We spent some time tonight looking at locking the PRMI with REFL165 vs. REFL33, while reducing the CARM offset.
We were not able to lock the PRMI on REFL165 I&Q at small CARM offsets. When locking at larger CARM offsets (about 100 counts, which is about 100nm) and then re-adjusting the REFL165 demod phase as I reduced the CARM offset, I saw that I had to significantly rotate the phase. For PRMI only (no arms), the REFL165 demod phase was -138.5 deg. When the PRMI was locked with a -100 count CARM offset, the optimal demod phase was -123 deg. Then at -90 counts the phase was -113 deg. At -70 counts, the phase was -108 deg, at -50 counts it was -98 deg, and at -40 it was -93 deg. We want to go back and look at these more carefully, and in a more continuous way, by watching the sensing matrix calibration lines. It's unclear to me right now why we're seeing this, but it's possible that we're getting some kind of extra 55MHz resonances.
REFL DC looks like it should be good - same slope and gain as sqrtTR, extra 20 or 30 deg of phase margin, so we think that we should be able to transition over to it, and then try engaging the AO path. Tonight we had Den's new 1kHz lowpass engaged, and with this, everything looks nice and stable.
Game plan: Bring CARM in until transmissions are at about 10ish, then try keeping CARM on sqrtInvTrans for the DC part, and engage the AC AO part with REFL DC. We probably just need to try this for a while more to find just the right way to turn it on.
Need to think about demod phase rotation vs CARM offset as well as extra resonances, but this may take a while, and if we can just get the AO path engaged, that would be good.
Just a quick report on the REFL OSA.
The attached plot below shows the raw signal from the REFL OSA which Keiko installed in this afternoon.
When the data was taken the beam on the REFL OSA was a direct reflection from PRM with the rest of the suspended mirrors misaligned.
One of the upper and lower 11 MHz sidebands is resolved (it is shown at 0.12 sec in the plot) while the other one is still covered by the carrier tail.
The 55 MHz upper and lower sidebands are well resolved (they are at 0.06 and 0.2 sec in the plot).
One of the oscilloscopes monitoring the OSA signals in the control room has a USB interface so that we can record the data into a USB flash memory and plot it like this.
I swap an OSA at PSL and OSA at REFL. It was because the PSL-OSA had a better resolution, so we place this better one at REFL. The ND filter (ND3) which was on the way to REFL OSA was replaced by two BSs, because it was producing dirty multiple spots after transmitting.
I'm puzzled why the 11MHz peak can be such high considering 1.7~2 times smaller the modulation depth.
I was also wondering about the same thing, comparing with what Mirko obtained before with the same OSA ( #5519).
I placed the OSA (Optical Spectrum Analyzer) on the AP table and this OSA will monitor the REFL beam.
Tomorrow I will do fine alignment of the OSA.
- a new 90% BS in the REFL path for limiting the REFL beam power
- Squeezed the ABSL (ABSolute length Laser) path
- Modification of the AS OSA path
I am installing an OSA on the AP table and it's ongoing.
The OSA for the REFL beam is now fully functional.
The only thing we need is a long BNC cable going from the AP table to the control room so that we can monitor the OSA signal with an oscilloscope.
The attached picture shows how they look like on the AP table. Both AS and REFL OSAs are sitting on the corner region.
We measure the REFL OSA spectrum when (1) direct reflection from the PRM (2) CR lock at PRC (3) SB lock at PRC. When CR lock, both SBs are reflected from the PRC and when SB lock (ref line), some SB is sucked by PRM and looked lower than the other two lines.
After discussions during the meeting today, I removed the PBS from the REFL path, which gives much more light to REFL11, REFL33 and REFL55. Also, the ND1.5 in front of REFL165 was replaced with ND1.1, so that REFL165 now gets 50mW of light. REFL11 gets about 1.3mW, REFL33 gets about 13mW and REFL55 gets about 12mW.
No locking, and importantly no re-phasing of any PDs has been done yet.
Here is an updated diagram of the REFL branching ratios.
Some more words on yesterday's REFL path work.
The 90/10 BS that splits the light between REFL11 and REFL55 was placed back in August 2013, to compensate for the fact that REFL11 has a much larger RF transimpedance than REFL33. See elog 9043 for details.
We had been operating for a long time with an embarrasingly small amount of light on the REFL PDs. REFL11 used to have 80 uW, REFL33 used to have 400 uW and REFL55 used to have 700 uW. REFL 165 was the only sane one, with about 15 mW of light.
After yesterday's work, the situation is now:
As an aside, I was foiled for a while by S vs. P polarizations of light. The light transmitted through the PBS was P-pol, so the optics directing the beams to REFL11, 33 and 55 were all P-pol. At first I completely removed the PBS and the waveplate, but didn't think through the fact that now my light would all be S-pol. P-pol beam splitters don't work for S-pol (the reflection ratios are different, and it's just a terrible idea), so in the end I used the PBS to set the half waveplate so that all of my light was P-pol, and then removed the PBS but left the waveplate. This means that all of the old optics are fine for the beams going to the 3 gold-box REFL PDs. We don't have many S-pol beamsplitter options, so it was easier to use the waveplate to rotate the polarization.
I measured the spectrum of the REFL165 output using AG4395A. As this entry we put the directional coupler between REFL165 output and demod board input, so I measure the signal from the coupler during the PRMI was locked.
After measure REFL165, I also measured REFL55 output in order to make sure that the signal is not smaller than noise because of coupler. I terminated the couple output of coupler on the REFL165, and take signal from REFL55 output port directly. Both plots seems same except for around the resonant frequency of each PDs. From this plot we cannot say that the coupler reduce signal to spectrum analyser too much.
After this measurement I reconnected the REFL165 to analyser and reconnected the REFL55 output to demod board.
After Xarm and Yarm were aligned by Anchal et al, I aligned AS and REFL path in the AP table.
REFL path was alreasy almost perfectly aligned.
-REFL beam centered on the REFL camera
-Aligned so that REFL55 and REFL33 RFPDs give maximum analog DC outputs when ITMY was misaligned to avoid MICH fringe
-Aligned so that REFL11 give maximum C1:LSC-REFL11_I_ERR (analog DC output on REFL11 RFPD seemed to be not working)
-AS beam centered on the AS camera. AS beam seems to be clipped at right side when you see at the viewport from -Y side.
-Aligned so that AS55 give maximum C1:LSC-ASDC_OUT16 (analog DC output on AS55 RFPD seemed to be not working)
-Aligned so that AS110 give maximum analog DC output
The dump and some temporary mirrors were removed and now the REFL beam is available again.
I locked PRMI with REFL signals, it locked as usual.
Currently the REFL beam is bypassed by additional mirrors and blocked by a razor blade dump.
[Suresh / Kiwamu]
Therefore the signals associated with the REFL ports (e.g. REFL11, REFLDC and etc.) are unavailable.
Just be aware of it.
I moved some of the REFL optics on the AS table by a teeny bit to accomodate the new place that the REFL beam exits the chamber (none of this was done while we were at air....we were only dealing with the AS beam at the time, and were happy that REFL came out of the vacuum).
The REFL beam is now on the REFL camera (with PRMI aligned), and the beam is going toward the 4 REFL RF PDs, but it's not aligned to any of them.
I have some questions as to mystery optics on in the REFL path. There is a 90% BS, and I don't know where the 10% reflection goes....is it going to beat against the AUX Stochino laser?
I have to go, and I didn't fix the videocapture script today, so pix tomorrow, I promise.
According to the wiki, REFL 11 has a transimpedance of 4.08kV/A, and REFL 55 has a transimpedance of 615V/A. This is a ratio of ~6.5 . My optickle simulations from earlier this evening indicate that, at maximum, there is a ~factor of 2 more signal in REFL 11 than REFL 55. This is a factor of order 10-15. Then, REFL 55 has 15dB whitening gain, which is a factor of ~4. So, this explains why we're seeing so much more digital signal on REFL11 than REFL55.
