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

Magnus Danielson magnus at rubidium.dyndns.org
Sat Sep 15 10:13:46 UTC 2012


On 09/15/2012 12:08 AM, 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
>
>

PWM has the fantastic power of putting most energy into the lowest 
frequency, which makes analog filtering extra hard, so you need to move 
the bandwidth down or use higher degrees filter for a good filter slope. 
The filter bandwidth puts an upper limit to the PLL bandwidth.

I've done a inverse PWM spectrum modulation, which isn't all that hard, 
and it has significant improvements over PWM.

Another approach is to use the sigma-delta approach which smooths the 
frequency spikes out to noise.

Cheers,
Magnus




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