all is happy now. 

I want to hop over to cymac control now, since my error signal has a huge RMS and I want to use fancier servo TFs. As detailed in one of the Cryo ELOGs, there is an ADC noise of a few uV/rtHz, which would swamp my signal, so I drew up a rudimentary design for some whitening + AA. I used EAGLE+LISO to lay it out and estimate the noise that it would produce, then input-referred the ADC+circuit noise to compare to my noise budget, to make sure it actually would work in theory. First, the AA is a 2nd order butterworth LP at ~5k, and then the whitening is a unity gain DC path plus a 100x butterworth high pass at ~200Hz. It loks like it should be fine, dipping under the laser intensity noise at ~100Hz. I'm very open to suggestions for any improvements, however. 



Following some ideas we've discussed in "IFO class," I have some new transfer functions ready to try out once I make the flip to digitalland.

Finally, Ben's been working hard on the frequency noise Michelson. We borrowed a .5 lens from the ATF to adjust the beam size from the long arm, but the spots won't interfere despite their apparent matched size... More details should follow in his ELOG.

  645   Mon Jun 24 23:32:04 2013 Giorgos MamakoukasDailyProgressSUSPower PCB boards and design of the box

We wanted to wire up different PCB boards aiming to power our PCB boards that are connected to the strain gauge and amplify the signal. We looked at the schematics and picked resistors' values so that we have negative (using LM2991 adjustable regulator) and positive (with a LM2941 adjustable regulator) voltage inputs of -15V and +15V respectively. We built six of these PCB boards, missing only very few components, which we ordered.

Then, we looked at the box in which we will place the PCB "amplifying" boards. We also figured out how to drill holes on the cover of the box to get all the wires through. Our final measurements are shown below:Photo0446.jpg

  644   Mon Jun 24 17:55:57 2013 BenDailyProgressCrackleMeasuring beam width

 Today I used a razor blade mounted on a moving platform to measure the width of my laser's beam at five points along the beam. The moving platform was controlled by a micrometer, so I could easily record the position of my razor blade. I mounted the detector such that the entire laser beam was hitting it, and then I placed the razor blade assembly between the laser and the detector such that by using the micrometer, the blade could be driven across the beam (causing part or all of it to not hit the detector). As I drove the razor farther and farther across the beam, the output voltage of the detector would decrease. I recorded an output voltage at each razor blade displacement coordinate.  I preformed this process with the razor positioned 0.75, 10.75, 22.75, 36.75, and 57.75 inches away from the output of the laser.

The immediate goal with this data is to find the characteristic beam width at each of the these 5 points, so that the "A La Mode" software for MATLAB can make a good fit for the shape of the beam. Finding the characteristic beam widths will involve using MATLAB to help me to work out the math - the beam is a Gaussian distribution, not a circle! I have made some progress with this, and will continue tomorrow.

  643   Sun Jun 23 14:39:28 2013 Giorgos MamakoukasDailyProgressSUSFinished 6 PCB Boards

On Friday, we tried to get zero offset for the voltage output of the strain gauge, by replacing our fixed-value R2 resistors with adjustable ones. At R2=22.74kΩ, we achieved a 0.01mV offset, which we would then correct with our DC motors.

We measured the voltage required to drive our motors at a slow, steady pace; 5V seemed sufficient. To get that voltage, we used the voltage divider method. The internal resistance of the motors were roughly 100Ω, so we added a 500Ω resistor in series to get close to our desired Vout, using our 15V source. Our motors were "broken" and would however move at all times once connected and pause only when the control buttons were pressed; we ordered new ones.

Since we successfully built one circuit for one strain gauge and DC motor, we wired the rest 5 PCB boards. At the end of the day, we were happy to look at the following:


  642   Sun Jun 23 14:01:00 2013 Giorgos MamakoukasDailyProgressSUSThursday, June 20th 2013 - Gain of circuitry and calculations of proper resistors

We started by covering the basics of op-amps and their gain. We then talked about differential amplifiers and calculated Vout, in terms of the four resistances of the circuit and Vin1, Vin2. We found the gain (-R4/R3) and the offset, which should ideally be zero. For a gain of about 50, we introduced an uncertainty for the Rresistor, which in turns leads to an uncertainty for the output voltage:

ΔV/V=(ΔR2)/R2 ∙(1/G)

Then, we calculated the resistances for the desired outcome and wired the circuit on the PCB board for signal conditioning, powered it and made sure it gave sensible readings on a digital screen that read the output voltage.


