I spent some time trying to understand how touching the metal cage inside or bending the PCB board affected the photodiode response. It turned out that there was some weak soldering of one of the inductors.
Hartmut suggested a possible explanation for the way the electronics transfer function starts picking up at ~50MHz. He said that the 10KOhm resistance in series with the Test Input connector of the box might have some parasitic capacitance that at high frequency lowers the input impedance.
Although Hartmut also admitted that considering the high frequency at which the effect is observed, anything can be happening with the electronics inside of the box.
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?
REFL165 PD has been made from the old 166MHz PD.
As the required inductance was ~10nF level, the stray inductance of the circuit pattern was significant.
So, I am not so confident with the circuit functionality before the optical transfer function test.
I will test REFL33 and REFL165 with the Jenne laser to see how they work.
This REFL165 was good in terms of RF, but I forgot to make the DC path functioning.
I will try some ideas to fix this tomorrow.
REFL165 PD was made and tested. The characterization results are in the PDF file.
Resonance at 166.12MHz
Q of 7.3, transimpedance 667Ohm (Series Resistance = Z/Q2 = 2.5Ohm)
shotnoise intercept current = 4.3mA (i.e. current noise of 36pA/rtHz)
As the circuit pattern had ~10nH level strain inductance, some technique was needed.
Now the size of the loop for the resonant circuit is comparable with the size of SOIC-8 opamp.
(Left-Top corner of the photo)
This improved the resonant gain by factor of 8.5dB at the test with TEST INPUT. (Analyzer photo)
There is no tunable component.
The resonant freq was adjusted by a parallel inductance (270nH) to the main inductor (15nH).
These are the measurements for estimating the amplitude of the signal recorded in the CDS when a known amount of modulated light is incident on the photodiode.
I mounted the PD characterisation setup onto a small breadboard which could then be placed close AP table. I then placed position markers for REFL165 on the AP table before moving it onto my small breadboard. The AM laser was driven by an RF function generator (Fluke 6061A) at a frequency of 165.98866 MHz, which is 102 Hz offset from the 165MHz LO. The power level was set at -45dBm. This power level was chosen since anything higher would have saturated the AntiAliasing Whitening Filters. The counts in the CDS were converted to voltage using the ADC resolution = 20V per 2^16 counts.
When the 166MHz power is decreased by a factor of 2 the amplitude of 102Hz wave recorded in CDS goes down by sqrt(2) as expected. The RF AM power incident on the REFL165 was estimated to be 0.011mW(rms) (case #1 in the above table) using the DC power ratio and using the transimpedance of the 1611 BBPD to be 700 Ohms. This produces a 171 mV amplitude wave at 102 Hz. I then stepped down the power by factor of 2 and repeated the measurement.
(These numbers however are not agreeing with the power incident on REFL165 if we assume its transimpedance to be 12500. It will take a bit more effort to make all the numbers agree. Will try again tomorrow)
Here is a picture of the small black breadboard on which I have put together the PD characterisation setup. It would be great if we can retain this portable set up as it is, since we keep reusing it every couple of weeks. It would be convenient if we can fiber couple the path to the PD under test with a 2m long fiber. Then we will not have to remove the PD from the optical table while testing it.
To characterize the RF V to counts we need to know the state of the whitening filter board. Was the filter on or off ? What was the value of the whitening gain slider?
The filter was ON and the whiterning filter gain was 45dB
REFL165 removed from the table for the C(V) test
The PD was returned on the table.
The C(V) compensation path was modified and the change of the resonant freq was cancelled.
A more precise analysis comes later.
The original REFL165 had ~50MHz/A dependence on the DC photocurrent.
The resistr R21, which was 2670 Ohm contrary to the original drawing, was replaced to 532 Ohm
to increase the feedforward gain by factor of 5.
The resulting dependence is reduced to ~0.5MHz/A although it has Q reduction of ~20% at 6mA.
These transfer functions were measured between TEST IN and RF OUT while the diode was illuminated with the white light from a light bulb.
There looks some thermal effect on the resonant freq. If the white light illumination is suddenly removed, the bias compensation
is immediately removed but the resonance takes some time (~min) to come back to the original freq.
I am afraid that the light bulb gave too much heat on the surrounding PCB and lead unnecesarily high level dependence of the resonant freq on the DC current.
Or, if this thermal effect comes from the power consumption on the diode itself, we need to characterize it for aLIGO.
In order to check this, we need a test with the 1064nm illumination on the diode in stead of the light bulb.
