The following tasks will be finished within the next 24h:
we will then focus on (re-)measuring some things we need very soon for a more accurate NB (there are more but the following ones are my favorites for now. We are about a factor of 3 above the CTN @200Hz)
restarted the framebuilder process. Channel list needs to be updated, i didn't clean up the FSS channels so there are still old/unused channel names.
Current list of saved channels.
106 channels total
105 trend channels
chnum slow |name |rate |trend |group |bps |bytes |offset |type |active
0 0 C3:PSL-BOX_SENS1 16 1 0 4 64 0 4 1
0 0 C3:PSL-BOX_SENS2 16 1 0 4 64 64 4 1
0 0 C3:PSL-BOX_SENS3 16 1 0 4 64 128 4 1
0 0 C3:PSL-BOX_SENS4 16 1 0 4 64 192 4 1
0 0 C3:PSL-BOX_TEMPAVG 16 1 0 4 64 256 4 1
0 0 C3:PSL-BOX_HEATER 16 1 0 4 64 320 4 1
0 0 C3:PSL-BOX_SETPT 16 1 0 4 64 384 4 1
0 0 C3:PSL-BOX_KP 16 1 0 4 64 448 4 1
0 0 C3:PSL-BOX_KI 16 1 0 4 64 512 4 1
0 0 C3:PSL-BOX_KD 16 1 0 4 64 576 4 1
0 0 C3:PSL-BOX_ENABLE 16 1 0 4 64 640 4 1
0 0 C3:PSL-BOX_TIMEOUT 16 1 0 4 64 704 4 1
0 0 C3:PSL-BOX_SCALE 16 1 0 4 64 768 4 1
0 0 C3:PSL-ACAV_RFPDDC 16 1 0 4 64 832 4 1
0 0 C3:PSL-ACAV_RCTRANSPD 16 1 0 4 64 896 4 1
0 0 C3:PSL-ACAV_LOCKEDLEVEL 16 1 0 4 64 960 4 1
0 0 C3:PSL-RCAV_SENS1 16 1 0 4 64 1024 4 1
0 0 C3:PSL-RCAV_SENS2 16 1 0 4 64 1088 4 1
0 0 C3:PSL-RCAV_SENS3 16 1 0 4 64 1152 4 1
0 0 C3:PSL-RCAV_SENS4 16 1 0 4 64 1216 4 1
0 0 C3:PSL-RCAV_TEMP 16 1 0 4 64 1280 4 1
0 0 C3:PSL-RCAV_TEMPAVG 16 1 0 4 64 1344 4 1
0 0 C3:PSL-RCAV_S1CAL 16 1 0 4 64 1408 4 1
0 0 C3:PSL-RCAV_S2CAL 16 1 0 4 64 1472 4 1
0 0 C3:PSL-RCAV_S3CAL 16 1 0 4 64 1536 4 1
0 0 C3:PSL-RCAV_S4CAL 16 1 0 4 64 1600 4 1
0 0 C3:PSL-RCAV_SETPT 16 1 0 4 64 1664 4 1
0 0 C3:PSL-RCAV_KP 16 1 0 4 64 1728 4 1
0 0 C3:PSL-RCAV_KI 16 1 0 4 64 1792 4 1
0 0 C3:PSL-RCAV_KD 16 1 0 4 64 1856 4 1
0 0 C3:PSL-RCAV_ENABLE 16 1 0 4 64 1920 4 1
0 0 C3:PSL-RCAV_TIMEOUT 16 1 0 4 64 1984 4 1
0 0 C3:PSL-RCAV_SCALE 16 1 0 4 64 2048 4 1
0 0 C3:PSL-RCAV_RFPDDC 16 1 0 4 64 2112 4 1
0 0 C3:PSL-RCAV_RCTRANSPD 16 1 0 4 64 2176 4 1
0 0 C3:PSL-RCAV_LOCKEDLEVEL 16 1 0 4 64 2240 4 1
0 0 C3:PSL-FSS_HEATER 16 1 0 4 64 2304 4 1
0 0 C3:PSL-FSS_FREQCOUNT 16 1 0 4 64 2368 4 1
0 0 C3:PSL-FSS_VCOMON 16 1 0 4 64 2432 4 1
0 0 C3:PSL-FSS_VCOMON_CAL 16 1 0 4 64 2496 4 1
0 0 C3:PSL-FSS_VCOFREQ 16 1 0 4 64 2560 4 1
0 0 C3:PSL-FSS_RFPDDC 16 1 0 4 64 2624 4 1
0 0 C3:PSL-FSS_LODET 16 1 0 4 64 2688 4 1
0 0 C3:PSL-FSS_PCDET 16 1 0 4 64 2752 4 1
0 0 C3:PSL-FSS_FAST 16 1 0 4 64 2816 4 1
0 0 C3:PSL-FSS_PCDRIVE 16 1 0 4 64 2880 4 1
0 0 C3:PSL-FSS_RCTLL 16 1 0 4 64 2944 4 1
0 0 C3:PSL-FSS_VCODET 16 1 0 4 64 3008 4 1
0 0 C3:PSL-FSS_TIDALOUT 16 1 0 4 64 3072 4 1
0 0 C3:PSL-FSS_MODET 16 1 0 4 64 3136 4 1
0 0 C3:PSL-FSS_VCODETPWR 16 1 0 4 64 3200 4 1
0 0 C3:PSL-FSS_MIXERM 16 1 0 4 64 3264 4 1
0 0 C3:PSL-FSS_SLOWM 16 1 0 4 64 3328 4 1
0 0 C3:PSL-FSS_VCOM 16 1 0 4 64 3392 4 1
0 0 C3:PSL-FSS_TIDALINPUT 16 1 0 4 64 3456 4 1
0 0 C3:PSL-FSS_SW1 16 1 0 4 64 3520 4 1
0 0 C3:PSL-FSS_SW2 16 1 0 4 64 3584 4 1
0 0 C3:PSL-FSS_PHFLIP 16 1 0 4 64 3648 4 1
0 0 C3:PSL-FSS_VCOTESTSW 16 1 0 4 64 3712 4 1
0 0 C3:PSL-FSS_VCOWIDESW 16 1 0 4 64 3776 4 1
0 0 C3:PSL-FSS_INOFFSET 16 1 0 4 64 3840 4 1
0 0 C3:PSL-FSS_MGAIN 16 1 0 4 64 3904 4 1