In fact, for one board Vin=0.012V and Vout=0.058V and another Vin=0.014 and Vout=0.69 (where Vin(2)=5V), showing a gain of roughly fifty. At the end, we connected one strain gauge "wire" that supports the levitated plate to our Wheatstone bridge circuitry and read an output voltage of about 144mV (ideally -and with some feedback- it will be 0). Pressing on the strain gauge wire increased our voltage reading, clearly showing that our system responds to pressuring the strain gauge wire.

Tomorrow, we will replace R2 with an adjustable resistor, until we get our voltage reading to be 0 (or close to that). After we find our value, we will use a normal resistor as R2 and finish our system.

I am still trying to find how to insert equations

  641   Fri Jun 21 17:31:16 2013 BenDailyProgressCrackleUsing a highly asymmetrical Michelson to measure frequency noise

 This is an experiment I am working on to gain experience with some of the common techniques used here surrounding the detection of frequencies noise, the mechanics of setting up a michelson, and using lenses to adjust modes (beam size).

I began by creating a highly asymmetrical michelson on an optics table with arms of approximately 0.1m and 1.4m in length. After the laser, I used to mirrors to bring the laser down to a more useful height. This involved placing an iris both very close to the output of the second mirror, and very far from it. Adjusting the mirror orientations so that the beam passed through the center of the iris in both configurations yielded a beam parallel to the plane of the table.

I want to observe the interference pattern that will appear at the detector (which is placed at the output). This interference pattern will be able to give us a plot of V/√Hz as a function of frequency, but we are really looking for Hz/√Hz as a function of frequency (frequency noise). Luckily, we have just the tool to make this conversion. Eric has posted in "Calculating Frequency Noise" the constant of proportionality necessary to make the conversion. That being said, in order to measure the interference patterns, we must make the beams the "same size" when they hit the detector. Practically we will have to use lenses along the long arm of the michelson to manipulate the mode (or, as I understand it, change the beam size). On Monday, I'm going to learn how to use a program written in MATLAB called "a la mode" which will be able to do all sorts of cool calculations. By inputing the beam width at various points it will be able to extrapolate the position of the waist, and it will tell us where we need to place lenses in order to have the beam converge to a certain side at a certain point. Measuring these points accurately will be another important project. The laser beam, of course, is not just a defined cylinder with perfect boundaries - it is a gaussian distribution. We can put a razor blade on a micrometer platform and move it slowly through the beam, measuring the power output at each location. This will give us a distribution curve, and we can define a certain height on the curve to be the width.

Please let me know if I'm doing something wrong with ELOG since this is my first time posting!


  640   Wed Jun 19 11:42:51 2013 ericqHowToCrackleCalculating Frequency Noise

I want to include laser frequency noise in my noise budget, so we discussed making a very asymmetric Michelson in yesterday's meeting, to try and make a measurement of the laser I'm using. I sat down to do the calculation of relating the PD voltage PSD to frequency noise, but tripped up a bit. Looked for references, and couldn't find anything that didn't dive straight into optical cavities, etc. I realized that a Michelson is essentially a delay line, and found an old HP doc called "Phase Noise Characterization of Microwave Oscillators" which talks about delay lines.

The key difference was that I was initially modeling frequency noise as f(t)=f0 + df(t), whereas using a field that oscillates as sin(2 pi f t + d phi (t)) is much more fruitful. 

I translated the math into things I'm used to thinking about and wrote a quick note which I'm attaching here. Once we get the lab cleaned up after the hold drilling, I'm going to set up a measurement to quantify the frequency noise of my laser. 



Attachment 1: freqnoise.pdf
freqnoise.pdf freqnoise.pdf
  639   Tue Jun 11 21:22:47 2013 ericqDailyProgressCrackleLeak

 The fiber feedthrough leaks... Right through the middle of the sleeve. Will try to remake it.

  638   Thu Jun 6 19:22:33 2013 ericqDailyProgressCrackleUpdate

 Added the old Dan Chen / Mingyuan / Valera performance to the noise budget plot. Through contact with Dan, I corrected their loop gain correction. They were touching the laser intensity noise / shot noise of that arrangement occasionally above ~80Hz or so (see ELOG 344), whereas I don't for some reason (the current shot/intensity noise is quite lower from increased power, and isolated laser/fiber). They also were using passive damping (rubber blocks at the clamping points and magnets for eddy current damping), in contrast to the current shadow sensor damping. 


