[time-nuts] DAC for OCXO disciplining

[Mike Ingle]

Since for a PLL we generally want to steer in the direction indicated by
the error signal (phase detector), a relative output DAC rather than a
absolute output DAC seems more appropriate.  I would look at something like
this:
https://www.edn.com/dc-accurate-32-bit-dac-achieves-32-bit-resolution/ .
--mike

On Tue, Jan 14, 2020 at 2:33 AM Jim Harman <j99harman at gmail.com> wrote:

> The LTC1655 is a good option if 16 bits are enough. It has an internal
> reference which is a plus if your oscillator does not have a reference
> output. The DIP8 package is also a plus for us hand solderers.
>
> The AD5680 is a good 18 bit choice, and a bargain at $9.22 qty 1. It is
> basically a 16 bit DAC with 2 additional bits via an interpolator, so it
> needs some filtering at the output.  It should be a good choice for
> applications like GPSDOs where linearity and settling time are not major
> concerns.
>
> One challenge with coarse/fine approaches is that the combined DAC must be
> monotonic for the loop to be stable. If you can tolerate a reduced
> automatic tuning range and make the coarse adjustment a manual setting or a
> setup mode, coarse/fine may be OK.
>
> On Mon, Jan 13, 2020 at 7:25 PM Lifespeed <lifespeed at claybuccellato.com>
> wrote:
>
> > I have been looking into all-digital PLL designs to discipline an OCXO,
> the
> > scope of which would of course include GPS, as well as other reference
> > sources.  A component of the block diagram would be the DAC converts the
> > digital control loop output for application to the OCXO tune port.  In
> > order
> > to use such a circuit to improve Allan deviation, rather than degrade it,
> > concerns like step size, Differential Non-Linearity (DNL), and possibly
> to
> > a
> > lesser extent voltage noise, are concerns.  Some thoughts that have
> > occurred
> > to me are coarse and fine DACs, possibly sigma-delta or pulse width
> > modulation (PWM).  Or some combination of all the above.
> >
> > Any suggestions on topologies and/or candidate parts?  One possibility
> for
> > fine tuning might be the AD5791.
> >
> > Lifespeed
> >
> >
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>
> --
>
> --Jim Harman
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