0 0 C3:PSL-FSS_FASTGAIN 16 1 0 4 64 3968 4 1
0 0 C3:PSL-FSS_PHCON 16 1 0 4 64 4032 4 1
0 0 C3:PSL-FSS_RFADJ 16 1 0 4 64 4096 4 1
0 0 C3:PSL-FSS_SLOWDC 16 1 0 4 64 4160 4 1
0 0 C3:PSL-FSS_VCOPWR 16 1 0 4 64 4224 4 1
0 0 C3:PSL-FSS_VCOMODLEVEL 16 1 0 4 64 4288 4 1
0 0 C3:PSL-FSS_TIDALSET 16 1 0 4 64 4352 4 1
0 0 C3:PSL-FSS_LOCK 16 1 0 4 64 4416 4 1
0 0 C3:PSL-FSS_SLOWLOOP 16 1 0 4 64 4480 4 1
0 0 C3:PSL-PMC_PMCTLL 16 1 0 4 64 4544 4 1
0 0 C3:PSL-PMC_RFPDDC 16 1 0 4 64 4608 4 1
0 0 C3:PSL-PMC_LODET 16 1 0 4 64 4672 4 1
0 0 C3:PSL-PMC_PMCTRANSPD 16 1 0 4 64 4736 4 1
0 0 C3:PSL-PMC_PCDRIVE 16 1 0 4 64 4800 4 1
0 0 C3:PSL-PMC_PZT 16 1 0 4 64 4864 4 1
0 0 C3:PSL-PMC_MODET 16 1 0 4 64 4928 4 1
0 0 C3:PSL-PMC_PMCERR 16 1 0 4 64 4992 4 1
0 0 C3:PSL-PMC_SW1 16 1 0 4 64 5056 4 1
0 0 C3:PSL-PMC_SW2 16 1 0 4 64 5120 4 1
0 0 C3:PSL-PMC_PHFLIP 16 1 0 4 64 5184 4 1
0 0 C3:PSL-PMC_BLANK 16 1 0 4 64 5248 4 1
0 0 C3:PSL-PMC_GAIN 16 1 0 4 64 5312 4 1
0 0 C3:PSL-PMC_INOFFSET 16 1 0 4 64 5376 4 1
0 0 C3:PSL-PMC_PHCON 16 1 0 4 64 5440 4 1
0 0 C3:PSL-PMC_RFADJ 16 1 0 4 64 5504 4 1
0 0 C3:PSL-PMC_RAMP 16 1 0 4 64 5568 4 1
0 0 C3:PSL-PMC_LOCK 16 1 0 4 64 5632 4 1
0 0 C3:PSL-PEM_RMTEMP 16 1 0 4 64 5696 4 1
0 0 C3:PSL-PEM_BOXTEMP 16 1 0 4 64 5760 4 1
0 0 C3:PSL-FSS_RFAM_RCAV 16 1 0 4 64 5824 4 1
0 0 C3:PSL-FSS_RFAM_ACAV 16 1 0 4 64 5888 4 1
0 0 C3:PSL-FSS_EOM_TSET 16 1 0 4 64 5952 4 1
0 0 C3:PSL-FSS_EOM_TACT 16 1 0 4 64 6016 4 1
0 0 C3:PSL-FSS_EOM_IMON 16 1 0 4 64 6080 4 1
0 0 C3:PSL-FSS_EOM_SETTEMP 16 1 0 4 64 6144 4 1
0 0 C3:PSL-GEN_DAQ1 16 1 0 4 64 6208 4 1
0 0 C3:PSL-GEN_DAQ2 16 1 0 4 64 6272 4 1
0 0 C3:PSL-GEN_DAQ3 16 1 0 4 64 6336 4 1
0 0 C3:PSL-GEN_DAQ4 16 1 0 4 64 6400 4 1
0 0 C3:PSL-GEN_DAQ5 16 1 0 4 64 6464 4 1
0 0 C3:PSL-GEN_DAQ6 16 1 0 4 64 6528 4 1
0 0 C3:PSL-GEN_DAQ7 16 1 0 4 64 6592 4 1
0 0 C3:PSL-GEN_DAQ8 16 1 0 4 64 6656 4 1
10001 0 C3:FB1-FB_DUMMY 16384 0 0 4 65536 6720 4 0
changed the channel list for epics channels which will be recorded. Modified file according to changes mentioned in previous post.
File contains also channels which physically don't exist at the moment, e.g. RF-AM channels and 2nd. temp stabilized box around chamber.
The beam height changed by 1/8". Current beam height is 5-7/8". The top stack plate is slightly off-centered towards the ACAV side but we don't want to re-open the chamber now to fix this (it's not much, but one can see it). We can do this the next time we open it anyway or if we see weird coupling between different stack modes. When we open it next time we will add some markings at the end faces of the top stack plate to better see if it's centered or not.
We will align the beams along the hole pattern as designed first and move the cavities to the right position. Once we add the air springs (hopefully soon) we have to change the beam height again.
cleaned up the VME stuff a little bit - removed all old channels we don't use anymore, mostly FSS stuff, added new channels for the in-vac sensors and renamed a bunch of others to match the current situation. The only thing i didn't touch is all channels from "RCAV", which is now common to both cavities but i didn't want to dig too deep and change all scripts etc. So that's left for the near future.
Current channels connected to 16bit ADC (VMIC 3123):
new/modified channels are available in real-time, but not saved at the moment!
old database-files are located in "20120227"-subfolder on the SUN in the psl folder and on the svn in /software
I estimate the noise sensitivity in the delay line technique. The calculation should tell us how good this method can be used to measure frequency noise in beat signal.
The calculation follows the setup in PSL:828. All relevant datasheets can be found here.
I try to compute how each component in the setup generates noise and shows up at the end of the stream. With the calibration factor, I can convert the noise back to its equivalent frequency noise at the input.
The only component that introduces noise in the setup is the ZHL-1A mixer. Its noise figure is ~ 8dB. Assuming that the only noise from the input side is thermal noise in 50 ohm, then the noise level after the amplifier is ~ 50nV, see details in the note below.
For each signal trace, the signal goes through 4-ch splitter (-6dB), cable, then the mixer (~3dB conversion loss). These components give a factor of (1/4) x (1/2) to the signal. I'm not sure how noise in both delay line will sum up at the mixer. So for now I just assume noise coupling from one side of the mixer. The noise level after the low pass should be 50nV/8 = 6.3 nV. The calibration from Voltage to frequency noise at the mixer output is 2.5 MHz/V (from measurement on 2012_02_22,160MHz, svn) Thus the absolute frequency noise is 6.2nV x 2.5 MHz/V = 15.5 mHz. This level is ~ a factor of 1.5 lower than the frequency noise of Marconi (10kHz tuning range) , see psl:834, psl:833. If this is the only limiting source, we should be able to measure coating noise upto ~500Hz (limited bandwidth due to the chosen delay time is not taken into account yet).
It would be nice to be able to measure the noise and compare it with the calculation. To measure the noise, we need a low noise input source (Marconi with 1kHz tuning range should be ok) with power as specified in the setup. However, the measured noise level is higher than the expected noise and we don't know what the cause is. So we can not verify the calculated noise level yet.
Noise figure = 10log10( Noise Factor). From the datasheet, noise figure of the amplifier ~ 8dB which corresponds to Noise Factor = 6.3.
Noise Factor = SNR from input / SNR from output. For our setup, the signal from input is 5dBm (0.4 V), with noise ~ 1nV flat (50 Ohm thermal noise). the output signal is 22.8dBm (3.1V). Thus the expected noise at the output = Noise Factor x (signal_out/signal_in) x noise_in = 6.3x (3.1/0.4) x 1nV ~ 50nV.
I realigned ACAV and found TEM00, but now the transmitted beam is completely missed the opening on the insulation, it is off from the center by ~ 1 cm.
I'm trying to re-align the beams to the cavities. Due to the new RTV springs for the seismic stack, the cavities' natural axes shift by ~1/4 " with respect to the previous position.
I had to adjusted the height of the top mirror of the periscope before I could align and lock RCAV (visibility ~ 95%) again. The pictures below show the position of the current beam. With the previous setup, the beam position was almost at the center of the holes. Now, for RCAV, the axis shifts closer to the edge. RCAV might yaw with respect to the previous position. Left picture shows the incoming beam position, Right picture shows the outgoing beam position.
For ACAV, however, it seems that the position changes a lot and the beam clips on the outer edge of the top mirror before I can even find TEM00. I think I'll have to add a spacer between the mirror mount and the vertical plate in order to re align the beam.
I think we can keep the stack position as it is for now, if I can lock both cavities and the transmitted beams can be adjusted on the breadboard for beat path. We might also have to increase the hole size on the insulation cap as well depending on where the beam position of ACAV will be.
the new AD590 sensors are physically connected to
measured across 30kOhm (+/-1%, 100ppm) to gnd, so voltage is ~9V at 30C. Had no precision resistors but doesn't matter as the channels are used for monitoring purposes only
Software channels are not assigned yet.
disconnected the turbo pump and enabled the ion pump - initial current was 1mA (value before venting was <0.1uA)
pinout of 9-pin PEEK in-vac D-SUB connector:
1(-) / 6(+) - AD590 no1 (ACAV)
2(-) / 7(+) - AD590 no2 (ACAV)
3(-) / 8(+) - AD590 no3 (RCAV)
5 / 9 - heater (155Ohms) (ACAV)
BE CAREFUL: AS THE FEEDTHROUGH IS A MALE/MALE CONNECTOR THE PINOUT GETS MIRRORED OUTSIDE THE VACUUM, SO PIN 1 is PIN 5, PIN 6 is PIN 9 AND SO ON...
window is back on and pumping down over the weekend. Thermal insulation for vacuum chamber is back in place, so we should be ready to go for a new measurement on Monday afternoon after installation of the beat breadboard
I added another noise budget,"beat_nb.m", on SVN. This noise budget is for beat measurement via PLL. It calculates both cavities' noise sources which will add up incoherently in the beat measurement. The code is still incomplete. More noise source will be added later. This noise budget is supposed to be the correct nb for our beat measurement.
This code is based on the previous one which is the noise budget of a single cavity. I remove a few traces e.g. noises from ambient temperature fluctuation coupling through thermal expansion. Since they are way smaller than the other. I still keep Brownian noise in substrates and spacers. These might be removed later so that the plot is not too crowded.
The current code includes noise from:
The other noise sources that will be added later are:
For noise from seismic, especially from vertical direction, we can measure the TF between seismic -> beat. This will tell us the real coupling. Koji and Frank measured the TF between the table and the stack in the previous entry, so there should be no problem with the setup. We can also try other directions (horizontal), but from vertical direction, we should be able to get the idea how seismic couples into beat signal.
For RIN induced noise, I'm not quite sure yet if the SNR will be large enough to be able to measure, but we can approximate.
[Frank and Koji]
1) Stack vertical transfer functions
We have attached a KISTLER accelerometer on the stack.
The accelerometer was epoxied on a low-outgassing kapton tape while the tape is attached on the stack.
The table was shook by either a PZT or impact-hammering by a fist.
The new resonant freqs are 10.5 and 35Hz in stead of previous 15Hz and 55Hz.
This provides us an additional isolation by factor of ~10 above 20Hz
2) Stack pitching mode identification
Location of the accelerometer on the stack was swept from the center to the edge of the plate.
The difference of the transfer functions gives us the idea where are thepitch resonances.
It seems that the resonant frequencies in pitch are 20 and 60Hz
3) Ringdown measurement of the other modes
In order to check the resonant freq of the other modes, the stack was excited by a finger
in longitudinal, yaw-rotational, and transverse directions.
The results: longitudinal 3.1Hz, rotational 5.2Hz, transverse 2.7Hz
after carefully thinking about options how to get better results and closer to the coating thermal noise within the next 10 days we decided to open the vacuum chamber and work on the seismic isolation. The the current sensitivity limit is kind of flat and seems to continue like that towards lower frequencies (which we can kind of see when floating the table).
As we could not clearly identify other sources which are limiting us at the moment we decided to improve the seismic isolation next, which might help us measuring CTN at lower frequencies where it is higher. In parallel we will also add the thermal shields and the heater. So we can reduce the drift between the cavities which then makes it possible to reduce the range of the Marconi and so lower the contribution of phase noise. We already locked it to the Rubidium clock which also lowers the phase noise and should make it possible to see CTN below 1kHz.
With the heater we can also tune the beat frequency towards lower frequencies which
We first replace the old springs of the stack with the new ones characterized here. Measurements will be posted in a separate entry.
In parallel we will work on the air springs to isolate the whole chamber. But we won't get those parts until mid/end next week so it will be kind of a last-minute change before the LSC meeting.
Things finished so far:
- incoming -
We did optimize the alignment, power levels etc and tweeked almost every knob of the system to get an idea where we have to look for the current limit in sensitivity. Didn't find anything dominant. A lot of already known things can limit if you intentionally make it worse / misalign things, but once optimized they are below the current measured noise performance. Detail later.
Tue Feb 28 21:28:53 2012
Beat measurement after optimization, floated table.
The beat noise is roughly a factor of 2 above the coating noise at 130 Hz. This gives us a good reason to change the springs for the seismic stack in order to get better sensitivity at lower frequency, as it is getting closer to the coating noise at lower frequency. At 2kHz and above, the noise spectrum's feature is similar to the noise budget, but with some offset. We might miss a few other flat noise sources( noise from RFPD, electronics) that we have to add into the noise budget. Most of the mechanical peaks around 100 - 1kHz are probably from the mirror mounts.
once again measured the Marconi noise with the delay line - this time without the amplifier (so using a 7dBm mixer instead of 13dBm) and at two different frequencies, 20MHz and 160MHz. Still have no clue where the flat noise floor is coming from which we've seen in previous measurements (see elog #833).
The measurement at 20MHz (left graph) was taken with the frequency tuned so that the DC offset is close to zero (0.1mV). The measurements show a consistent 1/sqrt(f) noise level at low frequencies, independent from the marconi phase noise. And again, the Marconi noise for 1k input range can't be measured.
The right graph shows the result at 160MHz, but this time with a slight DC offset, so that LO AM couples into the measurements. The slope is similar to the one seen at 20MHz, but with clearly more features which come from amplitude noise of the Marconi. The situation at high frequencies is the same, the phase noise of 10k range can be seen, for lower input ranges not.
Next i measured the coupling from AM into the signal for different DC offsets (only at 160MHz), this time again for the original mixer and power levels (the one we want to use in our setup). As before at zero DC offset we are insensitive to AM and the noise floor is somewhat flat. With increasing offset the coupling from AM into the measurement becomes more dominant and looks identical to the coupling which can be seen on the upper right plot. This looks similar at other frequencies but i didn't save those.
The question now is: If the (almost) white noise floor is not thermal noise, amplifier noise etc. and not from AM, where does it come from? Any ideas?
I've tried the following thing, but nothing worked:
tried to measure the frequency noise of the Marconi using the delay line. Setup is identical to the schematic posted in entry #832.
I've set the LO power to 13.64dBm as it is close to optumum value. The mixer output is terminated with 500Ohms. The slope is 1.1145MHz/V.
Measured the noise at the output using the SR785 and a SR560, gain 1000.
Plot shows the following:
Started characterizing the cable-delay setup with the right length of cable (134ft of RG58 for 160MHZ). After checking the change in sensitivity with load impedance i've changed the load to 500 Ohms (instead of the usual 50 Ohms). I think an additional low-impedance path for the 2f has to be put in parallel later (to have proper 50Ohms @ 2f) to not get it reflected at the input of the low-pass filter back into the IF port of the mixer. (see first schematic).
However, the following simple setup has been used for the measurements:
I've measured the output signal vs different LO power levels while keeping the RF signal strength constant (8.29dBm) to find out the optimum signal strength in terms of size (not noise at this point!).
The following plots show the result:
==Some intros about this measurement:==
We are using a mixer as a phase detector. Usually, mixers are not optimized to be used as phase detectors (that provide DC output), they do have 50 ohms impedance output. However, when we use a mixer as a phase detector, the output is DC, and the output behaves like a current source. We want to know how load impedance will affect the readout system, so we measured the output and varied the load impedance.
==Setup and Result==
The setup is shown here. We checked how much we can gain in the cable-delay readout scheme by using a different (higher) load impedance. Used the standard setup with a Marconi as the source, modulated the frequency with a triangular signal and looked after the LP filter with the scope for different termination impedances after the mixer. Plot is in arbitrary units as only the relative change is relevant. Signals are triggered at slightly different times.
The result shows that with higher load impedance, we have better sensitivity (steeper slope). This is as expected from a current source with load impedance ( V = IR). At this point we are using 500 Ohm load impedance in our regular setup because we are not sure if the higher load impedance will introduce any extra noise.
We still working on the breadboard setup. There will be several things we have to modify for the setup.
Now we can mount the mirrors on the three screw blocks, with beam on the hole pattern.
[add details about what to modify]
Note: I have been looking at frequency only around 100 - 3 kHz. Here is the beat for broadband with other noise. The seismic noise in the plot is for floated table, I'll edit that, but it is pretty similar for what we had before. So the breadboard setup has not introduced any noise at low frequency that shows up in the beat yet.
reduced our RG58C/U cable length to optimum value (134.2ft) and characterized it. Below the confirmation that it is what it should be.
loss is 8.6dB, delay 206.4ns
Mixer will be driven very hard to saturate it. To operate the mixer in the required saturated mode, the RF signal level should be at least:
so if we use the right (optimum) cable we would have ~8dBm, which should be perfect for a level 13 mixer.
Let's see if we can confirm the calculations...
I checked the mechanical peaks in breadboard setup. We get rid of the peak at 800 Hz from the periscope and a big peak around 200 Hz. However, there are some new peaks popping up which are not identified yet.
As the beams are not on the whole pattern, Frank suggested that I move the whole board and clamp it down instead. In order to do that, I had to remove the QWPs. I clamped it down with steel clamps around the legs. After that I inserted some damping posts (see below pictures). It turned out that the rubber damping does not help much at this point the noise spectrum does not change at all between with or without the rubber. I think it is because I cannot insert enough rubber between the board and the post, as I slid them in after I clamped the board. I should have placed the damping posts in their places then clamped down the board.
[add clamping fig]
fig: beat noise in different setup: Blue: beat path with periscope, Magenta: beat with breadboard setup, Green: with damping on mirror mounts. The data were taken with 2kHz tuning range, gain 200.
Since only the beat path that has been changed, all the peaks that popping out are contribution from the breadboard setup.
So things that are changed are:
There are a few new peaks due to the breadboard setup which have not been identified yet. It is very hard to check, since tapping with slight force already excite the peaks of the mirror mounts around 1 kHz. Once the mounts are damped, other peaks might be easier to be found.
now as we know that the optimum loss of the delay line is 8.68dB we can calculate the optimum cable length.
optimum length for 160MHz are:
cables which introduce more delay for the same amount (8.68dB) of loss are better.
Now, we compare the minicircuits low-pass filter SLP-200 (datasheet) with the cables.
so we could add 22 filters for an optimum total delay/loss ratio. Total group delay would be 132ns.
If we compare now with the delays we get from the cables we see that even the simple RG58 gives us 50% more delay for the same loss ( and the price for the cable is the same as a single filter).
Using RG142 instead we get almost a factor of 2 more sensitivity and even more using lower loss cables.
So i don't see an advantage using those LP filters instead of cables.
started with some simple calculations for replacing the PLL with a delay line. Started with modeling the loss in the cable depending on frequency and length (separate matlab-function for different cables).
Below some first plots for our current "situation" (which probably changes in the near future but that's what it is right now) having a beat note @ 160MHz , 5dBm from the PD and an ZHL-1A amplifier (16dB gain) and a 4-way splitter (for two delay lines with different cable length):
files are on the svn in "CTNLab\simulations\noise_budget\delay_line_readout".
The optimum sensitivity is reached when the decrease in output signal is compensated by the increase in 2*pi*tau, which happens with a total loss of 8.68dB (factor 1/e) of the cable.
We don't win with adding delay if we make the cable longer, even if we increase the power going into the cable! That also explains why i had such a poor sensitivity with the 500ft of RG58 (which had 33dB of loss). Using 15ft of cable instead would have given the same sensitivity!
UPDATE 2/14/2012@8pm- files on the SVN contain now also data for RG405 and LMR-400 !
We removed the periscopes in beat path and use breadboard setup instead. There are higher broadband noise in the beat around 100 - 3kHz. At least, the peak around 800 Hz is slightly small, since the contribution from the periscope in the beat path was removed.
We are trying to get rid of individual mechanical peaks in the beat signal. One of the major peaks comes from the periscopes and the associated mirror mounts. So instead of using periscopes to bring the beam height down, we use breadboard setup to bring the whole beat path up. This new setup gets rid out the periscopes, and 4 mirror mounts on it.
Frank has a solid work assembly file showing how the setup should look like. However, the beam coming out of the cavities are not exactly on the whole pattern, and without the periscope we have no way to steer the beam to the designed path. As a temporary solution, I use a post mounts which are clamped on the board, not screwed.
Fig1: The beam path is not on the whole pattern on the table, so the block mirror mount cannot be used.
fig 2: current breadboard setup. The mirrors behind quarter waveplates are mounted on a regular post and clamped down on the board. If I use the block mount, the beam won't hit the mirror. Note: I use the previous beam splitter post (with 1.4kHz resonant peak) for now, because we don't have anything that fits right now.
fig3: Comparison between original setup (in blue,with periscope) and breadboard setup (green). Both signals were taken with 2kHz tuning range, gain 200.
I think the broadband noise might come from the scattering on the PD. THe spotsize on the PD is not much smaller than the PD. I haven't found the appropriate lens for modematching yet. The peaks around 1kHz-2 kHz also seem to be more than the regular setup. This will be investigated.
measured the delay for the old cable (RG58): dPhi=180deg, df=600KHz
1.67ns/ft (value from datasheet: 1.53ns/ft)
typical values for other cables using the following dielectric materials:
Dielectric Type Time Delay (ns/ft)
Solid Polyethylene (PE) 1.54
Foam Polyethylene (FE) 1.27
Foam Polystyrene (FS) 1.12
Air Space Polyethylene (ASP) 1.15-1.21
Solid Teflon (ST) 1.46
Air Space Teflon (AST) 1.13-1.20
from Measurements of Earth-station delay instabilities using a delay-calibration device measured at 70MHz
other data (e.g. for RG223/U) can be found here : http://tmo.jpl.nasa.gov/progress_report/42-99/99E.PDF
or here : http://ivs.nict.go.jp/mirror/meetings/v2c_wm1/phase_stability.pdf
or here: http://tesla.unh.edu/courses/ece758/Handouts/cable-specs.pdf
Type of cable Temp coef (ppm/K)
RG-223 -40 to -100
Belden 9913 -21
Andrew FSJ4-50B -2 to +6
Andrew LDF2-50 -8 to +6
Andrew LDF4-50A +7 to +16
And for the LMR-240 which i would buy for future cable delay lines ~24ppm/K
trying again to measure the coupling from seismic to cavity length for the individual cavities. Measuring the coupling to differential length (beat signal) is not a problem at all, but to the individual length.
The problem arises from the small coupling coefficient from vertical acceleration to changes in length. to not measure any effect from the filtering from the stack we have to measure at low frequencies. The first mechanical resonances occur at around 6Hz, the stack itself has it's first resonance at 16Hz. So we have to measure below 5Hz. The coupling to changes in length is small, about 1e-9 * Length of the cavity [units: m/(m/s2)], so about 2e-10 m/(m/s2). The signal from shaking the table around 1Hz is estimated to be a few hundreds of Hz/rtHz with maximum modulation. However the laser frequency noise is 10kHz/rtHz @1Hz, so we have to integrate very long to get a reasonable SNR. We cant use anything on the table to pre-stabilize the laser to reduce it's noise as this would be shaked as well and we don't really know which one we actually measure.
For the first cavity we have to look at the feedback to the laser pzt as this tells us how much the laser frequency has to be corrected. We assume that shaking the table at 1Hz does not mechanically modulate the laser frequency in any other way. For the second cavity we can't simply lock it the usual way as we would have two coupled cavities (that's what we measure using the beat signal already). So we have to lock the laser to the second cavity instead without using the FSS path by feeding back to the laser (fast actuator) instead of the VCO.
A first measurement showed that we have an additional mechanical resonance around 6Hz which we currently don't have in our current stack model (and actually don't know exactly where it's coming from).
We measured the Eigenmodes of the stack some time ago and have two candidates for it (see here)
So i will re-measure the TF below 10Hz to clearly identify which one it is.
resonance frequencies of the stack : 16.1Hz and 55.6Hz
other resonance frequency: 6.5Hz
coupling to cavity length: COMSOL model: 53 kHz / (m/s2)
measured: ~100 kHz / (m/s2)
All data and plots on the svn in /measurements/2012_02_12.
Notes for calibration:
TF to beat signal:
TF to fast actuator:
Did a little bit of peak hunting to clear our frequency span of interest from those massive mechanical resonances we currently have. After replacing the combining beam splitter mount we got rid of the 1.4kHz peak already. Yesterday i've focused on the mounts within the beat setup, but not the periscope, as we already know that this is very unstable and we will take care of that soon. I didn't want to replace things, just know where which stuff comes from.
I've found (only) one mirror mount which is currently clearly visible in our noise spectrum . Tapping the other mounts or damping the front plate or springs does not change the spectrum (at least i don't see any changes). Tapping (even slightly) is very difficult anyway as you also excite all the mounts surrounding your DUT, especially the periscopes and your whole spectrum changes and it's hard to figure out which is your primary resonance you are looking for. So i prefer damping it with a large piece of rubber and than compare it with a spectrum taken before with a reasonable.amount of averages.
Anyway, i found only one mirror mount (out of six) which i could clearly identify in our current noise spectrum. It's one of the mirrors right in front of the combining beam splitter.
Below a comparison before and after damping the front plate of the mirror mount. Resonance frequency is 544Hz. I have to check but i think we can replace this one with a non-adjustable turning mirror.
We still don't know where the 1.1KHz stuff is coming from.
we replaced the mount for the combining beam splitter in the beat setup as it caused a large, broadband peak in the spectrum around 1.4kHz. The new mount is one of the old, fixed turning mirror blocks they used in initial LIGO at LLO as far as i know. After replacing the mount the peak is entirely gone. I've used two springs instead of one to increase the pressure. We could not determine the resonance frequency of the new mount. Tapping the mount excites only known mechanical resonances from the surrounding mirror mounts. Tara posted a plot for comparison before and after replacing that mount (see here). He also has prepared a nice plot combined with a drawing which mount corresponds to which resonance we see in the spectrum. We will use this to start reducing (or even eliminating) those resonances starting with the most dominant ones close to 1kHz
Attached a copy of the drawing.
We try to identify the origins of each mechanical peaks in the beat signal from frequency between 100 up to a few kHz, so that we can damp it down below coating thermal noise. For today, we get rid of the peaks from the beam splitter in the beat path.
fig1: beat signal on linear x scale. Blue and red was taken with 10kHz and 5 kHz input range respectively. The broad band noise on Blue is from LO phase noise.
The beam splitter with a steel post and a clamp is replaced by ->
this block which holds the beam splitter inside. The whole pattern does not line up with the table, so we have to use clamps for now.
mixers (which we currently have (and use) in the lab):
cables calculator for cable loss, which has a huge amount of different cable types in it's database. : http://vk1od.net/calc/tl/tllc.php
velocity ~0.66 to 7*c
RG-58C/U: loss 32 dB for 500ft @160MHz
RG-142: loss 25 dB for 500ft @160MHz
4-way splitter ZBSC-413 (datasheet)
additional insertion loss 0.5dB
power Input: 1W max
- gain 16dB
- output power: 28dBm min. (1dB compression)
- noise figure: 11dB
- VSWR 2:2 (in and out)
- 2 to 500 MHz
- gain 24dB
- output power: 5dBm min. (1dB compression)
- noise figure: 2.9dB
- VSWR 1.5:1 (in) and 1.8:1 (out)
- 0.1 to 500 MHz
Noise calculation from PD in PLL: ( I actually asked Koji once and did this already, see psl:730 . The results are similar)
1) determining which setup gives the best performance:
fig1: OLG TF of PLL with different gain setup.
2) Measure electronic noise from readout system with the chosen setup. This noise will show up (after some correction) in the beat and determine what is the limitation of PLL readout technique.
The PD was blocked, the feedback signal (Vfb) to the actuator (LO) was removed and measured.
3) Block diagram
[add block diagram and calculation]
4) After Koji explained on how to calculated noise budget from electronic noise in PLL to us, here the nb with PLL noise. (note: the LO phase noise has updated to 1kHz input range)
With the electronic noise from PLL, the sensitivity of this technique will prevent us from observing coating noise above 1kHz.
I'll calculate the noise from cable delay technique later and compare which one will give us better sensitivity.
The noise budget is updated and plotted with today's beat measurement.
(*The electronic noise plotted in the graph is not correctly calibrated, see psl:816 for the complete calculation)
After we replaced the table's broken leg, we floated the table and measured the beat signal as a reference before modifying the seismic stack. The calculation agree with the measured data quite well.
We also measured the electronic noise from PLL. This was the signal which was fed back to the LO (with SR560 gain = 20). Apparently, we are sitting on it at 1kHz and above. We definitely need to work on PLL readout system to measure at lower sensitivity.
New traces in the noise budget:
I removed noises from room temperature fluctuation/ heater noise and spacer thermal noise because they are way smaller than coating noise and crowd the plot.
==Details about some traces in the noise budget==
The vertical seismic noise coupling is calculated by applying the seismic measurement times stack transfer function times cavity bending coupling
[ Frequency noise from seismic ] = [measured data] x [stacks TF] x [bending factor].
The peak around 6 Hz might be coupled from horizontal direction. This will be added soon.
RIN induced length noise is still an estimated. We have not been able to measured the real coupling yet, as the SNR is so low.
LO phase noise: This is from measurement. I'm not quite sure if I miss some calibration factors. The phase noise does not show up in the beat yet even though it is very close together right now.
We measured Q and k of RTV springs. Currently, there are 9 of them. The results are ok
An aluminum block with accelerometer (0.398kg) was placed on a spring. The block was tapped, and the ringdown response was measured.
* the piece is broken, so it is softer and more lossy.
** the piece is made of other material.
Mean Q (piece 1 to 7) = 13.29 +/- 1.11
Mean k (piece 1 to 7) = 21,286 +/- 1,500
list of finished items:
all parts needed for upgrading the stack and adding radiative shields/heaters are tested/ready or currently beeing baked. Installation can start Wed/Thu
unfinished things left for tomorrow:
TF from all components except the sevo is measured (collectively). This will help us to determine what kind of servo(UPDH) we want for ACAV.
TF from other components in ACAV loop except the servo (collectively) (frequency discriminator, marconi, amplifier) and fit]
TF from ACAV loop without servo. The red curve is fitted with 1pole at 50kHz with 70kHz delay (exp(-i*f/70kHz)) . Since our bandwidth of interest extends upto only a few hundred kHz, this fit is good enough for a model. It starts to diverge from the data at 300kHz.
Note: The TF looks ok, it is flat as expected from most of the parts (frequency disc/Marconi/amplifier). The pole and time delay is from the AOM. We can see the phase changes as we change the AOM position so that the beam is closer to the PZT side. We gain ~5 degrees from adjusting the position.
[current updh schematic]
The TF has a pole around 50kHz. C18 with 3300pF gives a zero at that frequency and cancels the pole. We are designing the TF of the servo that is suitable for our need (UGF ~100kHz, with 1/f roll off at UGF, and ~45 degree phase margin, and, 1/f^2 at low frequency)
Problem with UPDH, on stage 2, the resistors' values might be wrong. We will check and fix it tomorrow.
I checked the TF of the amplifier used in the ACAV loop because I did not measure that yesterday. The amplifier's TF is flat at least up to 200 kHz. The Bode plots between the loop with and without the amplifier are pretty much the same. Thus, it is ok and won't cause a problem in the loop.
The setup is similar to what I did in the previous entry, except the output of the marconi is connected with the amplifier. See blue arrow.
The TFs between the two cases have similar shape. So the amplifier will not be the limiting component. The magnitudes (in arbitrary unit) are slightly different because I did not attenuate the power by the same factor it was amplified.
The UPDH box will be modified next to see if we can increase the loop bandwidth.
We measured the TF of marconi using PLL loop. Marconi has flat response up to around 200kHz. This is quite good and we can certainly use it in ACAV loop.
==block diagram of ACAV loop==
The whole OLG TF was measured in PSL:... This time we looked into the marconi to see if its TF has bandwidth high enough for ACAV loop or not. We know that Marconi has lower phase noise than LIGO homemade VCO (Megan's elog), but we have not learned about its bandwidth yet.
==PLL setup for Marconi TF==
The actual magnitude TF at DC can be determined by using a voltage calibrator to inject DC signal and measure how much the frequency of the output changes. This depends on the tuning range setup on the Marconi. However, we don't know the bandwidth of the TF, so we use PLL to find out. The setup is shown below. The gain from SR560 was set to be low, so that the signal at high frequency will be the TF of the Marconi + frequency discriminator.
Since the mixer output gives 1/f response (flat in [rad/rtHz] unit), we corrected the TF by multiplying back with f to get the TF of the marconi. The magnitude on the plot below has arbitrary unit. We are only interested in its shape. We tried 100Hz, 1kHz(not shown), 10kHz tuning range. The magnitude varies with the tuning range as expected. The phase does not change that much.
If we want to have phase margin of 45 degree, assuming other components in the loop have no phase lag. The best UGF we can do is upto 200kHz, according to the phase response of the Marconi (the phase drops by 135 degree around 200 kHz). Therefore, using a Marconi as an oscillator for driving the AOM is also possible because its bandwidth is high enough for measurement up to 1kHz.
Note: we will check what is the TF of the amplifier (H) used in our setup to make sure that it is not the limiting component.
We can definitely use Marconi as a VCO in our ACAV loop.
NExt: The next step is checking the UPDH box. At a glance, we found that the TF shape of the current UPDH is not suitable for our requirement.
I'm working on characterizing ACAV loop to check if the loop is good enough for the coating noise measurement. The results show that there we need better improvement on this part.
I realigned the cavity, and the visibility is only 85%. I have not figured out yet why the visibility decreases from 90%. Then I measured dark noise @ error point, inloop noise with different gain setting, and error signal's slope ( all with 50 ohm system).
Error slope (with 2V DC on RFPD) =122.7 kHz. The visibility is @ 85%
1) Dark noise and inloop noise: The in loop noise changes with gain setup on UPDH box. I changed the gain from 3 to 7 and made sure that there was no oscillation (oscillating @ gain7.5).
With the error signal slope, I can calibrate in loop noise to absolute frequency noise from ACAV loop and plot them on noise budget. On the figure below, we can see that at high frequency noise from ACAV loop is dominating.
Notice the bump around 3kHz. It comes from the phase noise of the marconi. If I change the tuning range from 10khz to 1kHz the in loop noise also change (blue to green in the below figure). It is a trade off between lower frequency noise, less gain. The peaks around 80 Hz go up because we have less gain to suppress the noise.
2) OLG TF at different gain setup
Back to the 10kHz input range. Since the in loop noise is getting worse with more gain, I look into the OLG TF of the loop. The UGF is around 3-10kHz, depending on the gain. TF looks ok at gain 3-5, the magnitude increases along with the gain. At gain 7, the TF starts to deviate at high frequency (it might be oscillating at high frequency).
The in loop noise should be suppressed with 1/(1+G) factor, but the results above do not tell the same story. The gain increases but the in loop noise gets worse. So the next step is to look deeper into this problem. I'll use SR650 instead of UPDH and check if the same problem occur or not. This should verify if the UPDH box is bad or not.
Note: I'll calculate what we need for ACAV loop, and check if LIGO homemade VCO will be good enough for ACAV loop or not.
These drawings are for RFPD's bases. The square plate will be made of Aluminum. It is for mounting RFPD on the table. The block is for RFPD legs which will be made from Delrin.
swap the RFPDs. The SN002 for RCAV was not working (see psl:803). So we use SN01 for RCAV and characterize it again.
1) dark noise in Vrms/ rt Hz
Dark noise is not particularly high. ~40 nV/rt Hz flat. In loop noise is only around 10 nV/rt Hz. It is about a factor of 2 above SR785 noise for the input range setup. When I measured the dark noise, I made sure that monitor screens in the lab were off, no cables connected to a scope to create any ground loop. So the measured signal this time does not have harmonic lines as much as the previous measurement does.
2) error signal slope : measured to be 34.4 kHz/V ( with 20dB attenuator on EOM. if I use 16dB attenuator, the calibration can be lower to 22 kHz/V). We will decide what should be the modulation index once we calculate the acceptable level of shot noise and compare with the gain we need.
Note that the numbers in psl:802 missed a factor of 2 (they should be twice as much).
3) absolute frequency noise from dark noise at error point and inloop noise: The gain setup for common/fast are 1000/750. It becomes unstable if I increase more fast gain.
The inloop noise lies below the estimated coating noise up to 100kHz. So if we can decrease any other technical noise sources, we should be able to reach coating noise level.
Next will be the complete OLG TF measurement.
just for reference which part is/was where for later...
Double checked the distance between the mirrors using the new periscope base Tara designed. Distance is 2.96in according to the 3D assembly, close enough to the 3" we want. CAD files are on the svn in the mechanical drawings folder.
Took me some time to figure out that the current RCAV RFPD (SN002) is broken, but not in a normal way. The functionality is still as it should be using standard measurements: RF-noise level is low, response is OK etc. , but some non-stationary noise process at RF frequencies happens from time to time, only visible in the demodulated spectrum. My guess is that the MAX4107 is about to die or close to be unstable and starts oscillating or so. Will check. ACAV PD (SN001) is fine.
Attached a movie of the demodulated noise spectrum. The "feature" does not show up on the oscilloscope. There you see only constant noise, no changes in amplitude or so. Only in the demodulated spectrum shows the effect, mostly visible between 10k and 100k. Rana, Koji: have you guys seen this before?
We need to characterize RCAV and ACAV loop, here is a list to do
Power adjustment: We now choose some appropriate and easier numbers of the setup for future reference.
The DC out from RCAV and ACAV RFPD are 2 V which correspond to ~ 1mW input.
The DC out from RFAM PD is 0.2 V which corresponds to 1mW input as well (this one has different R, hence different gain)
[add pic], and power to EOM is 0dBm. This will be our current standard setup.
RCAV loop characterization:
We use different RFPD, different TTFSS set, so this has to be done again.
The power to EOM is 0 dBm, 1mW input to RCAV:
The error signal slope:
ACAV 0.0149 MHz/V (I'm not sure why the error signal from ACAV is clipped,see below picture)
RCAV 0.0224 MHz/V
Dark noise level in Vrms/ rtHz
-> Dark noise level in absolute frequency:
It looks like the noise is low enough almost up to 1kHz. Yay!
The OLG transfer function as well as in loop noise, will be measured later once we optimize the gain.