[time-nuts] GPSDO control loops and correcting quantization error

[Bruce Griffiths]

Hal Murray wrote:
> djl at montana.com said:
>    
>> Michael: Actually implementing a 16 bit DAC to its 1-bit minimum resolution
>> will be headache enough. You will gain a real education in good grounding
>> practice, shielding, power supply stability and noise, and other Murphy
>> intrusion. A 32 bit DAC IMHO, is impossible, and that's the name of that
>> tune.
>>      
> The trick for this application is that you don't need full accuracy over the
> full range of the DAC.  All you need is roughly linear and stable around the
> operating point.  The PLL will take care of any offset.  Any gain error is
> just a minor change to the overall gain.
>
> This thread started with "16 bit PWM DAC".  I think that matches the
> requirements.
>
> I would expect a problem area would be filtering the PWM output.  Anything
> you don't filter out will turn into close in spikes.  It might be fun to try
> to measure them.
>
> 64K/72 MHz is about 1 ms.   32bits at 72 MHz is 60 seconds.
>
> Has anybody compared DDS style DACs with PWM?  The idea is to spread the
> pulses out to make the filtering easier.  Instead of 1111100000, you would
> get to work with 1010101010
>
>
>    
Using a synchronous filter for the PWM DAC eases the additional 
filtering required considerably.
24 bit resolution is readily achieved by combining the outputs of a pair 
of PWM sources sharing a single synchronous filter.

Bruce