Tomorrow, I need to replace the 50/50 beam splitter that splits the beam between REFL55 and REFL11 (33 and 165 have already had their light picked off at this point). I want to put in a 10% reflector, 90% transmission beamsplitter. Steve, can you please find me one of these, and if we don't have one, order one? This will give us a little more light on 55, and less light on 11, so hopefully we won't be saturating things anymore.
As I always tell everyone: Don't use a 10% reflector which produce ghost beams. Use a 90% reflector.
As I always tell everyone: Don't use a 10% reflector which produce ghost beams. Use a 90% reflector.
Hmmm, yes, I forgot (bad me). I'll find a 90% refl BS, and swap the positions of REFL11 and REFL55.
I have done the swap in the REFL path. First, I swapped the positions of REFL11 and REFL55. Then, I swapped out the 50/50 BS for a 90% reflection BS. (90% goes to REFL55, 10% goes to REFL11). I also changed the aluminum dump that was dumping the old REFL165 path into a razor dump.
Before: REFL11 had 4.0mW, REFL55 had 3.1mW. Now, REFL11 has 0.53mW, and REFL55 has 6.9mW. REFL165 still has around 61mW of light, and REFL33 has 3.3mW (the things that were changed were after 165 and 33 in the REFL path).
Now, the DC value of the REFL PDs are: REFL165 = 10.4V, REFL33 = 110mV, REFL55 = 232mV, REFL11 = 18.6mV.
As I was finishing aligning the beams onto all of the REFL diodes, Manasa asked for the IFO so she and Masayuki could continue their work on the Xarm, so I'll check the signals acquired a little later.
We need to calculate whether this level of astigmatism is expected from the new active TT mirrors, but I claim that the beam is not clipped.
As proof, I provide a video (PS, why did it take me so long to be converted to using video capture??). I'm just showing the REFL camera, so the REFL beam as seen out on the AS table. I am moving PRM only. I can move lots in pitch before I start clipping anywhere. I have less range in yaw, but I still have space to move around. This is not how a clipped beam behaves. The clipping that I see after moving a ways is coincident with clipping seen by the camera looking at the back of the Faraday. i.e. the first clipping that happens is at the aperture of the Faraday, as the REFL beam enters the FI.
Also, I'm no longer convinced entirely that the beam entering the Faraday is a nice circle. I didn't check that very carefully earlier, so I'd like to re-look at the return beam coming from TT1, when the PRM is misaligned such that the return beam is not overlapped with the input beam. If the beam was circular going into the Faraday, I should have as much range in yaw as I do in pitch. You can see in the movie that this isn't true. I'm voting with the "astigmatism caused by non-flat active TT mirrors" camp.
Let's wait for astigmatism calculation.
In either case(clipping or astigmatism), it takes time to fix it. And we don't need to fix it because we can still get LSC signal from REFL.
So why don't we start aligning input TTs and PRMI tomorrow morning.
Take the same alignment procedure we did yesterday, but we should better check REFL more carefully during the alingment. Also, use X arm (ETMX camera) to align BS. We also have to fix AS steering mirrors in vacuum. I don't think it is a good idea to touch PR2 this time, because we don't want to destroy sensitive PR2 posture.
Calculations need to be done in in-air PRMI work:
1. Explanation for REFL astigmatism by input TTs (Do we have TT RoCs?).
2. Expected g-factor of PRC (DONE - elog #8068)
3. What's the g-factor requirement(upper limit)?
Can we make intra-cavity power fluctuation requirement and then use PRM/2/3 angular motion to break down it into g-factor requirement?
But I think if we can lock PRMI for 2 hours, it's ok, maybe.
4. How to measure the g-factor?
To use tilt-and-measure-power-reduction method, we need to know RoC of the mirror you tilted. If we can prove that measured g-factor is smaller than the requirement, it's nice. We can calculate required error for the g-factor measurement.
To see how much of the light that comes out of the REFL port actually goes to the PDs, I measured the power immediately after leaving the vacuum (~575mW) and in front of REFL11 (~5mW) and REFL55 (~6mW).
So, 0.01 of the power leaving the vacuum actually goes to the REFL PDs. This number will be useful when calculating the actual signals (in volts) that we expect to see.
I have recalibrated the REFL signals.
I first adjusted the demod phases until the I-signals lined up with the I-phase in the sensing matrix plot:
I then balanced the ITM drives by pushing on -1*ITMX and +1.015*ITMY, and seeing a minimum of MICH actuation in the I-phase of REFL55 (the PD I was locking with).
I then took a nice long measurement with DTT, and measured the peak heights in I and Q for each REFL diode. I was driving PRM with 100 cts at 675.1Hz, and ITMX with 1000 cts at 452.1 Hz (and matching ITMY drive, to make pure MICH). Knowing these numbers, and the actuator calibrations (PRM elog 8255, ITMs elog 8242), I know that I was driving PRCL by ~4.3 pm, and MICH by ~23 pm.
For the I-phase calibrations, I find the peak height at the PRCL drive frequency, and divide 4.3 pm by that height. For the Q-phase calibrations, I find the peak height at the MICH drive frequency, and divide 23 pm by that height.
This gives me the following calibrations:
Calibration [picometers / count]
REFL 11 I
REFL 11 Q
REFL 33 I
REFL 33 Q
REFL 55 I
REFL 55 Q
REFL 165 I
REFL 165 Q
My calibrated REFL spectra then looks like:
[Jenne, Unni, Manasa]
I touched some in-vac steering mirrors, so we have REFL and IPANG coming out of the vacuum, not clipping. IPPOS was done yesterday. I re-checked a few optics in the AS path that were hard to see yesterday while the plastic light access connector was in place, and AS still looks good.
Except for POX, POY, POP, and putting the regular EQ stops back on PRM, I think we're done with the in-vac stuff.
[Rana, Jenne, Manasa]
POX is coming out of the vacuum. We'll do POY tomorrow. We were able to hold the Watec outside the chamber and focus it on the pickoff mirror, and make sure it was roughly centered. Then we took the lens off the camera, put the camera in the POX beam path, and I steered the pickoff mirror until we were hitting the camera. POY will be done the same way.
POP is more challenging, since the transmission of the G&H mirrors is so low. We're not able to see a beam on an IR card held in the POP beam path. I had thought of removing PR2, getting the beam out, then putting PR2 back (using the same dog clamping some alignment markers technique that we use for the test masses), but the G&H mirrors have a 2 degree wedge, so this won't work. It would be fine for pitch, since the arrow is on the side of the optic, but it wouldn't be correct for yaw.
Maybe we should do something similar to what Suresh et. al. did when they set POP up originally - I think they put a green laser pointer on the POX table, and aligned it such that they were hitting the correct spot on PR2 and PRM (correct = the same as the IR spot, which should be the center of the optics). If we can do that with the POP in-vac steering mirrors, then we're fine, and POP should come out when we're back to high power.
All video capture snapshots of tonights pictures are on the pianosa desktop.
After adding an inductor L=100uH and a resistor R=10Ohm in parallel after the OP547A opamp that provide the bias for the photodiode of REFL11, the noise at low frequency that I had observed, was significantly reduced.
See this plot:
A closer inspection of the should at 11MHz in the noise spectrum, showed some harmonics on it, spaced with about 200KHz. Closing the RF cage and the box lid made them disappear. See next plot:
The full noise spectrum looks like this:
A big bump is present at ~275MHz. it could important if it also shows up on the shot noise spectrum.
# gnd n22
# | |
# Rip Rw2
# | | |\
# nt- Rsi-n2- - - C2 - n3 - - - - | \
# | | | | |4106>-- n5 - Rs -- no
# iinput Rd L1 L2 R24 n6- | / | |
# |- nin- | | | | | |/ | Rload
# Cd n7 R22 gnd | | |
# | | | | - - - R8 - - gnd
# gnd R1 gnd R7
# | |
# gnd gnd
What??? I don't see any gray trace of Rs in the plot. What are you talking about?
Anyway, if you are true, the circuit is bad as the noise should only be dominated by the thermal noise of the resonant circuit.
From the measurements of the 11 MHz RFPD at 11Mhz I estimated a transimpedance of about 750 Ohms. (See attached plot.)
The fit shown in the plot is: Vn = Vdn + sqrt(2*e*Idc) ; Vn=noise; Vdn=darknoise; e=electron charge; Idc=dc photocurrent
The estimate from the fit is 3-4 times off from my analsys of the circuit and from any LISO simulation. Likely at RF the contributions of the parassitic components of each element make a big difference. I'm going to improve the LISO model to account for that.
The problem of the factor of 2 in the data turned out to be not a real one. Assuming that the dark noise at resonance is just Johnson's noise from the resonant circuit transimpedance underestimates the dark noise by 100%.
Putting my hands ahead, I know I could have taken more measurements around the 3dB point, but the 40m needs the PDs soon.
Something must be wrong.
1. Physical Unit is wrong for the second term of "Vn = Vdn + Sqrt(2 e Idc)"
2. Why does the fit go below the dark noise?
3. "Dark noise 4 +/- NaN nV/rtHz" I can not accept this fitting.
Also apparently the data points are not enough.
1) True. My bad. In my elog entry (but not in my fit code) I forgot the impedance Z= 750Ohm (as in the fit) of the resonant circuit in front of the square root: Vn = Vdn + Z * sqrt( 2 e Idc )
2) That is exactly the point I was raising! The measured dark noise at resonance is 2x what I expect.
I also admitted that the data points were few, especially around the 3dB point.
Today I'm going to repeat the measurement with a new setup that lets me tune the light intensity more finely.
Here's another measurement of the noise of the REFL11 PD.
This time I made the fit constraining the Dark Noise. I realized that it didn't make much sense leaving it as a free coefficient: the dark noise is what it is.
Result: the transimpedance of REFL11at 11 MHz is about 4000 Ohm.
Data looks perfect ... but the fitting was wrong.
Vn = Vdn + Z * sqrt( 2 e Idc ) ==> WRONG!!!
Dark noise and shot noise are not correlated. You need to take a quadratic sum!!!
Vn^2 = Vdn^2 + Z^2 *(2 e Idc)
And I was confused whether you need 2 in the sqrt, or not. Can you explain it?
Note that you are looking at the raw RF output of the PD and not using the demodulated output...
Also you should be able to fit Vdn. You should put your dark noise measurement at 10nA or 100nA and then make the fitting.
[Koji and Kevin]
I was trying to characterize the REFL11 photodiode by shining a flashlight on the photodiode and measuring the DC voltage with an oscilloscope and the RF voltage with a spectrum analyzer. At first, I had the photodiode voltage supplied incorrectly with 15V between the +15 and -15 terminals. After correcting this error, and checking that the power was supplied correctly to the board, no voltage could be seen when light was incident on the photodiode.
We looked at the REFL55 photodiode and could see ~200 mV of DC voltage when shining a light on it but could not see any signal at 55 MHz. If the value of 50 ohm DC transimpedance is correct, this should be enough to see an RF signal. Tomorrow, we will look into fixing the REFL11 photodiode.
I just wanted to remind you that the most up to date resource about the RF system upgrade, including photodiodes, is the SVN.
Because I was doing new things all the time, the wiki is not up to date. But the SVN has all I've got.
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.
These are the dark noise spectrum that I measured on the 11MHz and 55MHz PD prototypes I modified.
The plots take into account the 50Ohm input impedance of the spectrum analyzer (that is, the nosie is divided by 2).
With an estimated transimpedance of about 300Ohm, I would expect to have 2-3nV/rtHz at all frequencies except for the resonant frequencies of each PD. At those resonances I would expect to have ~15nV/rtHz (cfr elog entry 2760).
I have to figure out what are the sources of such noises.
Yesterday night I plugged back the REFL11 RF cable into the corresponding demodulation board.
I'm going to remove REFL11 demod for the noise check/circuit improvement.
First I checked the noise levels and the transfer functions of the daughterboard preamp were checked. The CH-1 of the SR785 seemed funky (I can't comprehensively tell yet how it was), so the measurement maybe unreliable.
For the replacement of AD797, I tested OP27 and TLE2027. TLE2027 is similar to OP27, but slightly faster, less noisy, and better in various aspects.
The replacement of the AD797 and whatever-film resistors with LTE2027 and thin-film Rs were straightforward for the I phase channel, while the stabilization of the Q phase channel was a struggle (no matter I used OP27 or TLE2027). It seems that the 1st stage has some kind of instability and I suffered from 3Hz comb up to ~kHz. But the scope didn't show obvious 3Hz noise.
After a quite bit of struggle, I could tame this strange noise by adjusting the feedback capacitor of the 1st stage. The final transfer functions and the noise levels were measured. (To be analyzed later)
Now the REFL11 LO cable was replaced from the soft low noise audio coax (Belden 9239) to jacketed solder-soaked coax cable (Belden 1671J - RG405 compatible). The original cable indicated the delay of -34.3deg (@11MHz, 8.64ns) and the loss of 0.189dB.
I took 80-inch 1671J cable and measured the delay to be ~40deg. The length was adjusted using this number and the resulting cable indicated the delay of -34.0deg (@11MHz, 8.57ns) and the loss of 0.117dB.
The REFL11 demod module was restored and the cabling around REFL11 and AS110 were restored, tightened, and checked.
I've removed the PD mon cables from the NI RF switch. The open ports were plugged with 50Ohm temirnators.
Attachment 1: Transfer Functions
The original circuit had a gain of ~20 and the phase delay of ~1deg at 10kHz, while the new CH-I and CH-Q have a phase delay of 3 deg and 2 deg, respectively.
Attachment 2: Output Noise Levels
The AD797 circuit had higher noise at low frequency and better noise levels at high frequency. Each TLE2027 circuit was tuned to eliminate the instability and shows a better noise level compared to the low-frequency spectrum of the AD797 version.
RXA: AD797 , all hail the op-amps ending with 27 !
- Found the inductor which shunts the positive input of MAX4107 was not touching the ground.
This left the positive input level undetermined at DC. This was why MAX had been saturated.
The PCB has a cut, so it was surprising once the circuit worked.
- Resoldered the inductor to the ground. This made the circuit responding to the intensity-modulated beam.
- But the resonances and the notches were totally off, and the 200MHz oscillation has resurrected.
- Attached 40Ohm+22pF network between the neg-input of MAX and the gnd. This solved the oscillation.
- Made the tuning and the characterizations. The PD is on Kiwamu's desk and ready to go.
More to come later
The symptoms were :
- a big offset of ~ -3 V on the RF output. No RF signals.
- The DC output seemed to be okay. It's been sensitive to light.
I did a quick check and confirmed that +/- 5V were correctly supplied to the op-amps.
It looks that the last stage (MAX4107) is saturated for some reasons. Need more inspections.
At the moment the REFL11 RFPD is on the bench of the Jenne laser.
I took REFL11 out from the AS table for a health check because it wasn't working properly.
The full characterization of REFL11 is found in the PDF.
Resonance at 11.062MHz
Q of 15.5, transimpedance 4.1kOhm
shotnoise intercept current = 0.12mA (i.e. current noise of 6pA/rtHz)
Notch at 22.181MHz
Q of 28.0, transimpedance 23 Ohm
Notch at 55.589MHz
Q of 38.3, transimpedance 56 Ohm
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?