The transfer function and current noise were measured. The location of the peak shifts with the amount of incident light power (RF or DC). The TF was measured at an incident 1064nm light power of 0.4 mW which produced a DC output voltage of 14 mV => DC photocurrent of 0.28 mA.
Many of the effects that Koji noted in the previous characterization are still present.
In addition I observed a shift of the peak towards lower frequencies as the RF power supplied to the AM Laser (Jenne Laser) is increased. This could create a dependance of the demodulation phase on incident RF power.
The plots are attached below.
To determine the amount of RF power in the AM laser beam at various RF drive levels I measured the RF power out of the Newfocus 1611 PD while driving the AM laser with a Marconi. During this measurement the DC output was 2.2V. With the DC transimpedance of 10^4 and a sensitivity of 0.8 A/W we have carrier power as 0.275 mW (-5.6 dBm). [Incidentally the measured carrier power with a power meter is about 0.55 mW. Why this discrepancy?]
Estimation of the signal strength at the REFL165 PD:
From the 40m Sensing Matrix for DRFPMI we see that the signal strength at REFL165 in CARM is about 5x10^4 W/m. Since we expect about 0.1nm of linear range in CARM length we expect about 0.05 mW of RF power. If the (DC) carrier power is about 10 mW at the photodiode (18mW is about the max we can have since the max power dissipation is 100 mW in the diode) then the RF : DC power ratio is 5x10^-3 => -23 dB
As this is lower than the power levels at which the PD transfer function was determined and where we noted the distorsion and shift of the resonance peak, it is likely that these effects may not be seen during the normal operation of the interferometer.
The shift due to the carrier power level (DC) change may still however pose a problem through a changing demodulation phase.
1) The REFL165 has been replaced onto the AS table.
2) When the PD interface cable is attached the PD shows a DC out put of 6mV and does not respond to a flash light. I changed the PD interface port in the LSC rack by swapping the other end of the cable with an unused (Unidentified PD) interface cable, The PD is working fine after that. There could be a problem with some binary switch state on the PD interface where the REFL165 cable was plugged in earlier.
Kiwamu mentioned that REFL165 is not responding and its DC out seems saturated at 9V. Koji and I checked to see if changing the power supply to the PD changed its behaviour. It did not.
I then look a close look at the PD and found that the front window of the PD was not clear and transparent. There was a liquid condensation inside the window, indicating an over heating of the PD at some point. It could have arisen due to excessive incident power. The pic below shows this condensation:
I also checked the current flowing through the reverse bias voltage line. There was a voltage drop of 3V across R22 (DCC D980454-01-C) indicating a 150mA of current through the PD. This is way too much above the operating current of about 20mA. The diode must have over heated.
I pulled out the old PD out and installed a new one from stock. The pic below shows the clear window of a new PD.
After changing the PD I checked the DC output voltage while shining a torch light on to the PD. It showed an output of about 30 to 40 mV. This seemed okay because the larger 2mm photodiodes showed ~100mA DC output with the same torch.Below is the current state of the ckt board.
I will tune the PD to 165 MHz tomorrow and measure its transimpedance.
The following tasks need to be done in the daytime tomorrow.
The REFL165 RF output was not reaching the Demod board. The RF cable was disconnected. I fixed that and then I put in a RF signal at 165MHz , 1.66 mVrms at the test input (100Hz off set from the 165MHz LO) and saw that the 100 Hz demodulated signal was visible in the dataviewer.
Will complete the Optical RF power -> CDS counts calibration tomorrow morning.
Old MZ PD (InGaAs 2mm, @29.5MHz) has been modified for REFL33.
There has been no choice for the 11MHz notch other than putting it on the RF preamp
as the notch in parellel to the diode eats the RF transimpedance at 33MHz.
I wait for judgement of Rana if the notch at the MAX4107 feedback is acceptable or not.
REFL33 is ready for the installation
Characterization results of REFL33 is found in the PDF attachment.
Resonance at 33.18MHz
Q of 6.0, transimpedance 2.14kOhm
shotnoise intercept current = 0.52mA (i.e. current noise of 13pA/rtHz)
Notch at 10.97MHz
Q of 22.34, transimpedance 16.2 Ohm
Notch at 55.60MHz
Q of 42.45, transimpedance 33.5 Ohm
I tried the REFL33Q for controlling MICH in the PRMI configuration (#6407)
The result was --
It was barely able to lock MICH in a short moment but didn't stay locked for more than 10 sec. Not good.
[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?
REFL55 was modified. The noise level confirmed. The PD is now ready to be installed.
