[time-nuts] Updated Shera controller

[Bruce Griffiths]

Instead of copying the Shera controller albeit with higher resolution 
its probably more cost effective to choose a microprocessor with built 
in time stamping capability.
The PIC24FJ128GA for example allows 30ns timestamping resolution via its 
external timer capture inputs.
Other microprocessors (or DSPs) are available with even higher 
timestamping resolution.
Since modern GPS timing receivers can have a timing noise of a few ns 
(after sawtooth correction) it may also be useful to use a simple time 
to digital converter (eg TAC + ADC) to achieve a resolution of 1ns or so.
Several microprocessors have built in ADCs with sufficient resolution 
and low leakage to allow trimpots, external buffer opamps etc to be 
dispensed with.
The trimpots being replaced by interleaved software calibration.
The microprocessor initiates a TAC calibration cycle after each external 
PPS event is timestamped.
The resultant sequence of calibration coefficients can then be filtered 
and used to correct the PPS fine time stamp sequence.
If the microprocessor also has a couple of PWM outputs then these can be 
used to implement a high resolution synchronously filtered DAC
The synchronously filtered DAC requires a stable reference, a couple of 
opamps, a few analog switches plus a few resistors and capacitors.
Neither the resistors or capacitors need to be extremely close tolerance 
parts.

Bruce

EWKehren at aol.com wrote:
> What could you help with?   Bert
>
>
> In a message dated 7/28/2010 3:05:26 P.M. Eastern Daylight Time,
> wpxs472 at gmail.com writes:
>
> Bert,
> I for one, would be interested in  that.
>
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