PLL Readout Noise and Open-Loop Transfer Function:
- I remeasured dark noise of wideband photodetector after mixing down with mixer when beat note frequency was around 46.985 MHz.
- The detector was behaving nicely with effective noise contribution to beat note spectrum of less than
 .
- The noise spectrum is almost flat above 100 Hz with a mean value of
 between 100 Hz and 10 kHz.
- I also remeasured PLL's open loop transfer function. It turned as a nice 1/f curve with unity gain frequency of 28.75 kHz.
Beat Note Frequency Noise Spectrum
I used the same measurement method as explained in (PSL:2272). Changes in the setup from before:
- FSS loops were tuned up. See (PSL:2292).
- During measurement of New FOcus 1811-FC wideband RF detector Dark Noise, we found that after disconnecting and connecting back the RF port, we found a significant decrease in dark noise. See (PSL:2288).
- In the noise budget notebook, I have updated the PLL Oscillation Noise (coming from Marconi) and PLL Readout Noise (coming from detector dark noise) as measured this week.
Conclusions:
- Good news first, we got rid of roll up near high frequencies and are now close to Nov 2017 spectrum.
- There are some disturbing bumps at 150-160 Hz, 300 Hz, 800 Hz and 900 Hz. I think these would be our new points of attack.
- I do think that higher noise <100 Hz is due to the way we are taking measurements. Our spectrum is just 16 Hz linewidth throughout the range and due to the marconi jumping points, the lower frequencies are wordt affected.
- I think we should do some correlation mesurements to search the sources of above mentioned noise bumps.
- There is also a new noise floor of roughly
 to push against.
Edit Tue Aug 13 17:08:46 2019 anchal:
The PLL Readout noise added on this plot was erroneous and I can't find where it came from either. So the noisebudget attached is wrong! I was a dumbo then. |