[time-nuts] yet another GPSDO design, or so

Hal Murray hmurray at megapathdsl.net
Tue Jun 29 18:39:43 UTC 2010

bruce.griffiths at xtra.co.nz said:
> Its possible to build a 24 bit resolution D/A using a synchronously
> filtered PWM circuit. A pair of PWM outputs and a few relatively low
> precision resistors and  capacitors together with a low noise low drift
> reference are required. The technique takes advantage of the fact that the
> required EFC voltage  changes slowly and isnt updated at a highg rate. The
> synchronous filter technique eliminates the very long time constant  RC
> filters required with an asynchronously filtered PWM waveform. 

24 bits is 16,777,216.  At a reasonable clock rate, that's one second.

Another approach is to distribute the individual bits rather than clump them 
together.  If you want 1/2, send 10101010 rather than 11110000.  You would 
have to do something like build a bit pattern in memory and use a serial port 
to send it out.

That shifts the frequency of the junk so that it's easier to filter out 
and/or reduces the amplitude.  If you send 10101010, you have lots of energy 
but it's at 8 MHz.  If you send 1000000, you have energy at 1 Hz, but it's 
only 1/16000000 as big.  Or something like that.  [Since this is a linear 
system, you will get that spur with any odd number of 1s.]

I can't determine if that's good enough.  I think the math is similar to the 
spurs you get from a DDS.

These are my opinions, not necessarily my employer's.  I hate spam.

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