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Entry  Mon Nov 3 15:16:28 2014, Dmass, Noise Hunting, Noise Budget, New beat spectrum: ErrSigs.pngBeatNz.png
    Reply  Mon Nov 3 16:15:21 2014, Dmass, Noise Hunting, Noise Budget, Beat spectrum not ltd by  
       Reply  Mon Nov 3 20:27:28 2014, Dmass, Noise Hunting, Noise Budget, ISS servo fail? 
          Reply  Wed Nov 19 14:54:36 2014, nicolas, Noise Hunting, Noise Budget, ISS servo fail?  
       Reply  Tue Dec 2 22:51:52 2014, Dmass, Noise Hunting, Noise Budget, Beat spectrum not ltd by  PDH_demod_chain.png
          Reply  Sat Sep 12 14:25:05 2015, Dmass, Noise Hunting, Noise Budget, Beat spectrum not ltd by  
    Reply  Mon Nov 3 20:36:14 2014, Dmass, Noise Hunting, Noise Budget, New beat spectrum: 
       Reply  Tue Nov 25 22:45:43 2014, Dmass, Noise Hunting, Noise Budget, New beat spectrum: 
          Reply  Wed Dec 3 17:02:31 2014, Dmass, Noise Hunting, Noise Budget, Noise budget cleaning up 
             Reply  Thu Dec 4 02:45:49 2014, Dmass, Noise Hunting, Noise Budget, Noise budget cleaning up PDH_sensing_nz.pngLB1005inputNoise.png
                Reply  Sun Dec 7 12:42:55 2014, rana, Noise Hunting, Noise Budget, Scatter Noise hump is RAM 
Message ID: 1172     Entry time: Tue Dec 2 22:51:52 2014     In reply to: 1163     Reply to this: 1329
Author: Dmass 
Type: Noise Hunting 
Category: Noise Budget 
Subject: Beat spectrum not ltd by  

Talked to Rana about the increase in sensing noise due to DC grounding in the PDH demod electronics (we were plugging the Gold PD directly into the mixer- he confirmed that we want transformers here)

He agreed that we want to not make monstrously sized ground loops and should use transformers for DC isolation like so:

PDH_demod_chain.png

I borrowed a pair of "balanced to unbalanced" 1:1 transformers in the PSL/CTN lab and stuck them in the readout chain - the noise went down.

I tried all four permutations of balanced / unbalanced on both the LO and RF inputs of the mixer - I saw no difference in noise between these.

There is no grounding done at RF in these mixers (I opened and looked). I have seen grounding put into the transformers in other demod chains.

 

Questions I need answered:

  • What kind of transformers do we want to buy for this?

  • What are the consequences of not grounding / grounding the transformer at RF (say with a 4 nF cap across the ground which gives => 1 ohm at 33 MHz)

 

Slightly downstream of this I (previously) made another change in the setup: here is a picture of what I added between the LB1005 box and the driver:

PDHoutput_attenuator.png

 

The LB1005 is a P-I with crossover at 30 kHz (near the cav pole)

The RC filter between the driver and LB acts as 40 dB of low frequency boost

Measurements/plots wanted:

  • Sensing noise for each path with transformers in play
  • LB1005 + atten input ref noise (I believe the atten doesn't change this)
  • *also need: in loop error signal + ctrl sig noise out to high freq
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