I finished the direct measurement of cable impedances.
Moreover, I wrote the cable replacement plan.
The reason I am checking the cables is for replacing the cables with impedance of 50 or 52 ohm by those with impedance of 75 ohm.
After I figures out which cable has not proper impedance, I will make new cables and substitute them in order to match the impedance, which would lead to better VIDEO signal.
Moreover, as Koji suggested, the VIDEO system will be upgraded for better interface.
I measured the cable impedance by checking the reflection ratio at the point connected to the terminator with 50 ohm or 75 ohm.
The orange colored cables are measured to be 75ohm so we do not need to replace them.
Combining the list of cable types and the list of desired length,
I need to make total 37 cables and to remove 10 cables from the current connection.
Detailed plan is attached below.
I currently ordered additional cables and BNC plugs.
From now on, I will keep making CCD cables for VIDEO upgrade.
Then, with your helps, we will replace the CCD cables.
In my opinion, I will finish VIDEO upgrade by this year.
[Koji, Rana, and Kevin]
I have been trying to measure the shot noise of REFL55 by shining a light bulb on the photodiode and measuring the noise with a spectrum analyzer. The measured dark noise of REFL55 is 35 nV/rtHz. I have been able to get 4 mA of DC current on the photodiode but have not been able to see any shot noise.
I previously measured the RF transimpedance of REFL55 by simultaneously measuring the transfer functions of REFL55 and a new focus 1611 photodiode with light from an AM laser. By combining these two transfer functions I calculated that the RF transimpedance at 55 MHz is ~ 200 ohms. With this transimpedance the shot noise at 4 mA is only ~ 7 nV/rtHz and would not be detectable above the dark noise.
The value of 200 ohms for the transimpedance seems low but it agrees with Alberto's previous measurements. By modeling the photodiode circuit as an RLC circuit at resonance with the approximate values of REFL55 (a photodiode capacitance of 100 pF and resistance of 10 ohms and an inductance of 40 nH), I calculated that the transimpedance should be ~ 230 ohms at 55 MHz. Doing the same analysis for the values of REFL11 shows that the transimpedance at 11 MHz should be ~ 2100 ohms. A more careful analysis should include the notch filters but this should be approximately correct at resonance and suggests that the 200 ohm measurement is correct for the current REFL55 circuit.
RF Transimpedance of 200Ohm means the residual impedance at the resonance (R_res) of 40,
if you consider the amplifier gain (G_amp) of 10 and the voltage division by the 50Ohm termination,
this corresponds to the thermal noise level of Sqrt(4 kB T R_res)*G_amp/2 = 4nV/rtHz at the analyzer, while you observed 35nV/rtHz.
35nV/rtHz corresponds to 7nV/rtHz for the input noise of the preamp. That sounds too big if you consider the voltage noise of opamp MAX4107 that is 0.75nV/rtHz.
What is the measurement noise level of the RF analyzer?
I measured the RF transimpedance of the POX photodiode by measuring the optical transfer function with the AM laser and by measuring the shot noise with a light bulb. The plots of these measurements are at http://lhocds.ligo-wa.caltech.edu:8000/40m/Electronics/POX.
I measured the noise of the photodiode at 11 MHz for different light intensities using an Agilent 4395a. The noise of a 50 ohm resistor as measured by this spectrum analyzer is 10.6 nV/rtHz. I fit this noise data to the shot noise formula to find the RF transimpedance at 11 MHz to be (2.42 ± 0.08) kΩ. The RF transimpedance at 11 MHz as measured by the transfer function is 6.4 kΩ.
The last time(Friday) I made an arrangement for RF distribution unit.
I am making RF distribution unit for RF upgrade which is designed by Alberto.
To reduce a noise from loose connection,
I tried to make the number of hard connect as much as possible while reducing the number of connection via wire.
This is why I put splitters right next to the front pannel so that the connection between pannel plugs and splitters could be made of hard joints.
I attached the arrangement that I made on the last Friday.
Next time, I will drill the teflon(the supporting plate) for assembly.
Any suggestion would be really appreciated.
The last time(Moonday) Jenne and I worked on the RF distribution unit's structure.
We are making RF distribution unit for RF upgrade which is designed by Alberto.
Rana, Koji, Jenne suggested a better design for RF Distribution unit.
So Jenne and I gathered information of parts and decided what parts will be used with specific numbers.
Specific circuit is shown in the attached picture.
Kevin was working on characterizing all of our RF photodiodes for the upgrade, and he discovered that REFL11 didn't work, as described in elog 3890. Rana was working on fixing it, but then he went off to Japan.
I visually inspected the components inside the RF cage on the REFL 11 circuit board inside the PD. Most of them were okay, but the connection between L5 and C33 (the big tunable inductor and the next capacitor in the path) was totally flaky. the leg of the inductor had been soldered directly to the trace on the PCB, and the inductor was a little bit tipped over, and pulling the trace off the board. I wiggled it a little while trying to see what was going on, and the trace broke. Since there is nothing going on between L5 and C33, just the trace, I used a piece of resistor lead to attach the two. The connection now seems very robust. I'm a little worried about the connection between the inductor and the board on the other side, but I can't see it since it's under the inductor itself.
Also, the soldering of L4 (a standard surface mount component type body) to the board seemed totally shoddy. I was desoldering the first side, and the whole inductor popped off. It was clear that the inductor was making a physical connection to the board, but not a nice solid electrical connection. So I resoldered it on. (On Alberto's schematics, it is listed as a 633nH inductor. I can't find any of this value, so I just put the same one back on. The best I could do to confirm the component was still okay was measure its resistance, and compare that to a similar inductor of a similar value. It seemed okay.)
After that I powered up the PD, and took an electrical transfer function, just to have a look-see. It seems kind of okay, although the resonance seems to be closer to 13MHz than 11MHz.
Since we would like to remove the capacitor that is in parallel with the diode itself, which will then change all of the resonant conditions on other components, I didn't worry too much about the resonant peak for tonight. We're going to have to look in on this though.
Also, I'm leaving the optical check-out for Kevin, so he will let us know if I magically fixed the PD, or if it needs some more work.
Photos of the circuit board (mostly Alberto's mods) before and after I fitzed with it are on Picasa
More testing. Probably more fixing.
[Koji, Jenne, Kevin]
Jenne worked on fixing REFL11 last week (see elog 4034) and was able to measure an electrical transfer function. Today, I tried to measure an optical transfer function but REFL11 is still not responding to any optical input. I tried shining both the laser and a flashlight on the PD but could not get any DC voltage.
I also completed the characterizations of POX. I redid the optical transfer function and shot noise measurements. I also took a time series of the RF output from the PD when it was powered on with no light. This measurement shows oscillations at about 225 MHz. I also measured the spectrum with no light which also shows the oscillations at 225 MHz and smaller oscillations at ~455 MHz.
The plots can be found at http://lhocds.ligo-wa.caltech.edu:8000/40m/Electronics/POX?action=show.
This is looking better, but the fit data for the TF should be plotted along with the data. The data should be made up of points and the fit a line.
For the fit, we should have the Q of the main resonance as well as the peak height of the main resonance and the values of the gain at the notch frequencies.
Also the peak as well as the notches should have the frequencies fit for and labeled. In principle, you can make the plot on the wiki have all of the data. Then in the end we can print the plot in a small size and glue it to the PD's backside.
I used 50 mA to drive the laser diode. The light is split 50/50 between the DUT (Device Under Test) and the New Focus 1611 (1 GHz BW) diode used as the reference.
This measurement is the TF of DUT/(New Focus). The resonances are there, but clearly there's an issue with instability around 200 MHz. The setup is still powered up, so please be careful around the RFPD testing table (don't stomp around yank the cables out of the power supplies).
I looked at the RF Photodiode wiki that Alberto has started - most of the TF features are replicated there. Todo:
* Update the 'schematic' with a real schematic instead of the cartoon.
* Change the circuit to remove the resistor in the RF path.
* Add compensation to avoid the 200 MHz instability.
* Make sure to include opamp current noise in the noise model (it is the dominant noise source but has been left out in the noise estimation plot).
* Make the output into a true 50 Ohms.
I redid the optical POX transfer functions and updated the wiki at http://lhocds.ligo-wa.caltech.edu:8000/40m/Electronics/POX.
I measured each transfer function several times to calculate uncertainties for each measured point. There is one large transfer function from 1 MHz to 500 MHz showing a resonance peak at 11 MHz and notches at 22 MHz and 55 MHz. I also made more detailed measurements around each of these resonance peaks. These measurements were fit to a resonance curve to determine the resonant frequency, transimpedance at resonance, and Q for each peak. These measurements agree with the shot noise measurement for the transimpedance at 11 MHz taken earlier considering that this measurement was made at 11 MHz instead of at the resonant frequency of 11.14 MHz.
I measured these transfer functions with the Agilent 4395a using the netgpib.py script last week. I realized that when using this script to save multiple copies of the same measurement after setting up the instrument, the first and second measurements are saved but all measurements saved after are identical to the second measurement until the instrument is physically reset. This happens because the analyzer switches the trigger from continuous to hold after making a measurement using this script. Kiwamu said that the script can be modified to return the trigger to continuous after saving the data so that multiple measurements can be saved without being at the analyzer physically. I did not want to waste more time figuring out how to modify the script to do this so I used one of the netbooks and sat at the analyzer manually returning the trigger to continuous after each measurement.
TF looks fine except for the large peak at around 200MHz which has been reported by Rana. The time series and the spectrum without the light are pathetic...
I still prefer to see the fit by LISO as the pole/zero fitting of LISO as the fit result is more physically understandable.
Anyone can ask me about the instruction how to use LISO
I guess Idc of 24mA would be just a mistake. It looks like ~0.2mA from the plot that sounds normal for the transimpedance of 2kOhm.
Question: What is the HWHM of the reesonance when you have f0 and Q.
The value of I_dc was a mistake. The value should be 240 µA.
The widths of the resonance peaks are listed below the fits to each peak on the wiki.
We fit the entire POX optical transfer function from 1 MHz to 500 MHz in LISO. The fit is on the wiki at http://lhocds.ligo-wa.caltech.edu:8000/40m/Electronics/POX. Using LISO's root fitting mode, we found that the transfer function has five poles and four zeros.
I will work on making plots of the residuals. This is difficult because by default, LISO does not calculate the fitting function at the frequencies of the data points themselves and I haven't figured out how to force it to do this yet.
[Rana and Kevin]
I measured the optical transfer function of POY and fit the data using LISO. The fit can be found at http://lhocds.ligo-wa.caltech.edu:8000/40m/Electronics/POY. POY was missing the RF cage and back cover so I took those parts from AS55 in order to make these measurements.
POY does not have the unwanted oscillations at 225 MHz that POX has. Attachment 1 shows the transfer functions of POX and POY.
To measure the transfer functions, I used a 50/50 beam splitter to send half the light from an AM laser to POY and half the light to a New Focus 1611 reference photodiode. The transfer function for POY was measured as the transfer function of the signal from POY divided by the signal from the 1611. When I was measuring the transfer function for POX, I failed to ensure that the photodiodes were operating linearly. Before making the measurements for POY, I varied the RF power modulating the AM laser and recorded the magnitude of the transfer function at the 11 MHz peak. Attachment 2 shows these values. The measurements for POY were made in the linear region at an RF power of -10 dBm. The measurements for POX were made at 0 dBm and were most likely not in the linear region for POX.
[Koji and Kevin]
I measured the shot noise of POY and fit the data to determine the RF transimpedance at 11 MHz and the dark current. The transimpedance is (3.860 +- 0.006) kΩ. I realize that there are not many data points past the dark current but I did not want to take any further data because the light bulb was getting pretty bright. If this is a problem, I can try to redo the measurement using a lens to try to focus more of the light from the bulb onto the photodiode.
I also measured the spectrum and recorded a time series of the RF signal with the light to the photodiode blocked. These measurements do not show any large oscillations like the ones found for POX.
The plots of the measurements are on the wiki at http://lhocds.ligo-wa.caltech.edu:8000/40m/Electronics/POY.
We added the following two new DAQ channels into the c1:GCV model. The daq:analog input channels are on card ADC0 and correspond to channels 3 and 4 on the card.
c1:GCV-EXT_REF_OUT_DAQ Sampling rate=2kHz acquiring a 1Hz sine wave from the SRS Function Generator DS345. This is using the Rb 10MHz signal as an external frequency reference.
c1:GCV-PLL_OUT_DAQ Sa.rate=2kHz acquiring the demodulated signal from the PLL servo.
This work is connected to the study of VCO PLL loop noise at frequencies below 0.1Hz. We are trying to measure phase noise in the VCO PLL servo at low frequencies as this noise would result in arm length fluctuations in the green-locking scheme.
I wonder why POY11 has the dark noise level of 90nV/rtHz that is 5 times larger than that of POX (18nV/rtHz)
even though the Q are the same (~15) and the transimpedance is better (3.9k instead of 2k).
What cause this high noise level?
What is the expected dark noise level?
This diagram shows the setup of the analog Mixer-Frequency Discriminator (MFD).
The idea is similar to the one of the Schnupp Asymmetry for our Michelson interferometers. The signal from the PD (or any signal source for which you want to know the frequency) is split into two legs; one leg is much longer than the other. The two legs are recombined at a mixer/demodulator. The demodulator output varies sinusoidally with the phase difference of two legs, the same as when we try to measure the phase noise of an oscillator, for example. This is the same concept as the digital frequency discriminator that Aidan and Joe put into the GFD FE system recently.
With a ~1m cable length asymmetry, we get 180 deg of phase shift for a ~100 MHz signal (recall that the speed of light in most of our cables is ~2 x 10^8 m/s). The mixer gives a linear output at 50 MHz (and 150 MHz, 250 MHz, etc.).
This single mixer based setup is fine for most everything we do. In order to get even more resolution, one can just use 2 mixers by splitting the signal with a 4-way instead of 2-way mixer. One setup can have a 0.5-1 m asymmetry to have a large range. The other can have a ~10-30m asymmetry to get a comb of linear readouts.
Typically, we will have some kind of weak signal at the photodiode and will use a 20 or 40 dB gain RF amp to get the signal into the mixer. In this case, the mixer output noise will be at the level of tens of nV/rHz. Any usual low noise audio amplifier (SR560 variety) will be enough to read out the signal.
Why the 50 Ohm terminator? If you look at the specs of the BLP-1.9 filter from Mini-Circuits (its the same for almost all of their LP filters) you see that there's ~90 dB of attenuation above ~30 MHz (where our signals 2*f product will show up). If we use an RF input signal of ~0 dBm, this means that we get a high frequency product of -95 dBm (~10 uVrms) which is OK. But the return loss is 0 dB above 5 MHz - this means that all of the high frequency content is reflected back into the mixer! The 50 Ohm terminator is there to absorb the RF signals coming out of the mixer so as to prevent them from going back into the mixer and mixing with the RF/LO signals. The 50 Ohm terminator does attenuate the DC/audio frequency signals we get out of the mixer by a factor of two, but that's OK since we are not limited by the mixer's thermal noise.
To checkout the noise, we used a 6m RG-58 cable in one leg. We used the DS345 signal generator for the source. We adjusted the frequency to (~21 MHz) give a ~zero mean signal at the demod output. The 6m cable makes the demod output's peak-peak swing correspond to ~16 MHz. We then used an SR560, DC coupled, G=1000, low-noise, 2pole low pass at 1 kHz, to get the signal into the ADC.
The attached plot shows the noise. We have caibrated the digital gain in the channel to make the output into units of Hz. The high frequency noise floor is ~0.3 Hz/rHz and the 1/f knee is at 10 Hz. This setup is already good enough for all of the green locking work at the 40m. In order to make this useful for the reference cavity work or the gyro, we will have to use a longer cable and a lower noise audio amplifier.
As can be seen from the plot, the ADC noise is below the measured noise. The noise of the SR560 with the input terminated is shown in grey - the measured noise of the MFD is very close to this. In order to improve the performance, the next step should be to use a longer cable. There's clearly going to be some trade-off between the temperature dependent effects which come with long cables (dphi/dT gets bigger) and trying to use a high gain ~1 nV/rHz amplfier at the mixer output.
Temperature Drift of the long cable:
This 24-hour minute-trend shows the frequency wander as well as the room temperature. This is not proof of a temperature dependence, but if it is then we get ~3 kHz/deg for the sensitivity. If this is actually the cable and not the amplifier, then we'll have to hunt for a lower tempco cable and put it in a box to isolate it.
I swapped over to a 3x longer cable (old 65 ft. Pasternak cable from ancient 40m days). The old one was 6m, the new one is 18.2 m. It was already coiled up so I put it into a tupperware box to shield it somewhat from the HVAC wind.
The noise went down nearly proportional to the length (after I recalibrated the DAQ channel for the ~3x higher phase->voltage gain). With this length, the peak-peak mixer range is 5.5 MHz, so still enough to go an FSR here.
I give credit to the low frequency improvement entirely to Tupperware for their excellent containers. The current noise limit is most likely the SR560.
This is with reference to Kevin and Jenne's elogs # 3890, 4034 and 4048 .
While the electronics are working okay, there is no DC signal from the photodiode.
Since the solderings and tracks on the PCB were fine I took a close look at the exposed front face of the photodiode.
As we can see, one of the thin wires on the top surface of the photodiode is broken. We can see some wipe marks closer to the lower left edge..
Something seems to have brushed across the exposed face of the photodiode and dislodged the wire.
The new photodiode still has its protective can intact. Do we need to remove the can and expose the photodiode before istallation?
A new photodiode ( Perkin and Elmer Model no. C30642GH Sl No.1526) has been installed in the place of the old photodiode. The datasheet of this model is attached.
The 68pF capacitor which was present in parallel with the photodiode has been removed. Here is a picture of the PCB ( in all its gory detail!) and the photodiode after replacement.
I also checked to see if we have a DC output from the new PD. With 375mW of 1064nm light incident we have 15mV of output. Which matches well with the typical Reponsivity of 0.8V/A reported in the datasheet and our REFL11 ckt . The schematic of the ckt is also attached here for easy reference. The various factors are
V_dc = 0.375 mW x 0.8 V/A x 10 Ohm x 5 = 15mV
The last factor is the gain of the last stage on the DC route.
When I reassembled the box I noticed that there is problem with the SMA connectors popping out of the box. The holes seem misplaced so I enlarged the holes to remove this concern.
375 mW is way too much light. We must never put more than 100 mW on any of these diodes. We don't want to blow up more diodes like we did with the WFS. The InGaAs diodes often show an excess dark noise before they finally let go and completely fail. This one may show excess during the shot noise testing.
We should ensure that the beam paths are engineered so that none of these new detectors ever sees such high light levels.
The DC path should be made to let us see a 10V from the differential EPICS readout when there is 100 mA of photocurrent (i.e. an effective 100 Ohms transimpedance):
0.1 A * 10 V/A * 5 V/V * 2V/V
The last factor of 2 is from the single to differential conversion.
If we really only get 15 mV from 375 mW, then this diode or the circuit is broken.
Suresh is saying 375mW and 0.375mW. Let's wait for his update of the actual power.
Also he is not using EPICS, there may be the factor of two missing for now.
It is 0.375 mW as in the calculation. The total diode output is just 1mW and it is divided with a 50/50 beam splitter... There are a couple of lenses along the way which may account for the ~12% loss.
I used a handheld multimeter to measure the output.
We hooked up the VCO Driver output to the MFD. We adjusted the levels with attenuators to match up to the Level 7 mixer that's being used.
The mixer the input to the SR560 is going in to the XARM_COARSE_OUT channel and the SR560 (AC coupled, Low Noise, G=1000, LP@1kHz) 600 Ohm output goes into XARM_FINE_OUT.
We calibrated these channels by putting in a 10 mVpp sine wave at 0.22 Hz into the Wideband Input of the VCO Driver box (which has been calibrated to have 1.75 MHz/V for f < 1.6 Hz). This should correspond to 17.5 kHz_pp.
To increase the sensitivity, we also added a 140 ft. BNC cable to the setup. We also added some extra short cable to make the overall phase shift be ~90 deg and zero out the mixer output.
I used the time series data in DTT to then calibrate the channels by changing the GAIN field in their filter modules. So now the DAQ channels are both calibrated as 1 count/Hz.
There were several parts in this box which did not have shunting capacitors across their input power lines. Only the four RF amps (ZHL-2) had them.
I soldered two capacitors (100 microF electrolytic and 150pF dipped mica) across the power supply lines of each of the following units: 11MHz oscillator, 29.5 MHz oscillator, Wenzel 5x frequency multiplier and the 12x RF amplifier (ZHL-1HAD).
It was quite difficult to reach the power inputs of these units as some of them were very close to the inner walls of the box. To access them I undid the front panel and found that there were several very taut RF cables which prevented me from moving the front panel even a little.
I had to undo some of the RF cables and swap them around till I found a solution in which all of them had some slack. At the end I checked to make sure that the wiring is in accordance with the schematic present here.
The Distribution box is several steps nearer to completion.
1) Soldered capacitors and DC power lines for four units of the distribution box.
2) mounted all the components in their respective places.
3) Tomorrow we prepare the RF cables and that is the last step of the mechanical assembly.
4) we plan to test both the generator and distributon parts together.
Kevin took a transfer function of the newly assembled PD and noticed that the frequency has shifted to 14.99 freom 11. MHz.
We needed to find the current RLC combination. So we removed the ferrite core from L5 rendiring it to its aircore value of 0.96/muH. We then used this to find the Capacitance of the PD (117pF)
We used this value to compute the inductance required to achieve 11.065MHz which turned out to be 1.75microH.
This was not reachable with the current L5 which is of the type 143-20J12L (nominal H=1.4 micro Henry).
We therefore changed the inductor to SLOT 10 -3-03. It is a ferrite core, shielded inductor with a plasitc sleeve. Its nomial valie is 1.75 microH
We then tested the DC output to see if here is a response to light. There was nonel. l
The problem was traced to the new inductor. Surprisingly the inductor coil had lost contact with the pins.
I then replacd the inductor and checked again. The elecronics seems to work okay.. but there is a very small signal 0.8mV for 500microW.
There seems to be still something wrong with the PD or its electronics.
This is the 140 ft. MFD measurement of the VCO phase noise. It is open loop and so should be a good measurement. The RMS is 30 Hz integrated down to 2 mHz.
I don't know why this doesn't agree with Suresh's measurements of the same thing which uses the PLL feedback method.
In BLUE, I also plot the frequency noise measured by using a Stanford DS345 30 MHz func. generator. I think that this is actually the noise of the FD (i.e. the SR560 preamp) and not the DS345. Mainly, it just tells you that the PINK VCO noise measurement is a real measurement.
I calibrated it by putting in a 5 kHz_pp triangle wave on the sweep of the DS345 and counting the counts in DV.
Today I was working on RF distribution box.
So far I almost finished to electronically isolate voltage regulators from the box wall by inserting mica sheet, sleeve, and washers.
The problem I found is the resistance between wall and the voltage regulator is order of M ohms
I checked my isolation (mica sheet and sleeve and washer) but there is no problem there.
But I found that the power switch is not completely isolated from the wall.( around 800 kohm)
and that the resistance between the regulator and the wall is smaller for the regulator closer to the power switch
and greater for the regulator less closer to it.
So I think we need to put washer or sleeve to isolate the powersitch electronically from the box wall.
Suresh or I will fix this problem
[ To Suresh, I can finish the isolation when I come tomorrow. Or you can proceed to finish isolation.]
Frank put his low noise preamp info here.
I suggest that we build these (using Altium) but replace the cheapo transistors with the high class MAT03 matched BJT pair from Analog Devices.
This will allow us to have a pre-amp better matched to the noise of the mixers down to low frequency.
Most of the RF cables required for the box are done. There are two remaining and we will attend to these tonight.
We expect to have finished the mechanical assembly by Sunday and start a quality test on Monday.
The mechanical assembly of RF distribution box is 99% complete. Some of the components may be bolted to the teflon base plate if needed.
All RF cables and DC voltage supply lines have been installed and tested. I removed the terminal block which was acting as a distribution box for the common zero voltage line. Instead I have used the threaded holes in the body of each voltage regulator. This allows us to keep the supply lines twisted right up to the regulator and keeps the wiring neater. The three regulator bodies have been wired together to provide a common zero potential point.
I did a preliminary test to see if everything is functioning. All units are functioning well. The output power levels may need to be adjusted by changing the attenuators.
The 2x frequency multiplier outputs are not neat sine waves. They seem to produce some harmonics, unlike the rest of the components.
I will post the measured power output at each point tomorrow. The RF power meter could not be found in the 40m lab. We suspect that it has found its way back to the PSL lab.
Frank is recommending these PhaseTrack-210 as phase stable low loss rf coax cables.
We wish to have roughly 2 dBm of output power on each line coming out of the RF distribution box. So I adjusted the attenuators inside the box to get this.
I also looked at why the 2x output looked so distorted and found that the input power was around 17 dBm whereas the maximum allowed (as per the datasheet of Minicircuits MK-2) is 15dBm. So I increased the attentuation on its input line to 5dBm (up by 2dBm) The input power levels are around 14.6dBm now and the distortion has come down considerably. However the net output on the 2x lines is now down to 0.7dBm. We will have to amplify this if we need more power.
