[time-nuts] Software Sawtooth correction prerequisites?

[Dr Bruce Griffiths]

Tom
Tom Van Baak wrote:
> I would agree. A 1 ns phase detector is usually more than a
> single IC. Not sure what to say about the computer resource,
> though. The sawtooth correction is usually obtained though a
> low-speed (e.g., 9600 baud) serial message from the GPS
> receiver. Most any $2 microcontroller can handle a task like
> this.
>
> When someone finds a cheap single-shot 1 ns TIC-on-a-chip
> please let me know.
>   
Define cheap.
You can already get essentially single chip TICs with a resolution (and 
accuracy) better than 100ps for around 100 Euros or so.

> If you hare thinking of a GPSDO you need a phase detector
> so you have to ask yourself if hardware sawtooth correction
> with a good phase detector is better than an excellent phase
> detector with just software correction. You'd have to do the
> math to decide which was better.
>
>   
However sawtooth correction is cheaper than using a hardware variable 
delay line.
> The other factor that someone can comment on is what effect
> time averaging has on all this. Can a coarse phase detector do
> the job if you average long enough? Most GPSDO don't need
> to make decisions every second. If you have a good OCXO
> and tweak the EFC only once every 100 or 1000 seconds, can
> you get away with a cheaper phase detector design? 
> /tvb
>   

The answer depends on the sort term stability of your OCXO or other 
oscillator being disciplined.
If it has an ADEV floor of a few parts per billion then a relatively 
coarse phase detector without sawtooth correction will suffice with a 
relatively short loop time constant.
If, however you are trying to achieve the best you can from a standard 
with an ADEV floor of 1E-11 or better then it is prudent to do sawtooth 
correction and use a high resolution TIC.

However the optimal solution doesn't use a TIC at all, just use a good 
GPS receiver that makes the carrier phase observables available.
Quite a few of the older receivers used to make the carrier phase data 
available.
Even with a averaging time of a hundred seconds or less the effective 
receiver noise floor will be well below 1E-11, and the achievable 
performance will depend on just how "quiet" your OCXO is.
With a suitable receiver using a simple TCXO a stability of around 1E-11 
is possible even with an averaging time of a few seconds.
The catch is that the GPS receiver local oscillator must be phase locked 
to the OCXO.
Some receivers use a 10MHz crystal oscillator in which case this can be 
removed and replaced with an external 10MHz signal derived from the OCXO 
being disciplined.

Unfortunately there are as yet no relatively inexpensive off the shelf 
implementations of the GPS carrier phase discipling technique available.
The calculations involved for maximum performance are somewhat complex, 
however not too much computing horsepower should be required.
Magnus hopes to remedy this lack of suitable software and hardware 
sometime in the future.

> /tvb
>   


Bruce