[time-nuts] PICTIC II ready-made?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Apr 25 19:15:41 UTC 2012


Chris Albertson wrote:
> On Wed, Apr 25, 2012 at 9:37 AM, Don Latham<djl at montana.com>  wrote:
>
>    
>> I forgot to add that a simple redrafting of the II as an Arduino shield
>> with appropriate chips and chip passives would accomplish the desired
>> end without losing the very careful engineering and testing that has
>> already been done?
>> Would be nice to have a way to change caps without soldering as well,
>> maybe just some .1" jumpers?
>>      
>
> Yes, MOST of the design could be re-used.  As an Arduino shield there is no
> need for a PIC or RS-232 interface becusethe Arduino does that function.
>   You'd need to replace the 74ACT175 part but that is not hard.
>
> About changing the cap values without soldering.  I guess you could push
> the leads into a 0.1 inch header strip or install several and use a DIP
> switch to select which are "in".   But I don't know if the extra inductance
> al that wiring adds is enough to worry about.
>
>    
The time to digital converter (TDC) section is merely an interpolator 
that measures the delay of a synchroniser.
The TDC range should be about 2 clock periods to accommodate the range 
of synchroniser delays and to facilitate calibration.
Unless one is changing the synchroniser clock period there is no need to 
vary the TDC gain.
The SR620 uses a similar interpolator and has only a single interpolator 
range.
The range is extended by counting the number of synchroniser clock 
periods between synchroniser output transitions of interest.
When measuring the time interval between 2 signals a pair of 
synchronisers and interpolators are used.

Interpolator nonlinearity can be measured by using a statistical fill 
the buckets technique which uses nothing but a pair of noisy 
asynchronous oscillators with high reverse isolation to avoid injection 
locking.

If a suitable ADC is used the interpolator can be simplified 
considerably whilst improving its performance.
Minor nonlinearities are of little significance, as long as they are 
repeatable and relatively stable they can be easily corrected in software.

Bruce



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