[time-nuts] GPSDO using 100Hz

[Bruce Griffiths]

WarrenS wrote:
> Bruce
> It would seem we are now in agreement and on the same track in most areas.
>
>   
We probably always have been, but this certainly wasn't clear from the
original posting.
>    >B) A time constant of several hours is only useful with a very high quality OCXO.
> Agree, a High quality Osc needs a long TC or else it will degrade the noise performance.
> On the other hand if using a short time constant (for whatever reason) there is little 
> need for a high quality OCXO. A short term stable osc will give about the same results.
> AND if you don't care about short term noise such as when you are only averaging the counts   
> over say an hour or a day, to compare small phase shifts to get very accurate frequency results, 
> then any OSC even the most crappy VCO will do if it is updated fast enough to keep it from skipping counts.
>
>   >B) The variable pulse width of the 100Hz (and 10kHz) outputs do no favours
>      to an XOR phase detector, its better to use the leading edges of these signals.
> We agree again, That is why they MUST be divided by two first, using the correct edge.
>   
It is very important to give such detail on this list as the
knowledge/experience of the list members varies so widely.
The last thing one wants to happen is for someone to blindly rush ahead
and use an XOR phase detector without first dividing the the 100Hz or
10kHz by 2 and then wonder why the performance isnt that good. Providing
such detail also engenders more confidence in the soundness of the method.
>   >B) making 100 or 10,000 measurements of the phase error every second can, 
>         if the dither is of the right form, improve the effective resolution. 
>         However surely the timing quantisation error of the leading edges of the 100Hz (or 10kHz) 
>        outputs limits the potential improvement?  
> Yep, there is a limit to how much improvement is available, It can not get better than perfect.
> The rule of thumb is the improvement is the square root of the number of samples for random noise.
> For the non random noise of the 100 Hz the improvement can be anywhere from Zero to 1/number 
> of samples. Typically I'm seeing about a 50 to one improvement, with a worse case of no improvement 
> for short periods lasting under a minute (without the addition of a simple processor)
>
>   
If the phase error counter clock should ever injection lock to the OCXO
or the GPS timing receiver output, then the averaging will fail.
If this clock is sufficiently noisy or is phase dithered sufficiently
with random noise then this wont happen.
>    >B) One can do much better with an inexpensive processor...
> I completely agree. One can always do better with something.
> But my point is One can do 'good enough' for many applications with a lot less.
>
>  >B)  with little external hardware other than a high resolution DAC
>       (even that can be implemented in software and hardware within 
>       the processor together with a couple of opamps).
> Don't even need that much, most of the time, by providing a seldom 
> changed course adjustment along with the fine adjustment. 
>
>
> Warren
>
>   
Bruce