I’ve been trying to research this very topic!

Can you point to some of these papers?

I am trying to build the most accurate fee running, low power time base I can. I am using an MCU, 32768Khz watch crystals, 0.5C accuracy temp sensor, lots of thermal bringing between them, and mass around them. The idea is to measure the frequency shift at all temps in the range, and even in both directions (hopefully to capture some hysteresis) for each unit and then use that database to compensate in software once the system is free running.

I am trying to beat existing products like the Dallas DS3231 and Micro Crystal RV-8803-C7-32.768kHz-3PPM-TA-QC, which use (I think) a similar strategy. I’m hoping I can beat them by using more accurate temp tensing, longer and more exhaustive calibration effort, and anything else possible!

Can you give a quick explanation (or point to reference material) covering the fundamental limits to XTAL compensation accuracy, and how to get there?

That is, if I had an infinitely precise temp sensor and an infinite amount of time to characterize an XTAL, what would be limits to how accurately I could temp compensate it?

Also, what are the limits of characterizing and compensating for aging?

What other sources of inaccuracy would I need to consider?

Thanks!!!

-josh

How do those compare with vectron's part : ?

https://www.vectron.com/products/ocxo/mx-503.htm

There's also this patent.

http://www.google.sr/patents/US20020005765

I don't really know if that's valid - it seems to propose something similar
to the numerically-compensated oscillator in my rather old PM6666 frequency
counter.

On Wed, Nov 1, 2017 at 5:11 PM, tnuts@joshreply.com wrote:

Hi

The Frequency Control Symposium has papers going back > 50 years on crystals and how to
cut them from raw quartz bars. Plan on spending a few million dollars to get set up to do this well.
Something in the $5M is likely the “going rate” these days for a basic line, even with some of it
bought surplus.

Which are very low precision crystals by their very nature…..

Which is pretty loose by todays standards. You can get parts that will do better than that.

Keep in mind that your hysteresis runs need to be at multiple speeds and over multiple temperature ranges. The results vary with
both promoters.

Assuming you have a “normal” watch crystal, you can get into slopes well over 10 ppm / C without going to crazy extremes. That
and your 0.5 C resolution is going to have a pretty big impact.

I’d say it’s unlikely.

The FCS proceedings have a number of papers. The basics depend a lot on the type of crystal you have and the net result you are after. If you must
have the output on frequency that leads in a different direction than if you just want an accurate one second tick. For example, with the one second tick,
you can drop (or add) cycles in your divider. If the oscillator is at 10 MHz, the net result will always be within 100 ns. For something like NTP, there
is no real advantage going past that point. You can accumulate error over a long period. That allows very good stability over the long term.

If you need low power and proper frequency, a thermistor resistor network driving a varicap diode is the classic answer. You generate a cubic voltage
function over temperature. It is the “required signal” to tune the oscillator on frequency. The components in the network are evaluated and adjusted with multiple
temperature runs. The software to make it all work is generally the “top secret” IP of the people making the TCXO.

A brute force approach using a good temperature sensor (say an RTD) and a good ADC is possible. You can get into the low mili C range with this sort of
setup. Pairing that with a good low frequency overtone crystal can do pretty well. There are also multi mode ostillators that head in the same direction. There
are many twists and turns. A starting engineer generally takes something in the 5 year range to come up to speed on most of them.

Well, aging is one obvious issue. ADEV is another. A reasonable goal for aging would be in the low parts in 10^-11 per day range. Good ADEV at 1,000 seconds
would be at least that good. The electronics to make it all happen might use as much power as a low power OCXO …. Managing temperature in a TCXO like
device below 1x10^-10 is unlikely.  Figure the crystal used would be > $100 in volume. Single piece (if you could buy one) might be over a thousand dollars.

Low parts in 10^-11 per day on a “production” basis if cost is not a consideration. It also depends quite a bit on the constraints you put on the problem. (how many
days on power, what sort of environment, ….)

There are many. How many years do you have to study the various topics? Thermal gradients are one that will bother you.

Again, the FCS papers are a good start. Best to find a library  with free access. They are a bit expensive on DVD.

Bob

On 11/1/17 12:13 PM, Adrian Godwin wrote:

That part is interesting, but the phase noise (-124 dBc @ 100 Hz) isn't
particularly impressive compared to a middle of the road OCXO (-155dBc
at 100Hz)

The frequency stability is very nice, compared to a TCXO, and at a LOT
less power (40mW) than the OCXO (2W after warmup).

On Wed, 1 Nov 2017 13:11:00 -0400
tnuts@joshreply.com wrote:

You probably can get better, but is it worth the effort?
The problem with temperaure compensation is, that you have to measure
the crystal temperature with very high accuracy. Measuring something
close by means you have to model the crystals actual temperature using
the temperature measurements you made before. The more parameters you
add to your system to be identified, the more problems you get accurately
identifying them, as they are not completely independent and cannot be
directly measured.

Directly measuring the crystal temperature has been done and one quite
popular approach is MCXO. [1] and [2] are two early papers that describe
the approach in quite some detail. There are a few problems with this.
One is that you need a special crystal that is designed for dual mode
operation and the other is that you need quite a bit of electronics
around it to work. Oh.. and if you are living in the US, please be
aware that MCXO are covered by ITAR.

For general modeling of crystals, I recommend reading John Vigs
Oscillator Tutorial. It covers most of what we know about crystal
oscillators and gives lot of references.

			Attila Kinali

[1] "A Microcomputer-Compensated Crystal Oscillator Using a
Dual-Mode Resonator", by Benjaminson, 1989

[2] "Resonator Self-Temperature-Sensing Using a Dual-Harmonic-Mode
Crystal Oscillator", Schodowski, 1989

--
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor