[time-nuts] And, for my next trick, 50Hz

[Bruce Griffiths]

Steve Rooke wrote:
> Tom,
>
> Sorry to post in response to my previous post but perhaps this system
> could work if the input signal did have a quite a slow rise time. I
> was thinking more of coupling the output of the proposed 1KHz divider
> through quite a small amount of low-pass filtration. If the 1KHz was
> passed through a low-pass filter at the fundamental we would get a
> sine-wave with a relatively slow rise time and the ability to get more
> samples around the zero crossing point. Taking it a stage further, we
> could pass our square wave though an integrator to produce a linear
> sawtooth which would provide the best way to measure any phase shift.
> Now I think this could work to gauge 1ppm in 10 seconds but you would
> really need a short-term stable NTP disciplined clock to do it. I
> wonder what the average allan variance would be for this setup over a
> 10 second period.
>
> 73, Steve
>
>   
Steve

With interpolation you dont need to slow the signal transitions too much.
A transition suficient to allow 3 or 4 samples to be taken during the 
transition is adequate.
If the signal slew rate is too slow sound card input noise and the 
finite ADC resolution will increase the measuremnet noise.

Bruce