[time-nuts] GPS Disciplined TCXO

Gregory Maxwell gmaxwell at gmail.com
Sat Oct 24 17:43:33 EDT 2015

On Fri, Oct 23, 2015 at 11:08 PM, Nick Sayer via time-nuts
<time-nuts at febo.com> wrote:
>> On Oct 23, 2015, at 2:09 PM, Gregory Maxwell <gmaxwell at gmail.com> wrote:
>> On Tue, Oct 20, 2015 at 8:53 PM, Bryan _ <bpl521 at outlook.com> wrote:
>>> Saw this on the Hackaday site if anyone is interested.
>>> https://hackaday.io/project/6872-gps-disciplined-tcxo
>> Will this design that uses the output of the DAC directly not run into
>> problems with non-monotonicity and/or dead-zones in the DAC output?  I
>> would expect a PLL to behave very poorly if there is any
>> non-monotonicity in the least significant bit of the DAC.
> The datasheet claims the DAC is inherently monotonic. It’s a $7 part, so I don’t have much reason to look sideways at that claim.

Indeed!  However, the spec sheet shows (e.g. figure 10) a differential
non-linearity of 0.2 .. -0.2  LSB,  meaning that when the PLL makes a
single step the result may be 20% greater or lower than expected,
which probably isn't good for stability though not the PLL
breaking-ness of a non-monotone response.

> That strikes me as familiar - a little like how Arduino fakes analog output by running PWM into an LPF.

It's a common technique, (it and ones like it) also used internally in
high bit depth DACs.

> If you look at the AD5061 datasheet, there is unfortunately a relatively significant (to my eyes, at least) update glitch. I suppose it’s quick enough that the RC filter would get rid of most of it, but it is an extra noise source if you do it frequently, like you’re suggesting.

Ouch, that is a fairly substantial spike compared to 1lsb... it's
short at least, but if you are only updating once a second I'd wonder
if that would not have a measurable impact on stability.

A potential advantage of running at a constant high rate is that
rather than taking the impact of that glitch once per second, the
glitch happens constantly and so its effect can just be averaged out
by the PLL.  (e.g. it becomes equivalent to just scaling the output
voltage by its average effects).

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