[time-nuts] Thunderbolt? (re simple gpsdo.)

[Kasper Pedersen]

On 01/01/2012 12:23 PM, Tom Harris wrote:

> I too have been mulling over a minimal GPSDO.
> 
> Has anyone mentioned a PWM DAC? Even a "toy" microcontroller like the
> Atmel AVRs can generate 16 bit resolution PWM, which sounds like it

..

> So a minimal GPSDO looks like a simple microcontroller (I would use
> Arduino since we use these at work whenever we need to build something
> in an afternoon) with a filter for a PWM DAC, so one opamp and a few
> discretes. Of course as I write software for such beasts as a trade I
> am giving the software a zero cost.
> 


A 'bike light' sized microcontroller can do the job. Today I would have
used something larger, and saved development time. On the other hand,
having only room for 512 instructions prevented spending time on
non-essential features.

http://n1.taur.dk/simplexdo/

I used 8 bit PWM into a two-stage RC filter, making it easier to filter.
To get 16 bit resolution I added delta-sigma modulation on top of the PWM.

The ATTiny13 is not the right part for the job; It has no input capture.
So space- and time consuming tricks are necessary to get cycle-accurate
capture.

Recently I discovered that some of the microcontrollers in the drawer
would do nanosecond capture all on their own, but with a working tbolt,
need has been lacking.


/Kasper Pedersen