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Entry  Wed Apr 24 16:01:55 2019, anchal, DailyProgress, FSS, Time series and spectrum analysis of RFout of SCavReflRFPD near resonance South_Cavity_Reflection_RFout_Time_Series_Analysis.pdfSouth_Cavity_Reflection_RFout_Spectrum_Analysis.pdf
    Reply  Fri Apr 26 14:34:03 2019, anchal, DailyProgress, FSS, Time series and spectrum analysis of RFout of SCavReflRFPD near resonance 
       Reply  Sun Apr 28 22:31:39 2019, rana, DailyProgress, FSS, RF beats 
          Reply  Tue Apr 30 18:07:49 2019, anchal, DailyProgress, FSS, Spectrum of RFout from South Cavity Reflection RFPD SCavReflRFoutSpecAnalysis.pdf
             Reply  Wed May 8 20:03:43 2019, anchal, DailyProgress, FSS, FSS PDH Error Signal on South Path is in good health SCavPDHError20190508.pdfCavity_Refl_RFPD_TI_After_Modifications.pdf
Message ID: 2326     Entry time: Wed Apr 24 16:01:55 2019     Reply to this: 2328
Author: anchal 
Type: DailyProgress 
Category: FSS 
Subject: Time series and spectrum analysis of RFout of SCavReflRFPD near resonance 

I took time series and spectrum of RF output of South Cavity Reflection RFPD through a 20 dB coupler.


Time Series Measurement Setup:

  • The RFout port of South Cavity Reflection RFPD SN010 was connected to IN port of ZFDC-20-5-S+. OUT port of the coupler is connected to TTFSS box's PD input port.
  • The CPL port of the coupler was connected to CH3 in TDS3034C oscilloscope at 50 Ohm input impedance and probe set to 10x to compensate for 20 dB loss in coupling and have  a real estimate.
  • CH1 of the oscilloscope is connected to the output of Cavity Transmission Photodiode.
  • CH2 of the oscilloscope is connected to OUT1 port of TTFSS box, which is connector J1 in D040105. This is supposed to be 0.36 times the RF amplitude level measured with 50 Ohm termination (hence voltage divided after final RF opamp stage in RFPD).
  • CH4 of the oscilloscope is connected to Mixer output port of FSS which is at the output of the first common amplification stage in D040105. This is supposed to be 3.16 times OUT1 value.
  • Data from the oscilloscope is downloaded using its web portal.
  • I scan the laser pzt near resonance at 2 Hz with 2 V peak-to-peak sine wave. Incident light on the RFPD was measured as 2.95 mW.
  • Except for first two measurements which were triggered by transmission peak, I triggered the rest of the measurements by PDH error signal peak.

Analysis:

  • First, I just triggered the data capture with a wide 400 ms capture (Sampling rate of 25 kSa/s). I found that RFout level is 210 mVpp instead of expected 1.495 Vpp. OUT1 level was 53 mVpp (which ideally should be 75 mVpp given 210 mVpp RFout level). Mixer out level is as expected so the first stage common amplification is definitely good.
  • Next, I zoomed into 250 kSa/s because very clearly there were some oscillations. It seemed like there was a strong ~240Hz oscillation in RFout. This oscillation is quite big too, almost 100 mVpp. But at this point, I'm not really sure if this was an artifact of using a coupler.
  • Next, I zoomed in further in time scale going to 250 MSa/s sampling rate. In a 40 us snap, I saw another amplitude modulation. This time it corresponded to 290 kHz.
  • Next, wanting to see 37 MHz signal, I zoomed into 1 GSa/s where 10 us snap clearly shows this 290 kHz amplitude modulation.
  • Zooming into this 1GSa/s data, I can see 37 MHz sine wave. I saw some sharp features near the edges.
  • So to be sure, I zoomed into 5 GSa/s where I saw the sine wave better. Near the top, there are no sharp edges but still, it is not nicely curved. The bottom definitely looks sharper than top. But these minute defects in the RF signal could be due to oscilloscope quantization error or something else.

Clearly, I needed to see what other frequencies are there in this RFout signal. So I decided to take a spectrum of the signal.


Spectrum measurement setup:

  • I connected the transmission photodiode signal to the external trigger input of HP4395A (this is in the back). I couldn't get the analyzer to trigger with PDH error signal, so I had to use the transmission peak.
  • I connected the CPL port form the coupler to the spectrum analyzer directly which always has 50 Ohm input impedance.
  • Later I realized I do not need to do this as I am anyways not using down converted signal from FSS for triggering or anything. But I decided to keep going to keep the two measurements consistent.
  • Hence, in the analysis, I added 20 dBm in the measured output to compensate for coupler loss.
  • All measurements were taken with automatic IF bandwidth setting, so it changes according to span and is marked in the graphs. I took 100 averages in all the measurements which were triggered individually by transmission peak. So measurements were roughly taken over 100s.

Spectrum analysis:

  • First I took a wide scan to get a relative idea of important frequency peaks. I see that 37 MHz is the most dominant frequency (thankfully) at -30.71 dBm, which corresponds to 20 mVpp.
  • The reason this value is so small is that the FFT is taken over 281.3 ms which is over a quarter cycle in PZT scanning. So triggering from transmission peak, this must be the spectrum of the region where the PDH error signal has died out mostly. I know this measurement sounds stupid at this point, but I realized this in hindsight only. Still, we get an idea of other frequencies present. I have a better plan too which is about to come in the next post.
  • Second in the race is a peak at 36.72 MHz at -40.66 dBm. I think this is because the SN010 RFPD actually has its peak at 36.67 MHz and not 37 MHz and the EOM driver on the south path doesn't have a sharp resonance and has about 2-3 dB gain at 36.67 MHz, so that's why we are seeing this another peak (my guess).
  • Third, in line is 29.5 MHz peak at -45.62 dBm. This is the second harmonic of RFAM (RAM for people who prefer this) of EOM behind South PMC used of PDH of PMC. This surprisingly leaks through the PMC and I have not been able to tune it down passively.
  • Next, I took a board spectrum from 0-500 kHz to spot that 290kHz we saw in time series data. Indeed there is a peak at 281 kHz of -66.36 dBm. This looked like a bigger oscillation in time series (~50 mVpp => -22 dBm but again, this data taking method is not really good). There are other nearby peaks at ~269 kHz and 2~284 kHz. I'm not sure why these peaks are here. But they can not be some standing wave in the cable as it will require ~700m long cable.
  • There is some more activity near 60 and 70 kHz of ~-60 dBm again.
  • In the very low side, I took a broad spectrum up to 1 kHz and we see a horrifying comb of 60 Hz harmonics with 180Hz and 60 Hz being the dominant once. But the magnitudes in this measurement do not make much sense in my opinion.
  • And then I realized, I forgot to take spectrum near 14.75 MHz (the frequency of PMC PDH lock) but then the triggering stopped working. I guess the temperature in the lab changed and the intensity of laser changed slightly enough to make transmission peak low. That's my theory, but I do not know for sure why triggering stopped working. It was a very clunky way of doing science anyways :(

Some conclusions:

  • Well, clearly the RF output is indeed not strong enough from this RFPD (7 times smaller actually).
  • The shape of 37 MHz wave is also questionable. There are many dominant other frequencies in this signal. Need better measurements.
  • The draconian 60 Hz harmonics family is here as well. However, I hope after down conversion, it doesn't matter much.

Better measurements?

  • To be honest, I wished I took the spectrum in a cleaner way, exactly knowing where I am triggering and what part of time series data is being Fourier transformed.
  • So I started making a TTL triggering box for the same. See next post for details.
  • Next, I'll reduce PZT scan frequency and maybe FFT points and take a quick FFT around the PDH error signal peak only using my new trigger box.

DATA_DIRECTORY        ANALYSIS_CODE

Attachment 1: South_Cavity_Reflection_RFout_Time_Series_Analysis.pdf  475 kB  Uploaded Thu Apr 25 20:20:11 2019  | Hide | Hide all
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