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Entry  Sat Jun 19 19:20:29 2010, rana, Electronics, GYRO, IFR / Marconi Phase noise w/ Rubidium clock lock IFR.png
    Reply  Mon Jun 21 09:44:31 2010, Frank, Electronics, GYRO, IFR / Marconi Phase noise w/ Rubidium clock lock 
Message ID: 822     Entry time: Sat Jun 19 19:20:29 2010     Reply to this: 823
Author: rana 
Type: Electronics 
Category: GYRO 
Subject: IFR / Marconi Phase noise w/ Rubidium clock lock 

There is a 10x improvement in the Marconi phase noise spectrum when locking it to the FS725 Rubidium clock.

To see if we could improve on the Marconi spectra for use in the Gyro/RefCav measurements I set up two of the 40m Marconis for "Direct External 10 MHz Reference" lock.

In the attached plot you see the BLUE trace (free running Marconis) is higher than the PURPLE (locked to Rubidium) trace. The BROWN dots are the SSB phase noise specs

for the FS725 Rubidium clock. In this case, I have the FS725 locked to the 1PPS from our GPS receiver, but I think it doesn't have any effect above 10 mHz.

Some Notes:

  1. The 'Direct' lock mode of the IFR2023A implies that it locks directly to the external reference. The 'Indirect' mode means that it first locks an internal TCXO to the external reference and then uses the TCXO as the internal reference. Since the FS725 has a better SSB phase noise spec than the IFR2023's internal PLL, I used the 'Direct'.
  2. As you can tell from the noise improvement, it looks like the internal PLL has a ~1kHz UGF but only has a gain of ~10 below there. That's pretty strange if true. We should purchase another Rubidium clock to beat with this one to see if its really as good as its spec.
  3. As usual, the beat spectra shown are the quadrature sum of 2 oscillators, so divide by sqrt(2) to estimate the noise of an individual IFR/Marconi.
  4. In all my previous phase noise measurements, I had neglected to compensate for the 30 mHz AC coupling of the SR560 I am using as a pre-ADC preamp. I've corrected for this in the calibration of these measurements. Its almost a negligible effect but I include it anyway for my internal satisfaction.
  5. Because of the high input impedance of the 10 MHz inputs of the IFRs, I have daisy-chained the 2 by using a BNC T on the back of the first one. The cable connecting to the bottom one is terminated with an in-line 50 Ohm terminator.
  6. As described previously in the 40m elog, the measurement setup is using a ZP-3MH mixer as a phase detector. Looking at the peak-peak signal with free running oscillators allows us to calibrate the phase detector slope when locked. An AC coupled SR560 with G=1000 is used as the preamp between the mixer and the ADC. Between the mixer's IF output and the SR560, there is a 50 Ohm inline term and a 1.9 MHz BNC low pass filter from Mini-Circuits.
  7. Since the FS725 spec is better than the PURPLE trace, I am assuming that the PURPLE measurement is real. If the FS725 was noisier, we would be fooling ourselves by cancelling out the common-mode noise of the FS725, but in our case we are dominated by the internal noise of the Marconis.
  8. Simply taking the FS725's 10 MHz output and deriving a 95 MHz or 160 MHz signal wouldn't gain us much, since the phase noise would also increase by the multiplication factor. For the Refcav experiment, the best option may be to bring the beat down into a reasonable regime by using 2 lasers rather than an AOM. Then we may be able to put the beat frequency below the Nyquist of the digital system and forego any RF demodulation.
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