[time-nuts] GPS discipline oscillator vs phase lock

[Bob kb8tq]

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_speed at 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_speed at yahoo.com <mailto: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. 
>  
> 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
>