In other news, I've finished the fiber feedthrough for one of the small flanges, and made a little NIM interface box to get BNCs/Picomotor/Power into a 25pin Dsub connector to feedthrough into the chamber. I just need to finish the in vacuum wiring and then I can pump down. 

  637   Tue Jun 4 17:33:00 2013 ericqDailyProgressCrackleUpdated Budget

Updated the noise budget with current performance. Increased the gain on the PDs to get the dark noise under the shot noise. Measured each arm's intensity noise, added in quadrature. Measured servo noise by terminating both PD inputs on the NIM box. Estimated vertical seismic noise by some old seismometer data, estimates of the stack frequencies and known blade TFs. (Haven't accounted for horizontal seismic motion at all; blades are oriented differently, have swinging modes, etc.)


  636   Thu May 30 13:15:47 2013 ericqDailyProgressCrackleLocking again

 Locking again, and easier than ever. The new coil mounts and damping cut the low frequency motion down considerably. Still tweaking the balancing and such, but I took a measurement of the loop TF and it corresponds mostly to my model, the major difference being odd things happening around 150-200 Hz.


Update: Noise Spectrum


Much less of a "forest" than before, though the overall level is pretty much unchanged. (Unsurprising, as the main changes were in the mechanical structure). All that's missing for me to pump down is making the fiber feedthrough. 

  635   Fri May 24 16:25:42 2013 ericqDailyProgressCrackleSome improvements

I've been working on restoring lock for a while, first through modifying the damping circuit TF, which was completely unsuccessful. Then, I turned to redesigning the coil/shadow sensor mounting, as the skinny, tall, post holder was not an ideal situation.

I made some attempts at mounting the SS near the bottom of the suspended mass, to have a short, rigid connection to the stack. This resulted in a problem with some of the higher blade modes, as when the coil would push the top down, the bottom could swing up, thus switching the expected sign and turning damping into antidamping.

I've now created something more akin to the LIGO OSEMS, in that I've attached a flag directly to the actuation magnet to guarantee that the SS truly sees the magnet motion and nothing else. The whole thing is on a top of a thick post, with a thinner post attached as tight as humanly possible. (The blue thing is a plastic flag). The only remaining headache is that I have to be very careful not to snap the flags off. I should look for some stronger epoxy than the 5-minute stuff in the EE shop. Photo!

IMG_20130524_153023.jpg     May24Damping.pdf

I then took spectra of the blades while damped/undamped. This was performed outside of the chamber, on one stack. (Instead of the two stacks that are normally used inside the chamber. ) These spectra compare quite favorably to the earlier performance; the ~2Hz resonances have been damped by a factor of at least 10, whereas it was around 2-5 before. So, once I reinstall all of the optics, locking should be much easier!

In other news, I've modified the fiber coupling setup with a different coupler, for two reasons. A) The fiber chuck in the old coupler had a 2.0mm key slot, whereas the fiber had a 1.8mm key on the ferrule, leaving room for the fiber to rotate (and thus more susceptible to acoustic noise). B) The older coupler had tilt adjustments, and I feared the springs involved also exacerbated the acoustic noise susceptibility. 

The new couper (which was donated a good while ago by Koji, if I recall correctly) is more sturdy, having only translation adjustment. The chuck is 1.8mm, and the fiber sits solidly in it. Also, the translation screws on the stage seem much more stable than on the previous coupler. The coupling is a little more of a pain, but I reached ~80% coupling efficiency after careful adjustment, as compared to the 60-70% which was doable with the previous coupler. 

Punchline: The RIN has been very much improved! Especially heartening is the fact that the fiber RIN sits on the direct laser RIN from about 50-200 Hz. 

IMG_20130522_172533.jpg May22RIN.pdf

Also, I got ahold of a picomotor mount from Greg's old lab (with Eric Black's approval). Hooked it up the driver and hand controller that Koji pointed me to, and it works. This should make the final stage of alignment very much simpler. 

Altogether: I should be able to lock in the next few days. Next step will be closing up the chamber (i.e. making a fiber feedthrough)...

