[time-nuts] yet another GPSDO design, or so
bruce.griffiths at xtra.co.nz
Tue Jun 29 20:28:38 UTC 2010
Hal Murray wrote:
> 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.
Not if one uses a pair of 16bit PWM circuits to produce a DAC with 24
a few 0.1% resistors then suffice to achieve 24 bit linearity.
> 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.
With a synchronous filter the settling time (for small output changes)
is equal to the PWM period.
The synchronous filter uses a variation of a dual slope error
integrator, the output of which when sampled is equal to the desired output.
The effect of dielectric absorption in the error integrator can be
reduced by implementing a mutislope integrator rather than a dual slope
Its then possible to use a pair of 8 bit PWM signals to achieve 24 bit
> 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.
Simulated that, and Ulrich did some testing, the spurs can be problematic.
The synchronous PWM circuit is much easier to filter as the synchronous
output noise amplitude (with a constant input) due to sampling charge
injection need not be more than a few microvolts. That is there is a
small spur with an amplitude of a few microvolts at the PWM repetition rate.
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