Hoi Jim,

On Wed, 13 Mar 2019 08:39:24 -0700
jimlux jimlux@earthlink.net wrote:

I have never measured TCXOs, but I've seen data of other devices
that are adjusted at the manufacturer. What you end up with is
something between a uniform distribution and truncated Gauss.
It is also very likely that most are withing 1ppm or even less,
depending on how much margin the manufacturer put into the spec.
Aging and environment will spread the values in a Gauss like fashion
(think of a (anomalous) diffusion process).

Would it be rude to ask for more details on what you are planing to do?
It sounds interesting.

		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, Neal Stephenson

Hi

Ok, a bit of nomenclature:

TCXO = temperature compensated crystal oscillator = it has a compensation network in it
XO = crystal oscillator = it does not have a compensation network in it

Why does that matter in this case? They are different products targeted at different markets and
different applications.

A part with a +/- 50 ppm stability spec over 0 to 70C is almost certainly an XO. It might come in a
version that is +/- 50 ppm “all inclusive for X year(s)”. In all likelihood it does not have an adjustment
“port” on it.

A TCXO is mighty loose if it is +/- 2 ppm over 0 to 70C. It is highly likely to have an adjustment port
(voltage or trimmer) on it.

Since they are very different parts, it does matter more than a bit which one you are looking at.

XO’s may get manufactured with an “offset” but that’s not very common. It is more likely that the
“plate to frequency” process has an offset in it and you get a batch all to one side or the other
as a result. A part with a _/- 50 ppm overall spec might be plated +/- 15 ppm. (could be tighter
or looser depending on the outfit doing the part).

A TCXO might also have an offset, but it’s going to be a lot smaller. It’s TC likely will also be a
lot smaller.  Most low cost TCXO’s don’t get an offset since it is more hassle to track through
a high volume manufacturing process. They all should hit center frequency within a fairly tight
window. It’s not uncommon to see +/- 0.1 PPM spec’d.

In both cases, it would be highly unusual for the parts to get sorted in any way to “pick the good ones”.
It costs more to do that than it’s worth. ( = there are other ways to get “good ones”). The line
is focused and set up with a target range of spec's in mind. If you want a better approach, you get
them off a different production line.

So out of your list of specs:

Initial set = what you would expect your TCXO to read on your board, generally not an XO spec
Frequency after initial aging = same thing as initial set with some wiggle room added
Frequency over temperature = what happens if you run it 0 to 70C (or whatever the range is)

Long term aging = what happens over 5,10 or 20 years.

Again, it is not at all uncommon to roll all that up into a single “lifetime” spec on an XO. You
rarely see that done on a TCXO.

Simple answer would be to get a cheap oscillator with an EFC port and feed various voltages
into it. More or less - spend the $5 on the lowest phase noise tunable oscillator you can find.

Bob

On 3/13/19 9:17 AM, Attila Kinali wrote:

I'm looking at various radio issues with large swarms of tiny spacecraft

• specifically for making a radio telescope. Unlike terrestrial
  interferometers where you have reference distribution (local) or cesium
  clocks and masers (VLBI), we have to have some other way to solve the
  problem.

Fortunately, you can do a lot of post processing, so it's more a matter
of "knowledge" not "control" of the local frequency source on the swarm
nodes.

One scheme to propagate references is to radiate your reference
oscillator so that all the other nodes can receive it, and each node can
then say "I'm node A, and relative to me, nodes B is 1% high, Node C is
0.5% low, etc." and from that you can (maybe) solve for the ensemble.
One can also get some information about the relative positions of nodes
(by looking at the phase of the signal).

(one does need at least one single "good" reference to tie them all to..
in the case of most swarm schemes, there's a mother ship to relay data
back to Earth that can provide a broadcast reference, or if you're close
enough to Earth, you can radiate a high quality signal from Earth).

This whole scheme breaks horribly if too many signals are too close
together, and I suspect that this is the actual case.  So we need to
have a way to explicitly move the signals around.

when you're building 1000 nodes, you would really like them to be
a) all the same parts list and construction
b) not require any customization
c) and have the minimum number of parts on the parts list.

