A similar topic is in http://ieeexplore.ieee.org/document/7138792/ 73 de Ulrich In a message dated 7/22/2017 6:15:58 P.M. Eastern Daylight Time, mag...@rubidium.dyndns.org writes:
Fellow time-nuts, I thought that I should write a few lines about the EFTF-IFCS, so a good start would be the Cross-correlation workshop which is about addressing the complexities of the cross-correlation measurement and achieving stable and proper measurements. This session was curated by Prof. Enrico Rubiola of femtosecond and Craig Nelson of NIST. OK, for those that don't know about the issue, let me get a quick explanation here. For a long time cross-correlation have been a technique to remove the internal noise of two, by feeding the source into two channels using a power-splitter, and then each channel add its noise which is uncorrelated. The cross-correlation of the channels then sees the common signal and as one average over multiple spectrums the uncorrelated signals average out. Sounds splendid, but trouble showed up on the horizon when they had trouble getting stable readings from measurement to measurement as the noise starts to reach the thermal noise floor. Some measurements where about 20 dB below the noise floor, some where about the noise floor, but there was a dip in the response. As one look closer on historic measurements using cross-correlation it's been there for ages, since the measurement method started to be used. Looking closer on it, it looks like the thermal noise flips from 300 K to -300K and the "dip" is where the noise goes through zero. Oups! In 2015 there was a similar workshop, at that workshop Joe Gorin contributed an explanation for where the anti-correlated noise was generated. In a Wilkinson splitter, the power is divided into two ports. However, to achieve isolation between the ports, a resistor of twice the impedance is needed, i.e. 100 Ohm. It just happens that the noise that generates is two times the noise of the source, and it is completely anti-correlated, thus subtracting. OK, great, now we know why it breaks this way. What to do about it? At NIST Archita Hati and Craig Nelson where pulling their hair and at the FSM8 conference in Potsdam, Archita presented a poster which was interesting exercise in failure to solve the problem with a whole range of splitters, including a Wilkinson where they out of desperation had pulled the resistor, and still it was problematic. During late evening discussions over wine at FSM8 we had the most interesting discussions. Good teachings where had for everyone involved. At EFTF 2016 in York, I met with Craig and Archita to discuss some ideas, which they tried and found useful and presented by Archita at IFCS 2016 in New Orleans a month later. My contribution was to ask about what isolation gives us, and it turns out that lack of isolation causes channel noise to back-propagate and reach the other channel, being now correlated noise between the channels and then acting as correlated noise, with a complex response after FFT depending on the delay between the channels. Given that the source noise pops up on the real axis due to equal distance to the channels, insertion of variable delays allows one to steer the vector of the channel noise onto the imaginary axises where it as a secondary effect collapses as they cancel. Sound great huh? Yeah, in theory, but it is hard to balance this to maintain the full property for the length of the full measurement, but at least some attempt on it. Now, with that back-story it was time for another workshop on cross-correlation at the EFTF-IFCS-2017 in Besancon. Enrico asked for my contribution, so I volunteered to contribute. We ended up being some 30-40 people in the room, with people like Dave Leeson present. A variety of presentations where made, illustrating the width of different insights. NoiseXT showed a source that could generated AM and PM noise, in hope that it could prove useful in provoking different scenarios and be useful to illustrate AM-to-PM conversion for instance. Jason Breibarth of Hollingsworth presented an array of cases where AM-to-PM was evident. Enrico presented an approach to estimate the removed noise and simply add it back. Mike Driscoll discussed the potential of doing a different approach with mixers for higher amplitude level. Sam Stein made some good comments about how their equipment was built and pointed out some difficulties they had ran into, such as the cross-AVAR needed to avoid the flipping sine. Also, the data as presented out for post processing is a different decimation path than what is used for internal processing since, well, it was difficult to have them so close. As for myself, I made the point that even with ideal splitter, isolation is broken when noise from channels is reflected on the source, as some sources have far from flat 50 Ohm impedance, something that is known to be the case for some sources being troublesome to measure. Similar to the other cross-talk problem, it will create a partial collapse of the spectrum. My slides can be sent if anyone cares to have them, I will upload them to my server. As of this day, there is no real answer of how to do all this, we have multiple research groups and vendors seriously scratching their head on this one, but it is really amazing to be sitting on on this as observations and solutions are being tried. We now know better that we have a myriad of issues causing phase-noise readings to be of the mark, and some claim to be even non-physical. So, there is real research being done in what we used to consider a well-researched area, namely phase-noise measurements. This is one tough cookie to crack. Hope it was an interesting read. Cheers, Magnus _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.