Kevin's measurement report told us that something was wrong with REFL55 PD. The transimpedance looked OK, but the noise level was terrible (equivalent to the shotnoise of 14mA DC current).
Rana and I looked at the circuit, and cleaned up the circuit, by removing unnecessary 11MHz notch, 1k shunt resister, and so on.
I made a quick characterization of the PD.
The transimpedance ws measured as a function of the frequency. The resonance was tuned at 55MHz. The notch was tuned at 110MHz in order to reject the second harmonics. The transimpedance was ~540V/A at 55MHz. (For the calibration, I believed the DC transimpedance of 50V/A and 10000V/A for the DC paths of this PD and #1611, respectively, as well as the RF impedance (700V/A0 of #1611.
Output noise levels were measured with various amount of photocurrent using white light from a light bulb. The measurement was perforemed well above the noise level of the measurement instruments.
The measured output noise levels were converted into the equivalent current noise on the PD. The dark noise level agrees with the shot noise level of 1.5mA (i.e. 22pA/rtHz). In deed, the noise level went up x~1.5 when the photocurrent is ~1.4mA.
I used a matlab code written by Koji to analyse the transimpedance and current noise data of REFL55. The details are in the attached pdf file.
Resonance is at 55.28 MHz:
Q of 4.5, Transimpedance of 615 Ohms
shot noise intercept current = 1.59 mA
current noise =21 pA/rtHz
Notch at 110.78 MHz:
Q of 54.8 Transimpedance of 14.68 Ohms.
I measured the optical and electrical transfer functions for REFL55 and calculated the RF transimpedance. To measure the optical transfer function, I used the light from an AM laser to simultaneously measure the transfer functions of REFL55 and a New Focus 1611 photodiode. I combined these two transfer functions to get the RF transimpedance for REFL55. I also measured the electrical transfer function by putting the RF signal from the network analyzer in the test input of the photodiode.
I put all of the plots on the wiki at http://lhocds.ligo-wa.caltech.edu:8000/40m/Electronics/REFL55.
I upgraded the old REFL199 to the new REFL55.
To do that I had to replace the old photodiode inside, switching to a 2mm one.
Electronics and optical transfer functions, non normalized are shown in the attached plot.
The details about the modifications are contained in this dedicated wiki page (Upgrade_09 / RF System / Upgraded RF Photodiodes)
We could not find problems with any individual piece of the REFL55 electronics chain, from photodiode to ADC. Nevertheless, the PRMI fringes witnessed by REFL55 is ~x10 higher than ~two weeks ago, when the PRMI could be repeatably and reliably locked using REFL55 signals (ETMs misaligned).
Discussion and next steps:
Q: Koji asked me what is the problem with this apparent increased optical gain - can't we just compensate by decreasing the whitening gain?
A: I am unable to transition control of the PRMI (no ETMs) from 3f to 1f, even after reducing the whitening gain on the REFL55 channels to prevent the saturation. So I think we need to get to the bottom of whatever the problem is here.
Q: Why do we need to transfer the control of the vertex to the 1f signals at all?
A: I haven't got a plot in the elog, but from when I had the PRFPMI locked last year, the DARM noise between 100-1kHz had high coherence with the MICH control signal. I tried some feedforward to try and cancel it but never got anywhere. It isn't a quantitative statement but the 1f signals are expected to be cleaner?
Koji pointed out that the MICH signal is visible in the REFL55 channels even when the PRM is misaligned, so I'm gonna look back at the trend data to see if I can identify when this apparent increase in the signal levels occurred and if I can identify some event in the lab that caused it. We also discussed using the ratio of MICH signals in REFL and AS to better estimate the losses in the REFL path - the Faraday losses in particular are a total unknown, but in the AS path, there is less uncertainty since we know the SRM transmission quite precisely, and I guess the 6 output steering mirrors can be assumed to be R=99%.
Today I tried debugging the mysterious increase in REFL55 signal levels in the DRMI configuration. I focused on the demod board, because last week, I had tried routing these signals through different channels on the whitening board, and saw the same effect.
Based on my tests, everything on the Demod board seems to work as expected. I need to think more about what else could be happening here - specifically do a more direct test on the whitening board.
I did a quick check by switching the output of the REFL55 demod board to the inputs normally used by AS55 signals on the whitening board. Setting the whitening gain to +18dB for these channels had the same effect - ADC overflow galore. So looks like the whitening board isn't to blame. I will have to check the demod board out.
All connections have been restored untill further debugging later in the evening.
I don't have a good explanation why, but I too measured similar numbers to what Koji measured. The overall conversion gain for this board (including the +20dB gain from the daughter board) was measured to be ~5.3 V/V on the bench, and ~16000 cts/V in the CDS system (100Hz offset from the LO frequency). It would appear that the effective JMS-1-H conversion loss is <2dB. Seems fishy, but I can't find anything else obviously wrong with the circuit (e.g. a pre-amp for the RF signal that I missed, there is none).
I also attach the result of the measured noise at the outputs of the daughter board (i.e. what is digitized by the ADC), see Attachment #2. Apart from the usual forest of lines of unknown origin, there is still a significant excess above the voltage noise of the OP27, which is expected to be the dominant noise source in this configuration. Neverthelesss, considering that we have only 40dB of whitening gain, it is not expected that we see this noise directly in the digitized signal (above the ADC noise of ~1uV/rtHz). Note that the measured noise today, particularly for the Q channel, is significantly lower than before the changes were made.
There were multiple problems with the REFL55 demod board. I fixed them and re-installed the board. The TFs and noise measured on the bench now look more like what is expected from a noise model. The noise in-situ also looked good. After this work, my settings for the PRMI sideband lock don't work anymore so I probably have to tweak things a bit, will look into it tomorrow.
After this work, I measured that the orthogonality was poor. I confirmed on the bench that the PQW-2-90 was busted, pin 2 (0 degree output) showed a sensible signal half of the input, but pin 6 had far too small an output and the phase difference was more like 45 degrees and not 90 degrees. I can't find any spares of this part in the lab - however, we do have the equivalent part used in the aLIGO demodulator. Koji has kindly agreed to do the replacement (it requires a bit of jumper wiring action because the pin mapping between the two parts isn't exactly identical - in fact, the circuit schematic uses a transformer to do the splitting, but at some unknown point in time, the change to the minicircuits part was made. Anyway, until this is restored, I defer the PRMI sideband locking.
A new hybrid splitter (DQS-10-100) was installed. As the amplification of the final stage is sufficient for the input level of 3dBm, I have bypassed the input amplification (Attachment 1). One of the mixer was desoldered to check the power level. With a 1dB ATTN, the output of the last ERA-5 was +17.8dBm (Attachment 2). (The mixer was resoldered.)
With LO3dBm. RF0dBm, and delta_f = 30Hz, the output Vpp of 340mV and the phase difference is 88.93deg. (Attachment 3/4, the traces were averaged)
0 dBm ~ 0.63 Vpp. I guess there is ~4dB total loss (3dB from splitter and 1dB from total excess loss above theoretical from various components) between the SMA input and each RF input of the JMS-1-H mixer, which has an advertised conversion loss of ~6dB. So the RF input to each mixer, for 0dBm to the front panel SMA is ~-4dBm (=0.4 Vpp), and the I/F output is 0.34Vpp. So the conversion loss is only ~-1.5 dB? Seems really low? I assume the 0.34 Vpp is at the input to the preamp? If it's after the preamp, then the numbers still don't add up, because with the nominal 6dB conversion loss, the output. should be ~2Vpp? I will check it later.
Missed to note: The IF test was done at TP7 and TP6 using pomona clips i.e. brefore the preamp.
The attached plot shows that also the behaviour of the REFL11 and 55 signals is qualitatively equal to the simulation outcome.
Beautiful double peaks. I don't see the triple zero-crossings. Is this because you adjusted the phase correctly (as predicted)?
Don't you want to have a positive number for POP22? Should we set the demod phase in the configuration script for the positive POP22, shouldn't we?
All the details and data will be included in the wiki page (and so also the results for AS55). Here I just show the comparison of the transfer functions that I measured and that I modeled.
I applied an approximate calibration to the data so that all the measurements would refer to the transfer function of Vout / PD Photocurrent. Here's how they look like. (also the calibration will be explained in the wiki)
The ratio between the amplitude of the 55Mhz modulation over the 11MHz is ~ 90dB
The electronics TF doesn't provide a faithful reproduction of the optical response.
REFL55 has been installed on the AP table. REFL11 has been moved to make space for a 50% beam splitter. The reflected beam from this splitter is about 30% of the transmitted beam power. The reflected beam goes to REFL11 in the current configuration. The DC levels are 1.2V on REFL 11 and 3.5V on the REFL55.
I redid some of the cabling on the table because the we need to choose the heliax cables such that they end up close to the demod board location. As per the 1Y2 (LSC) rack layout given here, some of the PD signals have to arrive at the top and others at the bottom of the LSC rack.
Currently the PDs are connected as follows:
REFL11 PD --> Heliax (ASDD133) (arriving at the top of LSC rack) --> REFL11 Demod Board
REFL55 PD --> Heliax (REFL166) (arriving at the top of LSC rack) --> AS55 Demod Board
AS55 PD --> Heliax (AS166) (arriving at the top of the LSC rack) --> not connected.
We are waiting for the Minicircuits parts to modify the rest of the demod boards.
The heliax cables arriving at the LSC rack are not yet fixed properly. I hope to get this done with Steve's help today.
As the POP55 demod board is actually demodulating the REFL55 signal, I have connected its outputs to the REFL55 ADC inputs. Now, we can go back to using the REFL55 input matrix elements, and the data will be recorded.
I have changed the relevant lines in the locking script to reflect this change.
We did an ingenious checkup of the whitening board tonight.
I've restored all connections at that we messed with at the LSC rack to their original positions.
The TT alignment seems to be drifting around more than usual after we disconnected one of the channels - when I came in today afternoon, the spot on the AS camera had drifted by ~1 spot diameter so I had to manually re-align TT1.
I repeated the usual whitening board characterization test of:
Attachment #1 suggests that the steps are equal (3dB) in size, but note that the "Q" channel shows only ~half the response of the I channel. The drive is derived from a channel of an unused AI+dewhite board in 1Y2, split with a BNC Tee, and fed to the two inputs on the whitening filter. The impedance is expected to be the same on each channel, and so each channel should see the same signal, but I see a large asymmetry. All of this checked out a couple of weeks ago (since we saw ellipses and not circles) so not sure what changed in the meantime, or if this is symptomatic of some deeper problem.
Usually, doing this and then restoring the cabling returns the signal levels of REFL55 to nominal levels. Today it did not - at the nominal whitening gain setting of +18dB flat gain, when the PRMI is fringing, the REFL55 inputs are frequently reporting ADC overflows. Needless to say, all my attempts today evening to transition the length control of the vertex from REFL165 to REFL55 failed.
I suppose we could try shifting the channels to (physical) Ch5 and Ch6 which were formerly used to digitize the ALS DFD outputs and are currently unused (from Ch3, Ch4) on this whitening filter and see if that improves the situation, but this will require a recompile of the RTCDS model and consequent CDS bootfest, which I'm not willing to undertake today. If anyone decides to do this test, let's also take the opportunity to debug the BIO switching for the delay line.
This problem has re-surfaced. Is this indicative of some problem with the on-board VGA? Even with 0dB of whitening gain, I see PDH horns that are 10,000 ADC counts in amplitude, whereas the nominal whitening gain for this channel is +18dB. I'll look at it in the daytime, not planning to use REFL55 for any locking tonight.
Last night after checking cabling and turning on ISS, we tried several times to handoff to REFL_DC but it didn't work at all.
Still no success tonight
My previous eigenfrequency analysis was incorrect by two orders of magnitude due to the misuse of Young's Modulus information for Viton. After editing this parameter (as documented on 7/14 19:00), the eigenmodes became much more reasonable. I also discovered the Deformation option under the Surface Plotting Options, which makes the eigenmodes of the single stack much more apparant.
Attached are pictures of the first four eigenmodes:
First Eigenmode: y-translational, 7.49 Hz
Second Eigenmode: x-translational, 7.55 Hz
Third Eigenmode: z-rotational, 8.63 Hz
Fourth Eigenmode: z-translational, 18.26 Hz
Today I measured the max output power at the EOM output of one of the RF AM Stabilizers that we use to control the modulation depth. I needed to know that number for the designing of the new RF system.
When the EPICS slider of the 166 MHz modulation depth is at 0 the modulation depth is max (the slider's values are reversed : 0 is max, 5 is min; it is also 0 for any value above 5, sepite it range from 0 to 10).
I measured 9.5V from the EOM output, that is 32 dBm on a 50 Ohm impedance.
We pulled out the RF AM stabilization box from the 1X2 rack. PSL shutter was closed, marconi output, RF distribution box and RF AM stabilization box were turned off in that order. We had to remove the 4 rack nut screws on the RF distribution box because of the stiff cables which prevented the RF AM stabilization box extraction. I've left the marconi output and the RF distribution boxes off, and have terminated all open SMA connections with 50 ohm terminators just in case. Rack nuts for RF distribution box have been removed, it is currently sitting on a metal plate that is itself screwed onto the rack. I deemed this a stable enough ledge for the box to sit on in the short run, while we debug the RF AM stabilization box. We will work on the debugging and re-install the box as soon as we are done...