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Entry  Tue Jan 22 19:04:51 2019, anchal, DailyProgress, PLL, New Focus 1811-FC Dark noise Measurement NewFocus1811-FC_DarkNoiseSpectrum.pdfNewFocus1811-FCDarkNoiseSpectrum.ipynb.zipNewFocus1811-FC_DarkNoiseSpectrum_New.pdfNewFocus1811-FCDarkNoiseSpectrumNew.ipynb.zip
    Reply  Wed Jan 23 11:43:02 2019, awade, anchal, DailyProgress, PLL, New Focus 1811-FC Dark noise Measurement 
Message ID: 2291     Entry time: Wed Jan 23 11:43:02 2019     In reply to: 2288
Author: awade, anchal 
Type: DailyProgress 
Category: PLL 
Subject: New Focus 1811-FC Dark noise Measurement 

The first measurement here seems like a smoking gun for limiting noise.  A mixer down converted noise of 1.3 µV/rtHz (directly out of the mixer) with a S*A (VCO + SR560) gain of 200 kHz/V would be 0.26 Hz/rtHz. That would be consistent with the noise floor of the more recent BN PSDs. The PLL open loop gain will change this estimate, but the 200 kHz/V gives a lower bound. This initial measurement of NF1811 noise was a factor of ten more than spec for this detector. It should be ~100 nV/rtHz (equivalent to input ref noise of 2.5 pW/rtHz at peak responsively).  We should have traced back to the source of this earlier, but now that we know its a potential problem we should double check if we're having issues with BN noise floor not matching up with the noise budget.

Its weird that the second measurement of the same NF1811 seems to show it performing closer to its spec value. The setup for measurment was exactly the same.  I should note that when we were making these measurements we switched the BN cable from the NF1811 to the resonant 27.34 MHz detector and back again.  I also wiggled the power cable at the time. I've seen in the past that when there is a power connector issue, the NF1811 can produce something that looks like signal but with RF oscillations if there is a problem with one of the power rails.  We probably should have checked at the time with an oscilloscope to see if there were any issues with the RF signal coming out of the NF1811.  It could be that there was some kind of oscillation or other bad thing going on and the act of unloading and reloading the output with 50 Ω and wiggling the power connector some how got it back to proper operation.  

For now we should should recheck the NF1811 dark noise (after mixing down) before making another BN measurement and not touch connectors on the detector side in case there is a flaky connection.

You definitely​ saw the higher noise state, so I wouldn't dismiss it yet. I guess we won't know if the reduced noise second measurement will bring down out noise floor until we see a BN measurement.

Quote:

We suspected if the wideband RF detector New Focus 1811-FC could be the source of noise floor through its dark noise. So I took dark noise spectrum measurements today of this photodetector.


Method:

  • I blocked the photodetector with a beam dump.
  • First I took the spectrum directly from the RF port using HP 4395A in spectrum analyzer mode, from 0 to 130 MHz with 3kHz ResBW and 50 averages.
  • I converted dBm to V_rms using 10**((P_dbm-13)/20) and then divided by np.sqrt(3e3) to get spectrum in Vrms/rtHz.
  • Next, I measured the dark noise spectrum by mixing it down with a local oscillator using the same ZX05-1MHW mixer we use in the PLL.
  • First I used the Wenzel crystal oscillator at 24.483 MHz as LO.
  • Then I used Marconi at the same frequency for comparison.
  • I also took mixed down measurements for 27.34 MHz(Resonant RFPD peak point), 50MHz, 75 MHz, 100MHz and 125MHz.
  • All these later measurements were taken by SR785 from 0 to 102.4 kHz (in 3 steps) with rms averaging of 100 factor and BMH windowing.

Conclusions:

  • Contrary to our expectation, mixed down dark noise is more than the direct dark noise from RF port. The mixer should have same conversion loss for dark noise and should also be suppressing one quadrature, so the output from the mixer should be less noisy.
  • I think I have missed something in these measurements. It looks too bad to be true. In past, we have been running beat note measurements with S*A (Actuator Slop x Amplifier gain) of 200-500 kHz. With the measured noise after mixing down, it would correspond to 0.6 - 1.5 Hz/rtHz.
  • We do have a noise floor around these values but we also have measurements going below 1 Hz/rtHz, so something seems fishy.
  • At the time of writing this post, we quickly did a dark noise measurement with the resonant RFPD at 27.34 MHz with Marconi and its dark noise after mixing down is around 12 nV/Hz. Much much better!

Edit Tue Jan 22 20:06:00 2019

Correction:

  • When we measured resonant RFPD, measurements made more sense. The direct noise from RF port was higher and the noise after mixing down was lower by conversion loss and factor of sqrt(2) due to the suppression of one quadrature.
  • So we decided to measure the wideband RFPD again. To our surprise, this time the measurements made sense. PFA the new plots with new measurements.
  • The mystery is still on for what caused the measurements to be bad earlier. We can not find anything that we changed except for switching RFPDs back and forth.
  • So from new measurements, it seems like mixed down dark noise is at the level of 100 nV/rtHz. It is higher at other frequencies mostly because of Marconi's inherent noise. We should trust the result with crystal oscillator (black curve) more.
  • This would correspond to 50 mHz/rtHz of flat noise contribution from the dark noise. This is way below the measured noise floor in our beat note spectrums. So phot detector dark noise is off the hook (for now).
  • We need to continue our search for the culprit.

 

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