[time-nuts] BC637PCI and OCXO

[Tom Van Baak]

> Aside of that OCXO implementation, I would like to ask some direction 
> from time-nuts community about the relatively simple way to compare two 
> 1PPS signals using some MCU. Lets say I have T-Bolt and BC637. Each has 
> 1PPS and I would like to see the phase difference between of that two. 
> Of course it is some instruments already available for that, but I have 
> not own that one (yet). I am thinking to connect both 1PPS to Timers 
> input in "capture" mode on MCU. Then register first raising edge, start 
> counting of ticks until second timer will not catch another signal 
> raise. Then number of ticks will indicate the difference. But may be it 
> is more "elegant" way to do that comparison ? Thanks !
> 
> Regards,
> 
> V.P.

Yes, that's very simple and works well, up to the period of the MCU clock driving the timer, e.g., a 200 MHz MCU gives you 5 ns resolution on the capture register.

If you need under 5 or 10 ns, the next level is adding analog interpolation to the signal. There have been many discussions about this on the list. Search for Richard's PICTIC, for example. Depending on the complexity you want and the amount of calibration or re-calibration you employ, you can get down to hundreds or even tens of ps this way.

Most of us just buy surplus time interval counters on eBay. You can get 1 or 2 ns counters for $50 to $200 on a good day. Or for casual time interval work, a two channel digital oscilloscope.

/tvb