  634   Wed May 8 15:32:55 2013 ericqDailyProgressCrackleDCC document

 I've uploaded a document to the DCC detailing the experiment (some background, design, performance, analysis), borrowing some language from T0900167 wherein the current experiment, and other crackle investigations were proposed. It will be fleshed out more, as time goes on....

The DCC number is: T1300465

  633   Thu May 2 18:17:43 2013 ericqDailyProgressCrackleAnalysis pipeline

I continued this analysis with a "more realistic" background noise profile. I.e. I included some features that my real noise curve looks like; a low frequency ramp up, some 60Hz harmonic lines and a few "humps". The same demodulation code was used.

I chose a couple of 50Hz bands and did the analysis in the same way as in the previous post (simulated various levels, look for the point where the demodulated signal rises above the variation in the demodulation of the background alone).

Here are two plots showing about the same thing, that this analysis seems to be able to detect crackle noise power about a factor of 6 under the background noise power. (a ratio sqrt(6) in m/rtHz)


Additionally, I simulated the scenario of switching on the drive halfway through the data with the above background, which has the effect of no crackling being present until that point. Here is how the demodulation signal responds. In this scenario, the "integration" of the newly present crackle signal only takes a few seconds to settle. 


Attachment 1: Res1.pdf
  632   Fri Apr 26 16:04:56 2013 taraNoise HuntingNoise Budgetfrequency noise requirement for laser used in crackle experiment

I made an estimate for frequency noise requirement for a laser that can be used in crackle experiment. With some assumptions, I came up with df = 3x102 [Hz/rtHz ] for the requirement.

 The two beams from both arms are recombined at the output port of a Michelson interferometer. If it is operated at dark port, the output signal will be linear with the differential length between the two arms.

some assumptions in the calculation:

  • Operating at darkport
  • The laser has frequency = f0 + df  (carrier + noise)
  • mismatch between the two arms is ~ 1mm
  • aim for SNR = 1, no integration time.
  • crackle signal is ~10-15 m/rtHz, this is actually the shot noise limit of the current setup.


This will be a requirement for the planned ecdl.

Is a HeNe laser good enough? I'm not sure about HeNe frequency noise level, and I haven't found it in literature that much. I checked here,see fig 5, HeNe f noise is not so bad compared to NPRO noise (10^4 /f Hz/rtHz).This feels a bit counter intuitive. But if it is real, it should be ok for the measurement around 100 Hz and above.

  631   Mon Apr 8 13:17:46 2013 ericqComputingCracklesvn directory

A folder on the 40m svn server has been created to store crackle related files

The location is: /trunk/crackle

This currently includes the latest poster, a MATLAB subdirectory which holds all of the code from my last elog, a Lit subdirectory with a couple of papers in it, and an ExpChar directory which houses the bulk of the measurements I've made regarding the experiment.

Specifically listing what is in each sub-dir


  • Blades: transfer functions and ringdowns of the blades via shadow sensor
  • Laser RIN: measurements of RIN in different configurations (fiber, no fiber, dark noise, etc.)
  • Loop: Measurements of the Loop TF
  • Mich: spectra of the Michelson error signal
  • Servo: LISO files of the servo circuit, and transfer functions (liso and measured)
  • ShadowSensor: Circuit of the shadow sensors and TF of the damping
  • circuit.graffle: OmniGraffle drawing of the crackle NIM box circuitry
  630   Thu Apr 4 09:12:16 2013 ericqDailyProgressCrackleAnalysis pipeline

 (As of yesterday, I have been summoned for jury duty at the downtown courthouse, so I haven't been able to come into the lab.)

I've been taking a closer look at the demodulation analysis technique, in order to get a better idea of what is detectable. Specifically, up until now I would generally compare the demodulation of the driven michelson signal to the demodulated undriven michelson signal. Now, however, I am starting from the ground up, comparing the demodulation of purely white, gaussian noise to the demodulation of gaussian noise + 1/f crackle noise modulated by the drive frequency. I.e., the signals look like this:


The demodulation process looks like this (code attached in demodulate.m):

  • Bandpass region of interest (100-1000Hz in this case, can be narrower when we want to look at specific low noise bands)
  • Square to convert to power
  • Band pass filter around 2Fdrive
  • Multiply by sin2f, cos2f to get I, Q
  • Lowpass I and Q signals (corner freq of Fdrive) to smooth out

I simulate various levels of crackle noise injection, with a parameter alpha that affects the displacement as y(t)=background + alpha*sin(2pi Fd t)*randn. The demodulation Q signals (where crackle shows up) look like this:


I then compared the results from the different alphas to the gaussian background, with the goal of discerning the point where one can conclude that a crackle signal is present within the noise. To this end, I use the following logic:

As evident from the time series of the demodulation signals, the Q continues to vary randomly about its mean value. These variations are normally distributed about their mean, to reasonable accuracy. Thus, if the mean of a demodulation signal is over two standard deviations higher than the demodulated background signal (which contains no crackle), it is very likely that the signal contains a crackle component. In the first plot below, the green trace shows the 2sigma variation of the background signal. When the red trace, which is the background + crackle signal, crosses this point, we can be confident a crackle signal has been measured. The second plot compares the histogram of the demodulation signal amplitude of the background with that of the background+crackle signal at the point where the red trace crosses the green, about alpha=2e-13.


Finally, I evaluate the power spectrum of the crackle component of the signal at this threshold value of alpha, to see what level of crackle noise is detectable with this kind of analysis. 


 The next steps will be to bring this simulation closer to reality. First, by modifying the background to rise at low frequencies, as the real spectrum does due to seismic noise, etc., then by approximating my real noise spectrum by injecting lines, broad peaks, etc, and doing the analysis on a narrow band of locally low noise. 

All of the matlab code used for creating these plots is attached.

In addition, I also looked at the "double fft" of the signal, as Jan and I have thought about before. This involves taking the PSD of the displacement signal at various times, and then looking at the variations of one frequency's power in the PSD with time. I.e. a PSD of each frequency bin in the signal's PSD. The crackle signal is a periodic variation in the noise power, and thus shows up in the double FFT as a line.




Attachment 7: demodulate.m
function [ Iout, Qout ] = demodulate(sig,f1,f2,fdrive,fs)
%Demodulate Bandpasses sig between f1 and f2, then demodulates at 2fdrive

if nargin<5
if nargin<4;

... 28 more lines ...
Attachment 8: noisegen.m
function sig = noisegen(vecsize,beta)

    if nargin < 2
        beta = -1; % 1/f noise default
    if numel(vecsize)==1
        vecsize = [1,vecsize];
... 17 more lines ...
Attachment 9: NoiseSim.m
%% Parameters, create datastreams


... 89 more lines ...
Attachment 10: dubfft.m
function [dub,f1,f2] = dubfft( sig,fp1,fp2,fs )
%dubfft calculates power spectrum of power variations in sig after
%bandpassing between fp1 and fp2 (I.e. "double fft")

if nargin<4
if nargin<3
... 37 more lines ...
  629   Mon Apr 1 16:51:08 2013 ericqDailyProgressCrackleLatest Locks

Last week, I spent time tweaking the PD and actuator balancing. The apparatus can be locked straightforwardly. I compared the displacement noise of the driven michelson to that achieved last winter. One thing that I am glad to see is that the spectrum of the driven vs. undriven system (not plotted) do not differ significantly. On the contrary, with the previous setup, driving the blades introduced considerable broadband noise (see ELOG #597)





 This data was taken with the cymac, I used a SR560 to whiten the signal, which was unwhitened in MATLAB. Unfortunately, I didn't have the presence of mind to record the ADC noise. This will be included in the noise budget shortly. As for the noise budget, it is unclear to me why my noise floor over 100Hz is an order of magnitude higher than the PD noise. Acoustic noise is surely a component of this. 


My current plan is to clean up my signal by getting in shape to pump down the chamber, and using the new, longer fibers I ordered that have now arrived. To get there I need to do the following things:

  • Get a picomotor mount to be able to align without shaking the stack and with the chamber pumped down. (already have a driver)
  • Rewire the wire feedthrough for the 2nd PDs power and signal and the picomotor. 
  • Create a fiber feedthrough from a fiber mating sleeve + flange + epoxy

I also need to make sure my PD balancing circuit isn't introducing too much noise into my signal.

My other big concern at this point is the discrepancy between the model of my loop and what I've measured it to be. The measurement consists of injecting a differential drive on the arms, and taking the transfer function of the error signal to the actuator signal.


It seems I effectively have no phase margin at all, which doesn't make sense to me. I will recheck / redo the measurement to see if I can learn anything.

  628   Mon Mar 25 21:27:54 2013 ericqDailyProgressCrackleBalancing

I want to achieve good balancing of the PD signals and actuator gains, as I think this is currently a significant source of noise. After having some troubles, I realized my heretofore undefined polarization state caused the power in the two arms of the Michelson to be substantially different, and thus the combined beams at each PD are some odd mix of the two arms, thus making the subtraction scheme inefficient.

The polarization in the Michelson, and thus the power in each arm, is affected by two angles: a) the angle between the HeNe laser and the fast axis of the fiber at the input end b) the angle between the fast axis of the fiber at the output end and the plane that the beamsplitter sits on.

So, I placed a polarizing beam splitter (transmitting p polarization) before the fiber coupler (set up with the fast axis aligned to p as best I could), then rotated the HeNe tube to maximize the fiber output / minimize the power reflected out of the PBS. This should ensure that the light coming out of the fiber is in a well defined polarization state, and incidentally seems to have cleaned up my high frequency RIN a bit (plot will follow).

Next, I put the fiber output through my main Michelson beamsplitter (which oddly seems to somewhat polarization dependent, but not as much as a normal PBS) and rotated the fiber output collimator until the power transmitted and power reflected were as equal as I could get them (within 3%). My first thought was to use a half wave plate for this, as I thought it would give me more fine grained control, but this introduced a substantial amount of "wander", i.e. sub 1Hz variation. 

In my circuit, before the subtraction of the PD signals occurs, each signal was passed through a unity gain buffer. One of these buffers has been modified with a variable resistor to change the relative gain between the two. Now that I am more confident that each PD is getting the same incident power, I should be able to nicely balance the PD signals tomorrow once I realign. 

I took some RIN measurements as well, today. The first was directly from the fiber output. For the next, I treated the two PDs and subsequent buffers and subtraction as one "PD," in the interest of comparison.


We can see the suppression of the fiber's RIN in the reduction of the peak just above 200Hz. Turning the relative gain dial of the subtraction circuit causes this peak to rise, which makes sense. However, given the power coming out of the fiber, I am above being shot noise limited. I didn't make this comparison until later, so I don't yet know whether PD noise or the electronics noise of the subtraction circuit is to blame. In any case, I expect/hope that I will be able to measure improved locked Michelson noise levels tomorrow. 

  627   Thu Mar 14 17:29:33 2013 ericqDailyProgressCrackleNoise Budget

 Spent some time cooking up my current noise budget. Notable omissions: seismic and acoustic noise.



 In these plots, the noise due to laser RIN is suppressed quite a bit, by the antisymmetric / symmetric PD subtraction. However, if I plot the RIN effect as if it were not suppressed...


 One thing to note is that I have not set up proper PD subtraction, and there is a small difference in their gains.This is accounted for, as a RIN coupling for the suppressed case, but may be adversely affecting the servo performance. I probably need to recheck the actuator balancing as well. 


  626   Tue Mar 12 15:08:49 2013 ericqThings to BuyCrackleFiber thoughts

It is clear that the fiber situation needs to be improved. Rana says that the way to reduce the fiber's intensity noise is to use a long (5m?) length of fiber inside of the chamber, tied solidly to the stack; as the beam jitter induced intensity noise is a result of the jittering beam coupling into higher order modes. I am working on calculating the attenuation of higher order modes in a single mode fiber as a function of length, as this should indicate how much fiber we would need. 

I have also been in email contact with Dan Clark at Stanford, who did some fiber feedthrough work for a seismic platform interferometer. He sought to achieve a design that would be fully compatible with aLIGO standards, and thus considerably more stringent than the requirements for our situation. He got ahold of a metalized polarization maintaining fiber, and soldered it directly to a flange blank, with pigtails on each end. This has the advantage of only having metal-to-metal vacuum seals, so UHV can be achieved. However, he told me that this all lives out of the way, and doesn't need to be touched often.

Our situation differs somewhat, because we occasionally need to work near the flange, and thus have a greater risk of breaking a fiber that is semipermanently mounted to the flange. Additionally, our vacuum requirements are much looser. I think a feedthrough that has female fiber connectors on each side along with patch cables such as the one we're using would be a robust solution. This way, the fiber can be detached from the flange while leaving the coupling intact, if things need to be moved around. 

My current thought is that we can make a feedthrough out of something like this Thorlabs mating sleeve, since an assembled feedthrough (like from CeramTech, which may not even be polarization maintaining) is quite expensive.

