[time-nuts] GPS discipline oscillator vs phase lock
life_speed at yahoo.com
Sat Jun 17 01:55:47 EDT 2017
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.
On Jun 16, 2017, at 7:24 PM, life speed <life_speed at 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.
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.
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