The schematic and the power output are now like this:
[Kevin, Rana, Koji]
I calculated the dark noise of POX and POY due to Johnson noise and voltage and current noise from the MAX4107 op-amp using nominal values for the circuit components found in their data sheets. I found that the dark noise should be approximately 15.5 nV/rtHz. The measured dark noise values are 18.35 nV/rtHz and 98.5 nV/rtHz for POX and POY respectively. The shot noise plots on the wiki have been updated to show these calculated dark noise sources.
The measured dark noise for POY is too high. I will look into the cause of this large noise. It is possible that the shot noise measurement for POY was bad so I will start by redoing the measurement.
This experiment deals with measuring the total harmonic distortion (THD) contribution of mixers in a circuit.
(a circuit diagram is attached) where:
Mixer: ZFM-3-S+ at +7dBm
Attenuator: VAT-7+, at +7dB
Low-pass filter: SLP-1.9+, which is set to DC-1.9MHz
The total harmonic distortion can be calculated by the equation:
where Vn represents the voltage of the signal at a certain harmonic n.
In this experiment, only the voltages of the first three harmonics were measured, with the first harmonic at 400Hz, the second at 800Hz, and the third at 1.2kHz. The corresponding voltages were read off the spectrum analyzer after it had time averaged 16 measurements. (picture of the general shape of the spectrum analyzer output is attached)
(results for this mixer's particular configuration are on the pdf attached)
There really isn't that much correlation between the modulations and the resulting THD.
We won't know how good these numbers are until more experiments on other mixers are done, so they can be compared. Since the rest of the mixers are relatively high levels (+17dBm, +23dBm in comparison to this experiment's +7dBm), an RF amplifier will need to be hooked up first to do any further measurements.
The previous measurement for the shot noise of POY had the dark noise at ~100 nV/rtHz. I redid the measurement and got 26 nV/rtHz for the dark noise. I think that when I made the previous measurement, the spectrum analyzer had automatically added some attenuation to the input that I failed to remove. This added attenuation raised the noise floor of the measurement making the dark noise of POY appear larger than it is.
The updated measurement can be found on the wiki at http://lhocds.ligo-wa.caltech.edu:8000/40m/Electronics/POY.
Finished calculations for harmonic distortion at each of the 10 outputs of the RF distribution box. The diagram can be found on Suresh's post http://nodus.ligo.caltech.edu:8080/40m/4342
THD calculation consisted of gather data on the dBm at harmonics of the fundamental frequency. These dBm values were converted into units of power and plugged into the appropriate THD equation pulled from Wikipedia:
On the table, the number 1-6 correspond to the harmonic number of the input frequency used. For example, the first five PD's listed used an 11MHz source, while the second set of five PD's listed used a 55MHz source. Values listed under certain harmonics are dBm measurements at the corresponding frequency. The P-subscript values are essentially the dBm measurements converted to units of power (Watts) for ease of calculation in the equation above. THD is then calculated using these power units; I have converted the ratios to percentages.
It should be noted that as with all THD calculations, the more data points collected, the more precise the THD % will be.
By the way, the outputs on the physical RF distribution box for REFL165 and AS165 are actually labeled as REFL166 and AS166.
Fast work indeed! It would be nice if we could have the following details filled in as well
a) A short title and caption for the table, saying what we are measuring
b) the units in which this physical quantity is being measured.
It is good to keep in mind that people from other parts of the group, who are not directly involved in this work, may also read this elog.
I measured the transfer function, shot noise, and dark spectrum of AS55.
From the shot noise measurement, the RF transimpedance is (556.3 +- 0.8) Ohms and the dark current is (2.39 +- 0.01) mA. The dark noise agrees with the approximate value calculated from the circuit components.
There are no anomalous oscillations when there is no light on the photodiode. I am working on fitting the transfer function in LISO but the other plots are on the wiki at http://blue.ligo-wa.caltech.edu:8000/40m/Electronics/AS55
We want to increase gain in the lower frequencies, so a circuit must be designed (a passive low pass filter).
First, measurements were taken at the X arm for impedance and capacitance, which were 104.5kOhms and 84.7pF respectively. Kiwamu decided to make the circuit resemble a voltage divider for ease of calculation, such that Vout/Vin would be a ratio of some values of the equivalent circuit reactance values. After a few algebra mistakes, this Vout/Vin value was simplified in terms of the R, C measured and the R', C' that would be needed to complete the circuit.
Since the measured C was very small and the measure R was fairly high, the simplified form allowed us to pick values of R' and C' that would make the critical frequency occur at 0.1Hz: set the R' resistance to 1MOhm and C' capacitance to 10uF, which would yield a gain ~1.
