Hi Nick,

There may be several threads in the time-nuts archives related to your question. The greater concept is a phase microstepper (aka microphase stepper). Imagine a small board that takes =10 MHz in and puts ~10 MHz out. Using RS232 (or SPI or I2C) you control the phase, or even the phase over time, which is to say, the frequency offset. Maybe do it with analog (EFC). Maybe do it with digital (DDS).

There are highly-prized commercial instruments that do this. But no amateur has tried yet. You should be the first. If you think about what we do with steering oscillators -- for GPSDO, or for dual-mixers, or for home time scales, or even sidereal or mars time -- having a device that cleanly steers phase and frequency with simple RS232 would be very useful. For example, it would allow anyone to steer a Rb or Cs standard, even though many of these lab instruments do not have analog EFC or digital tuning options.

The possibility of this at the amateur level occurred to me when I played with Bert's 9913:

http://leapsecond.com/pages/ad9913/

Read especially about the "programmable modulus mode" which can be used to avoid truncation errors and achieve perfect long-term phase; kind of like the difference between PLL and FLL in a GPSDO's 1PPS.

Look also at how the amazing FE-405 oscillator works:

http://leapsecond.com/pages/fe405/

And the idea of [mis]using a DDS as a high-resolution phase measurement technique was confirmed with the PicoPak project:

http://www.wriley.com/PicoPak%20App%20Notes%20Links.htm

So, yes, please take the bait and play with all aspects of your NCOCXO idea.

/tvb

----- Original Message -----
From: "Nick Sayer via time-nuts" time-nuts@febo.com
To: "Chris Arnold via time-nuts" time-nuts@febo.com
Sent: Thursday, July 21, 2016 10:05 AM
Subject: [time-nuts] NCOCXO anyone?

In message 4763643485B04450A76F7C04BA8CFB63@pc52, "Tom Van Baak" writes:

It would be more precise to say that no amateur has been willing to
talk about their results yet.

I personally know several have tried and failed at various levels
of performance.

My own personal experience, both analog and VHDL, is that there is
a particularly long and noisy way from theory to practice in this
space.

The one thing I have not tried, and the only one I think has any
realistic chances, is to use a DDS chip which has a phase modulation
register.

That should get you to a nanosecond without too much trouble.

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG        | TCP/IP since RFC 956
FreeBSD committer      | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

Oh my. That’s a bit more than I was originally considering… What I had in mind was adding a DAC front end to an OCXO so that you could tune the EFC with serial commands rather than analog and calling that a product.

A simple version of what you seem to be describing, however, sounds to me something like this:

The microcontroller has the same phase discriminator system the GPSDO has, except that instead of the reference signal coming from a PPS, it comes from an input reference. The microcontroller can get a phase difference reading between the oscillator output and the reference and in software can tune the oscillator DAC output to arrange for a certain rate-of-change, adjustable via serial commands.

Does that sound about right?

Perhaps a more traditional PLL approach - using the 4046 PC2 output with an RC and simply allowing the controller to sample that makes some sense (calibrating it may be painful). I’ll have to do some more thinking about it. :)

Hi

If you go the DDS route, it really needs a post filter to make it “fly right”. The narrower the
filter, the better it gets. Pretty quickly you are into an ovenized filter.  Is that better or worse
than an ovenized phase modulator? Not at all clear.

Bob

Hi

Ideally a phase micro stepper would have an ADEV floor that is lower than anything you would run through it.
That way the ADEV in would be the same as the ADEV out. Since there are things out there that are lower
ADEV than an OCXO, that’s not a good thing to put in the middle of the beast.

Bob

On 7/21/2016 4:56 PM, Nick Sayer via time-nuts wrote:

20 years ago when HP was working on a so called "smart clock"
GPS box, they decided to do what you said, use a DAC to
tune the EFC on the E1938A oscillator.  I
recommended to them NOT to try to do that, but they
didn't listen to me.  At that time, it
was nearly impossible to come up with a DAC, buffer
amplifier, and voltage reference that didn't degrade
the stability of the E1938A, which isn't even as stable
as a 10811.  What you need to ask yourself is:  in
2016, can I finally get analog components that are
an order of magnitude or two better than what was
available in 1996?  I don't know, without researching
it.  Certainly, we can't depend on Moore's law coming
to the rescue.  If anything, that works against analog
IC's by obsoleting older analog processes.

Also in 1996, phase microsteppers were already legacy
technology and didn't have a good reputation for spectral
purity.  Another non-panacea.

Rick

On Thu, 21 Jul 2016 18:47:24 -0700, you wrote:

Increased integration has only helped insofar as you can attempt
designs which would have been prohibitive before.

I keep trying to come up with a charge balancing design but what about
Linear Technology's solution from back in 2001?

A Standards Lab Grade 20-Bit DAC with 0.1ppm/°C Drift
http://www.linear.com/docs/4177

Hoi Rick,

On Thu, 21 Jul 2016 18:47:24 -0700
"Richard (Rick) Karlquist" richard@karlquist.com wrote:

If they are legacy, what is current state of the art?
And is that how your DDS approach came to be?

		Attila Kinali

--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson

On Thu, 21 Jul 2016 22:22:14 -0500
David davidwhess@gmail.com wrote:

You can already get 24bit DAC's off the shelf from TI (DAC1282).
I do not know how stable they are in reality. I looked into high
precision DAC's a year or two ago and figured out that once you
cross the 20bit line, all kind of weird stuff happens that is
hard or almost impossible to compensate for. The trick with using
an ADC (which are available up to 32bit) doesn't really work either,
as offset drifts, thermal voltage etc are hard to impossible to
compensate completely. If you go through the volt-nuts mailinglist,
you'll see how much effort it is to even keep 1ppm (~20bit) stability
of a voltage reference... and then to get that performance out of a DAC.

If anything, I think the better approach is to use high resolution DAC
like the DAC1282 or maybe the DAC1280 with a custom modulator and put
it inside the control loop such that the real measurement happens in
the frequency/time domain. The results from Sherman & Jördens[1]
seems to indicate that sub-100fs stability should be possible, though
there are a couple of open questions there.

		Attila Kinali

[1] "Oscillator metrology with software defined radio",
by Jeff Sherman and Robert Jördens, 2016
http://dx.doi.org/10.1063/1.4950898
(it's available from NIST as well)

It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson