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 Wed Jan 16 22:07:44 2019, awade, anchal, DailyProgress, PLL, PLL noise from the Marconi 2023A FG Fri Jan 18 13:10:02 2019, anchal, DailyProgress, PLL, PLL noise from the Marconi 2023A FG
Message ID: 2286     Entry time: Fri Jan 18 13:10:02 2019     In reply to: 2284
 Author: anchal Type: DailyProgress Category: PLL Subject: PLL noise from the Marconi 2023A FG

I took the noise spectrum of beat note in PLL when it is fed with a low noise Wenzel Crystal Oscillator of frequency 24.483 MHz and doubled frequency 48.967 MHz.

### Method:

• Instead of feeding Beat note from cavities, I fed crystal oscillator signal to the mixer in PLL.
• We tried to keep the strength of the crystal oscillator signal similar to beatnote strength from cavities which is around 0 dBm.
• At 24.483 MHz, the signal was -0.1808 dBm.
• I also took readings with a frequency doubled signal. Frequency was doubled by splitting the 24.483 MHz signal and mixing the two outputs at a mixer with an appropriate phase delay in one path.
• At 48.967 MHz, the signal was -1.16 dBm.
• Spectrum was taken for 4 different actuation slopes (S) at Marconi, 0.5 kHz/V, 1 kHz/V, 5 kHz/V and 10 kHz/V. The gain of SR560 (A) was changed accordingly to keep S*A = 500 kHz/V. This was the maximum we were able to achieve without overloading the SR560.
• Two different Marconi were tested. One is labeled '#539' and other one is labeled 'PD9020'. We tried to work with a third one from cryo lab but it was giving a weird GPIB error 'Fractional N-Loop High' without GPIB-Ethernet controller connected at all RF Level above -90 dBm.
• During all measurements, Marconis were using external frequency standard (direct mode) and were connected to Rubidium Frequency Standard SRS FS725 10 MHz sine wave output.
• Each measurement was taken in two parts, one with linewidth 16 Hz up to 1.28 kHz and one with linewidth 128 Hz up to 102.4 kHz. RMS averaging was done for an averaging factor of 100. BMH window was used.

### Conclusions:

• Marconi '#539' won the race with lower noise at all frequencies.
• The plots show noise for the beat note frequency but since the crystal oscillator is about 25 dB less noisy then Marconi, this noise is almost entirely due to Marconi and SR560. SR560 has ~ $2 \, mHz/\sqrt{Hz}}$ noise contribution above 10 Hz for all actuation slope values (this remains constant as S*A = 500 kHz/V always).
• Clearly lower actuation slope decreased the noise due to Marconi significantly. So we would like to keep drift of beat note frequency of cavities as low as possible.
• One alarming concern is the presence of a sharp peak in Marconi noise at ~900 Hz. This will mix with low-frequency noise of incoming beat note frequency and would create humps near kHz region.
• We need to figure out why this peak is there in the Marconi noise spectrum and what we can do to take it off.
• Possible candidates: Spurious cross-coupling with something else, reflections between the mixer and low pass filter in PLL due to impedance mismatch, inherent peak in SR560??
 Attachment 1: MarconiPLLNoiseAnalysis.pdf  584 kB
 Attachment 2: Marconi_PLL_Noise.zip  1.460 MB
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