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:

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:

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