(recognizing that I can probably do something like modulate the
reference broadcast with a single bit from some existing part and do
CDMA in some form - but it was a thought..)

--
It's funny - folks have spent a lot of time over the years making
oscillators that are "on frequency" - but with modern processing
techniques, a lot of times it's more about "good phase noise close in"
and "a way to measure and estimate the frequency" (the latter requires
decent ADEV if you're not making the measurement simultaneously) but no
real aging requirement.  (Which is good in space applications, because
radiation causes frequency shifts.)

Take the NTIA 20Hz carrier frequency control requirement for SSB voice
radios in HF (to provide natural sounding voice without needing a
clarifier control). At 30MHz, that's <1ppm, and tough with a cheap
crystal. But if you have an integrated GPS receiver with 1pps, you
measure the current crystal frequency and adjust the DDS or PLL as
needed to compensate.

On 3/13/19 9:45 AM, Bob kb8tq wrote:

That's kind of what I thought.. I didn't really think that XOs (of any
kind) were sorted like resistors of yore, where a 10% resistor meant it
was at least 5% away from nominal.

And I can see that in the XO case, there's a lot to lot variability
that's probably comparable to (or at least, in addition to), the "within
lot" variability.  And the whole thing (aging, temperature, among and
between lot variability) rolls up to the overall 50ppm spec.

So if you were building a "batch" of systems, they'd likely be clumped
closer together than the "spec" range.

Yeah, so I think I can't depend on 100 units being sufficiently
different to be distinguishable, no matter how cheap they are.

Thanks to the time-nut hive-mind...

jim

Heterodyne them up x100 and they will be less crowded?

On 3/13/19 1:49 PM, jimlux wrote:

Hi Jim,

If you have a mother-ship and presuming that it is going to be within
"RF" range of all little ones, how about sending a "PILOT" tone that
comes from the mother-ship to phase lock all the little ones internal
oscillators ???

If there is a master oscillator of any note (i.e., say a Rb unit) in the
mother-ship, then a reasonably decent XO circuit may suffice or a TCXO.
As for ranging with the signal using phase locking, well I would have to
think about that for a bit.

This process was used back in the 1970's to lock the master frequency
source of a few repeaters together so that they could simultaneously
transmit on the same frequency with little to no interference.  In this
case, if I recall correctly, a 1000 Hz pilot tone was used as the sync
signal.

In your case you could, perhaps, transmit on some frequency that is out
of the way and use whatever pilot tone you wish.  Depending upon the
transmitter bandwidth maybe even transmit a 10 MHz sideband and do
direct comparison to the local (TC)XO.  Many possibilities !!!

Bill....WB6BNQ

jimlux wrote:

On Wed, Mar 13, 2019 at 9:02 AM jimlux jimlux@earthlink.net wrote:

If you are buying 1000, you probably will get the manufacturer to test to
meet your specs. At 100, you will likely get what you get from them. I am
buying VCTCXOs 50 at a time and they have quite a variation. Basically they
meet spec for temperature variation, but many just barely meet spec. They
typically are 1-2 ppm off before adjusting them on frequency and drift 1-2
ppm in the weeks after soldering too. After that, maybe 1 ppm per year.
This is only one example of one manufacturer in a custom frequency for me.
If you buy a standard frequency, you will likely get the benefit of better
manufacturing control they can afford for high volume parts. I am buying
parts specified at +/- .28 ppm over 0 to 70 degrees and they just meet
that, but the frequency is anywhere at any temperature. It is often not a
linear function.

I've also seen hysteresis, frequency jumps, etc, with only about 80% of the
parts acceptable to me. Those effects are not considered or tested by the
manufacturer at this low volume, so I have to do the temperature testing
and cull the unacceptable ones. You will probably have to do the same, age
and test yourself to get acceptable results.

Regards,

Mark

On 3/13/19 2:28 PM, wb6bnq wrote:

That's basically the idea. But you need multiple source locations at
multiple frequencies to solve for position, velocity, and local clock
offset.

The process is very much like GPS (or Transit) but, of course, there is
no GPS in deep space.