With these values a circuit we can start actually making the circuit.
[Steve, Suresh, Larisa]
The following cables were laid today: ETMYT, ETMY, IFOPO, MC1, OMCR, AS Spare, and MC2T.
Though the paper suggested 135' for the MC2T, we used a 110'. This is too short: need at least another 15' for the MC2T.
The RCR cable wasn't crossed off on the list, but a cable exists at the RCR cable which is black and is labeled (old label, 75 ohms)
There was no indication of which length was needed for MC1, so a 95' cable was used.
This is the continuation of http://nodus.ligo.caltech.edu:8080/40m/4402
The first picture is of the actual component, where the resistor is 1M and capacitor is 10uF.
But before the component can be put into place, its transfer function had to be checked to make sure it was doing what we calculated it would do. The results of these are in the graphs generated: frequency vs. gain, and frequency vs. phase.
According to these graphs, we are not achieving the targeted cutoff frequency: need to recalculate and compensate for the extra 100k resistance being encountered.
For bode plot:
USE LOG-LOG plot for the amplitude
USE LOG-LINEAR plot for the phase
Search "Bode Plot" on web
I have prepared several diagrams outlining the current state of the RF System.
These are uploaded into the svn40m here and will be kept uptodate as we complete various parts of the task. These plans have taken into account
the new priorities of the LSC (set out by Koji here )
We (Koji, Kiwamu and I) took stock of the RF cables which we have inherited from the earlier RF system and have made new plans for them.
I took stock of the filters purchased for the modifying the demod boards. We have pretty much everything we need so I will start modifying the boards right away. The following table summarises the modifications
LP Filter (U5)
We seem to have a spare SHP-175. I was wondering where that is supposed to go.
This is the status and tentative schedule for completing the various tasks. I have put the dates based on priority and state of the hardware.
The RF Cable layout plans are drawn on top of a Lab Layout. The various subsystems are drawn (not to scale) on separate layers. The graffle files are located here . I thought they might come in handy for others as well.
[Steve, Kiwamu, Larisa]
Having finished laying new cable last week, we moved on to connecting those on PSL table and AP table.
--RCR, RCT, PMCR (all three are blue)
--OMCR (blue cable, ***now has a camera***), PMCT, IMCR, REFL, AS (white cable), OMCT (***now has camera***)
Unless otherwise noted, the cables are black on the AP table. Also on the AP table: cables were connected directly to the power source.
The wiki has been updated accordingly.
Steve noted that MC2T and POP cameras are not there.
Ooh. Can you explain the purpose of the resistors which are connected to the (+) inputs? It looks like some real electronics ninjitsu.
51 Ohm for CLC409
The datasheet of CLC409 uses 25Ohm there. This is to cancel the input bias current of the two inputs of the opamp.
The source impedance (series) of SGD444 is 50Ohm. So I used 50Ohm for the + input shunting.
However, I could probably use anything between 0-50Ohm as the datasheet itself tells that the bias currents are
not related between the two inputs. In addition, I am not sure how much the real series resistance of the PD is.
1kOhm for OP27
This resister is to ensure the (+) input to have a high impedance at high frequencies.
As far as OP27 is behaving as an ideal opamp, the (+) input has a high impedance.
Also if the inductor behaves as the ideal inductor, no photocurrent comes to the AF path.
However, if both of the op27 and the inductor show similar impedances to the RF transimpedance of 240Ohm,
the AF path absorbs some photocurrent and affects the RF transimpedance of the RF output.
We know that the inductor has a self resonance where the shunt capacitance take over the impedance of the coil.
Above that frequency, the inductor is no longer the inductor. The self resonant freq of this inductor is ~300MHz. It is OK, but not
too far from the freq of interest if we like to see clear cut off at around f>100MHz.
Also OP27 is an AF amplifier and I had no confidence about the input impedance of the OP27 at 100~300MHz.
If I put 1k in the (+) input of the OP27, I can ensure the entire AF path has the impedance of ~1k (at least 500Ohm even when L and OP27 are shorted).
I think the chip resister easily works as a resister up to 1GHz.
I brought TTFSS set #7 to 40m and kept it in the electronic cabinet.
note that Q4 transistor has not been replaced back to PZT2907A yet. It's still GE82.
Q3 is now pzt3904, not PZT2222A.