Hi If you also need the phase noise of the OCXO’s to be quite good when operating, the PLL approach has some issues. If you are after -100 doc / Hz sort of numbers at 1 Hz offset at 100 MHz, then a PLL to GPS is not your friend. At GPS will degrade that by many 10’s of db. If the phase similarity requirement of the two OCXO’s extends out into the 100’s of Hz (you need them phase coherent to 10 MHz maybe..), you will not be able to do that with any PLL approach. Again, the “record a set of modulated signals” approach is more likely to do what you need to get done. In some ways the modulation *helps* in this case. It lets you better estimate cycle slips and other odd occurrences.
Bob > On Jun 17, 2017, at 1:55 AM, Lifespeed <life_sp...@yahoo.com> wrote: > > Not too surprising to read locking two crystal oscillators together without > using a physical cable is difficult to impossible. Essentially what I am > looking for is the phase alignment accuracy (and phase noise) one would get > PLL’ing one oscillator to the other using a cable, but over a longer > distance. Some modest phase noise degradation might be acceptable, but not > an order of magnitude. Clearly not a trivial problem. Yes, the jitter (phase > noise) typically accomplished from a PLL phase comparing at 100MHz is better > than what one could get “locking” to GPS. It was just a thought, apparently > not a realistic one. Thanks for disabusing me of that notion. > > Sorry I can’t go into a lot of detail about the overall system block diagram, > but this one aspect of the design does just reduce to phase-locking two > oscillators over a distance. > > Bob, I think I understand your post processing method refers to the reality > that all broadcast signals from which phase information could be extracted > are modulated, introducing complications that would not be present with a > simple carrier. > > Lifespeed > > Hi > > >> On Jun 16, 2017, at 7:24 PM, life speed <life_sp...@yahoo.com >> <mailto:life_sp...@yahoo.com>> wrote: >> >> That sounds like phase-locking the oscillators to a local radio transmitter. >> Not sure there is any difference post-processing vs. real time. > > The advantage is that you capture a much wider bandwidth signal than you can > lock to. That lets you extract better “instantaneous phase” information. With > the narrow band loop normally used for locking, loop dynamics get into the > picture. That on top of the RF propagation issues is a bit of a mess. It also > is quite possible to capture multiple radio (or TV or …) transmissions and > post process against all of them. > > The bottom line is still that “many degrees” at 100 MHz is far more practical > than “tenth of a degree”. There are very few options if your application > really does need roughly a tenth of a degree. > > Bob > > > > > - Lifespeed > > > Hi > > A far more common approach is to let the two oscillators free run and to > record something like a local broadcast station. > You then post process all of the data to give you the phase accuracy. One of > several gotcha’s is the stability of any > radio link at the level you are looking for. > > Bob > _______________________________________________ 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.
