[time-nuts] 5 MHZ PIC PPS Divider?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Apr 16 20:50:16 EDT 2008

Bruce Griffiths

Correction added:  

> Said
> Not this particular implementation but have built similar circuits 
> decades ago.
> Tempco isnt an issue.
> There's plenty of time to interleave calibration cycles of say 100ns, 
> 200ns, 300ns, 400ns duration between PPS measurements.
> It should even be possible to correct residual low order non linearity.
> With some elaboration is easy to construct a BJT current source with a 
> residual tempco of around _100 ppm/C_ or so due to the BJT's hfe (~100) tempco.
> Replacing the polystyrene cap with a C0G/NP0 cap is probably worthwhile 
> especially if one wishes to avoid hand soldering the polystyrene cap 
> using heat sinks on the cap leads.
> This technique and variants thereof have been in use for decades in 
> nuclear instrumentation.
> Another widely used technique is the Wilkinson converter wher the 
> capacitor is charges by a large current during the time interval to be 
> measured and the discharge time with a current perhps 1/1000 of this is 
> measured using a comparator and a counter clocked at 10 or even a 100 MHz.
> Bruce

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