Thus, the solution could be made of the following things:


  625   Mon Mar 11 14:56:48 2013 ranaDailyProgressCrackleActual progress

 I used the following line to change your PDF into PDF/A format so that the ELOG could make a thumbnail of it:

gs -dPDFA -dBATCH -dNOPAUSE -sDEVICE=pdfwrite -sOutputFile=out-a.pdf lockcomp.pdf

  624   Mon Mar 11 14:01:49 2013 ericqDailyProgressCrackleMeasured loop TF

 I was able to make a measurement of a portion of the loop transfer function today. Fitting with a power law gives a UGF of 59.4Hz. 



  623   Thu Feb 28 17:23:44 2013 ericqDailyProgressCrackleActual progress

With Koji's help, I was able to lock the interferometer. Noise level is compared to the last lock in the following plot:


I'm not sure what's happening at ~100Hz, and am perplexed to see the high frequency noise be not much different. Koji made a variety of suggestions of what I should do to improve the performance of the servo, which will be my current focus. 

  622   Sun Feb 24 12:26:20 2013 ericqDailyProgressCrackleActual progress

Nic and Dmass helped me lift the plate into the chamber, but I unwisely had some things hanging over the plate's edge, which meant the whole thing didn't fit...

Came in yesterday, changed the layout slightly, aligned outside, lifted in, touched up the alignment, saw a little bit of fringing. Then, I placed the coils + shadow sensors and turned on damping. Better fringes were achieved: 


Tomorrow, I will take some time to align with good contrast, and then I hope to be able to lock!

  621   Thu Feb 21 17:41:01 2013 ericqDailyProgressCrackleActual progress

 Long radio silence due to cymac work. Came back to crackleland, and have been beating my head against alignment woes. Today I have achieved fringes on both ports! 

This was finally possible by taking the plate out of the can and putting everything together from scratch. Because I am outside of the can, the cables for shadow sensors + actuators don't reach. Thus, I couldn't damp, and so the fringes are sporadic, since the masses are free to swing. Nevertheless, in the absence of a squishy wobbly table, I was able to align both ports.

Pictures of the layout and scope showing some fringing. (Ch1 is antisymmetric port PD, Ch2 is symmetric port PD)

layout.jpg scope.jpg


Procedure looked like this:

  • Take everything out of the can
  • Use iris, fiber output and two steering mirrors to produce a level horizontal beam. 
  • Use 45 degree inclined mirror and improvised plumb bob to produce vertical beam. 
  • Put end mirror (on the bottom of a mass) into vertical beam. Retroreflection consistent across different rotations == level end mirror. Repeat for other end mirror.
  • Arrange first mass + inclined mirror to get acceptable wedge angle that will fit in BS cube. Use iris to ensure inclined mirror is at right pitch angle. 
  • Position cube. Use iris to confirm its leveling. (Two reflected beams are coming out of the cube, two tilt degrees of freedom.)
  • Position second mass + inclined mirror, adjust BS rotation as neccesary. Use iris to fix mirror pitch.
  • From here on out, I touched second inclined mirror and BS only as my degrees of freedom. Pitch should be mostly aligned from iris use.
  • Align by touching 2nd inclined mirror yaw and BS rotation, and looking at near field (card near AS port) and far field (letting Sym port shine across the room)
  • Position PDs, tweak alignment until fringes pop up. 

Next steps:

  • Recalibrate shadow sensors, since I repaired one a little while back. 
  • Unplug PDs. Arrange a clever way to lift the plate into the can without destroying everything...
  • Damp masses, align to good contrast. 
  • Lock! (Either with slight modification of old control circuit, or bring a cymac over.)

In cymac land, I'm facing some trouble retrieving written frames, but the DAC and ADC seem fine, but this is on Jamie's prototype that will stay at the 40m. The future 050 cymac doesnt' have ADC or DAC cards in it yet, but it is all set up to receive them, but has the same frame reading issue. I will continue to work on these in parallel with crackle work, but it'll probably be more productive to modify my old circuit for the immediate future before completely jumping ship to cymac control. 

  620   Mon Jan 21 22:33:26 2013 haixingSummarySUSfront and rear panels for signal conditioning boxes

Here are the front and rear panels for the signal conditioning boxes. The front-panel files are attached.

