[time-nuts] DAC for OCXO disciplining

[Jim Harman]

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