Hi At least with 60 Hz power (which may indeed behave differently than 50 Hz) everything I have seen so far has been in the OCXO's I'm testing. The TimePod it's self seems to be very immune to the usual ground loops and power supply noise issues.
As you play with phase noise testing - be sure to take a look at the neat little file that lets you fiddle the FFT's. It's a very neat way to stress test your computer... Bob -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Magnus Danielson Sent: Sunday, June 17, 2012 8:19 PM To: Discussion of precise time and frequency measurement Subject: [time-nuts] TimePod, cross-correlation fun and measurements Fellow time-nuts, As you know, I spent last week in Boulder. NIST T&F seminar, fellow time-nuts and much fun. I also picked up my TimePod out of the hands of no other than John Miles himself. Being sleep deprived from the travel, progress have been slow, but I have now come to the point where I got a bunch of measurements done. This has been greatly aided by the excellent cables and adapters provided by Tom Knox. I did a comparison of my OSA8601-02 SN1314 and OSA8600-1 SN817. Some material to let you know what these are, see: http://rubidium.dyndns.org/~magnus/time/oscilloquartz/8601/ Essentially the 8601 is a different interconnection mounting than the 8600. I tend to refer to them as 8600s as they are essentially the same. However, my 8601 is an older generation than my 8600. We will come back to that. I have made three essential measurements: OSA8600-OSA8601 pair (one as input and one as reference) OSA8601 with dual reference (OSA8600 and HP5065A) OSA8600 with dual reference (OSA8601 and HP5065A) With the first measurement, I get the sum of the noise-levels. With the second and third, the noise of the other sources cross-correlates out to some degree and the individuals noise should remain. It works to some degree. Let's see how the noise-floor measurements look: http://rubidium.dyndns.org/~magnus/time/timelab/OSA8600_HP5065A_OSA8600_2012 0618_1.png The green trace is the sum of them both. This is best seen from 10 Hz and above. The blue trace is the OSA8601-02 AT-cut BVA. The purple trace is the OSA8600-1 AT cut BVA. The overall plot shows the typical f^-3 noise, some f^-1 noise and the white noise. The white noise is around -155 dBc or better and is essentially flat from 1 kHz to 100 kHz, except for the 20 kHz bump. This bump I do not know the source of. In the 7 Hz to 500 Hz region, the f^-1 noise of the 8600 dominates the sum (green), where as below 7 Hz the 8601 f^-3 noise dominates. Considering that their Q values should be roughly the same, and hence their break-point, it is fair to assume that the white noise of the 8600 buffer amp is worse than the 8601 buffer amp, but the amplifier core of the 8600 is better than the 8601 amplifier core. Doing a Hadamard analysis with linear drift: http://rubidium.dyndns.org/~magnus/time/timelab/OSA8600_HP5065A_OSA8600_2012 0618_2.png and without linear drift: http://rubidium.dyndns.org/~magnus/time/timelab/OSA8600_HP5065A_OSA8600_2012 0618_6.png Shows kind of OK values, with the flat part of ADEV around 3-5 E-13. It's obvious how the 50 Hz breaks through, and that spurious was clearly visible in the phase-noise plots too. To maintain that low I kept lights off in the lab (with me walking around the disaster area with few visual cueues). Obviously I need to work on that. The ADEV is clearly school-book with a 9E-14/tau slope and then level out at about 4E-13 and then go steep up at tau or tau square depending on the linear drift being removed or not. This prooves that the frequency drift is not linear, and that Hadamard is not able to remove the effect. Looking at the phase difference: http://rubidium.dyndns.org/~magnus/time/timelab/OSA8600_HP5065A_OSA8600_2012 0618_3.png and frequency difference: http://rubidium.dyndns.org/~magnus/time/timelab/OSA8600_HP5065A_OSA8600_2012 0618_4.png Reveal that this is not matching up very well with the linear drift assumption, but there is clearly a systematic effect in here. A better frequency drift model should be applied, and it can be assumed that it pollutes the xDEV data, especially in the long-term. Finally a look at TDEV http://rubidium.dyndns.org/~magnus/time/timelab/OSA8600_HP5065A_OSA8600_2012 0618_5.png For short times, the time-stability can be as low as below 20 fs RMS. It is clear that 50 Hz noise is a threat for that performance. At 1 s they all remain below 300 fs. 1 ns stability is in the range of 100-400 s or so. Actually, there is many improvements in how these measures are being done, but it is a nice example of how cross correlation allows one to measure below onces oscillators noise. The HP5065A is even noiser. I could for instance not trim the OSA8601 for optimum frequency, so the measurements of the OSA8600 may suffer from tracking issues. This could be the explanation that the f^-3 noises isn't as good as they should be. Also, the OSA8600s wasn't separated physically or power-wise. It's pretty good numbers in there. -121.9 dBc and -147 dBc for 1 and 10 Hz is pretty amazing numbers, for a home lab. Suggestions for improvements is welcome. The long measurements was for 1 hour. Now, nobody donating a H-maser or two? Cheers, Magnus _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