For the coils:



For the QPD:



For the Hall-effect sensors:



For the linear motors [using simple DC control]:
Small panel-mounted voltage meter reads out the force gauge signal that indicates the weight of the floating plate, from which we know roughly how far we are away from the working point where the gravity is balanced by the DC magnetic force. We use two on-mom switches to control the linear motor (up and down).



Attachment 9: front_and_rear_panel_files.zip
  619   Thu Jan 17 23:40:12 2013 haixingSummarySUSGeneral signal conditioning circuit for maglev


- I forgot why you don't have a voltage reference (cf. AD587) for the offset subtraction.

- Don't you want to put an output impedance at the output of the current driver?


I implemented both of your comments in this revision. The altium file is also attached. Thanks.

Attachment 2: general_signal_conditioning_board_20130117.zip
  618   Wed Jan 16 19:41:15 2013 haixingSummarySUSGeneral signal conditioning circuit for maglev


- I forgot why you don't have a voltage reference (cf. AD587) for the offset subtraction.

- Don't you want to put an output impedance at the output of the current driver?

 > For your first comment, you are right. I am diverting some voltage from the power, which has a huge noise. This is rather bad.

> For your second comment, it turns out that my coils have quite high impedance, e.g., of the order of hundred Ohm. The maximal current, given the maximal voltage 15V, is smaller than what the buffer can provide. But it is good to add an output impedance for flexibility and also limits the current.

Thank you. I will implement these comments in the next revision of the PCB.

  617   Wed Jan 16 16:19:25 2013 haixing, kojiSummarySUSGeneral signal conditioning circuit for maglev

- I forgot why you don't have a voltage reference (cf. AD587) for the offset subtraction.

- Don't you want to put an output impedance at the output of the current driver?

  616   Wed Jan 16 12:40:39 2013 haixing, kojiSummarySUSGeneral signal conditioning circuit for maglev

Here is a general signal conditioning circuit for whitening and dewhitening of the signal from the sensor and actuator (multiple channels) in maglev. Previously, I was designing different circuits for whitening and dewhitening. Koji pointed out that by manipulating the zero and poles, we can realize them with the same circuit by choosing the proper values for the resistors and capacitors. In addition, by bypassing certain stages, we can use one type of PCB board for the sensor (the hall-effect sensor and quadrant photodiode), and the actuator (the coil).

The pcb board and the associated alitum file (altium_file.zip) are attached.


This circuit contains five stages, and each can be bypassed by using a three-terminal header and jumper:

The first one is to set the DC offset.


The next three are generic filters, each with one zero and two poles.


(different footprints for the capacitor for a generic purpose)

I will explain the detail in the additional information part appended to this elog.

The last stage is a current boost for coil drive.


-------Additional information----------

In the below, I will briefly explain the idea of the filter part:


The transfer function for this circuit is given by


s0: 1/C1(R1+R2)

s1: 1/(C1 R2)  

s2: 1/(C2 R3)

At very low frequency, the gain is determined by -(R3/R1), which is chosen to be -1 in our case. Given different values for s0, s1, and s2, we can have high-pass filter (s2>s1>s0) [cut-off above s2], or a low-pass filter (s1>s0>s2) [cut-off above s1].

For example, in our case, we have chosen

  1. A high-pass filter (more precisely, a bandpass): s0/2π = 0.5 Hz, s1/2π = 5 Hz, and s2/2π = 200 Hz [cut-off].
    The parameters for the components are: R1 = R3 = 28.6 KΩ, R2 = 3.2 KΩ, C1 = 10μF, C2 = 27.8nF
    The amplitude is shown by the figure below:

  2. A low-pass filter: s2/2π = 0.5 Hz, s0/2π = 5 Hz, and s1/2π = 200 Hz [cut off].
    The parameters for the components are: R1 = R3 = 32 KΩ, R2 = 820 Ω, C1 = 0.97 μF, C2 = 10 μF.
    The amplitude is shown by the figure below:

  3. To balance the whitening and dewhitening in the entire loop, we have chosen the zero and poles such that the product of the above two filters is close to unity at low frequencies, as shown by the figure below:



Attachment 1: altium_file.zip
  615   Mon Jan 14 20:31:51 2013 ericqDailyProgressCrackleAlignment is hard

Got sick. Got marginally better. Spent many hours trying to align. Consulted with Dmass, got some tips and got closer, but not there yet. 

The shakiness of the platform and inability to damp swinging modes or lock anything down continues to make things frustrating. 

ELOG V3.1.3-