On 10/31/17 1:47 PM, Bob kb8tq wrote:

Very much so - most of the TCXO curves I've seen tend to be "much"
better than the spec over the central part of the frequency range (which
makes sense, the underlying crystal is a cubic with temp, most likely)

Retrace and hysteresis might be your dominant uncertainty.
I've attached a typical TCXO data plot for your viewing pleasure..
(that's an expensive oscillator, because it's for space, but I don't
think space or not changes the underlying performance)

I'm stuck with a near ground level antenna site (~16" above grade?),
with half a sky view (thankfully to the SSE), less some low blocking
buildings with regular mutlipath, plus multipath bouncing off a taller
building to the SE that bounces sats from the NW at me from low over the
Bering Strait. The building I'm in is concrete with flat steel under
each floor from the construction method. As I write this I'm down to two
green sats in LH.

A number of times a day, it will drop to one sat, and there's a few
dropouts a day where it goes to none of sufficient signal. How many
times and for how long varies by the day. It's worse when it's wet out,
which it is right now. If I lower the signal strength threshold, then I
end up with tons of multipath signals.

If I can ever get a bios update to my NEO-M8T, then I'll have GAL in the
mix and should experience fewer dropouts, potentially none.

An RTC that +/- 3 PPM over 24 hours would be great for holdovers of one
to 20 minutes.

While I wrote this, LH was typically showing two or three green sats,
once up to five and once down to one. And I just hit a dropout... for a
minute and a half; the one remaining green sat went behind the corner of
the building's entrance canopy, then back out.

On 31/10/2017 10:30 PM, Bob kb8tq wrote:

Hi

For NTP levels of accuracy Glonas is quite fine. Combining that with GPS should
get you a pretty good “time source” even under your extreme conditions.

Bob

Hi

Unfortunately not all TCXO’s are created equal. It depends a bit on the
original intended use. I’d bet it also depends a bit on the original target
price. Perturbations  (frequency jumps) over temperature are one “feature”
that might be present. Hysteresis at half the temperature spec is another
“feature”.

Even within the same batch or same test run, some will be much better
than others. You stop the compensation process when they get “good enough”.
That will mean that a few are right at whatever the production target is and
others exceed the target by quite a bit.

While crystal curves are indeed cubic, there are higher order terms in the
curve. The “why” is something people get to write papers on.If you are trying
to compensate to tight specs, you will see all sorts of stuff. It is not at all uncommon
to see >9th order curves residual curves. Indeed some of that is from residuals
in the compensation circuit as well as from the crystal.

Why all this yack? A lot of people come from a background using OCXO’s. An
OCXO generally has a low order temperature characteristic. It also is rare to see
things like frequency perturbations in an OCXO. Moving from one to the other
can be a bit interesting.

Bob

On 11/1/17 6:01 AM, Bob kb8tq wrote:

Indeed - I was looking at algorithmically compensating some cheap TCXOs
and there's an amazing spread in the "details" of the curves - sure,
they all met the spec (several ppm, as I recall), but it was clear after
very little testing that there was no "one algorithm to fit them all"

As you say, good grist for a paper or thesis project.

That's why I wish they'd sell OCXOs, cheap, without the oven. Or maybe
look for regular XO (no TC).  Those might have a more "pure" (read lower
order) freq vs temp characteristic.

The problem I see with regular XO is that they tend to be designed to a
cost point and there might be more of the hysteresis and mechanical
effects - if you're not claiming better than 100ppm, then 50 ppm of
hysteresis isn't a problem.

A 1ppb OCXO is going to have to be a better mechanical design - so that
it can hit that 1ppb every time when you turn the oven on and go from
cold to hot.

Maybe this is just griping in general - why don't mass production
manufacturers make exactly the niche part I want to buy (that is of no
use to anyone else)for $3 each

I suppose if you were going to build little algorithmically compensated
modules, you'd bite the bullet and design a crystal oscillator and then
YOU get to choose what crystal in what mount etc.

When all is said and done, the production cost for a design that uses a
crystal in a can plus half a dozen discrete devices to make an
oscillator is probably not a lot different than the production cost for
a design using an oscillator in a can.  it's the "other stuff" in the
design that will add up.

Hi

The issue there is that the crystal is where the money is. The oven circuit is actually pretty far
down the list cost wise. Poke at this spec, poke at that spec and you have a $50 crystal (in volume).

It very much has a crystal that spent more time on the production line (somewhere) being processed than
it’s lower spec cousins. Time is money and that equipment isn’t cheap either. There then is the minor
issue of yield. Toss in a more expensive package while you are at it ….

If you are buying a few million of this or that a month, you most certainly can get people’s attention. Toss
in a willingness to pay a few dozen bucks a piece on top of that and you will get a lot of people’s attention.

Crystals are made and sold with characteristic data on them. The same is true of just about any type of
oscillator. Getting to the point that the data is useful takes a lot of engineering on both ends of the process.
There are a number of companies that have set up to do the characterization once the device is in the
end end product. To a great extent that gets done to speed up the engineering process ….

If the crystal is at some odd frequency or in an odd package … be careful. Experience counts in terms of making a good crystal.
Experience at 5.000000 MHZ in an HC-40 is unfortunately not the same as experience at 5.000005 MHz in the same package. I have a lot
of data on this ….

Bob

One can minimize temperature change which affects things like
regulated voltages, CMOS transition points for those converting
sine to TTL with gates, etc. in addition to the oscillators.

The November issue of Nuts and Volts Magazine has an article on an
Arduino based PID controller which might interest someone who wants
to experiment with reducing temperature effects by controlling
temperature.

Bob M

On 10/31/2017 8:42 PM, jimlux wrote:

I wish.

It's using GLO and GPS now, yet gets reception dropouts.
That's why I'm hoping to eventually get the firmware update that will
add GAL to the mix.

I had anticipated reception issues, which is why I went with the M8T for
its sensitivity, multi-constellation and it's a timing module so a good
PPS on a single sat - only to get surprised that my version didn't have
GAL enabled. But I didn't envision reception would be so bad that not
having GAL would be material.

I'm also too close to that tall building that is reflecting the sats
over the Bering Strait at me. It's a military computer site, which I
thought would be pretty tight on stray RF, but it has antennas. I asked
a friend who works there about my GPS issues and if RF from the site may
be influencing things. He hesitated, then said "'Yes'. That's all I can
say."
For first power up I had obtained an active antenna for
multi-constellation and a pre-filter that "provides protection from
near  frequency or strong harmonic interfering signals."

As it's just for NTP accuracy, I may be better off letting all the
multipath through than getting dropouts. I'm starting to think that part
of my problem is that I know the GPS is capable of getting better than I
need, so instead of working to get what I need, I want what it should be
capable of.

But I feel a better solution is a reliable holdover capability, which I
should have anyway for failsafe. So perhaps the reception dropouts are a
way to make me address holdover properly rather than limp in and get
surprised later.

Hence adding a 'precision' RTC to the Pi.

Michael

On 01/11/2017 8:45 AM, Bob kb8tq wrote:

Hi

Ok, local RF interference sounds like a significant part of the problem. I would
suggest that swapping antennas might make sense. Not all “super interference
rejecting” antennas are created equal.

Bob

Antenna is a Tallysman TW4722.

I'll try a different antenna later. If nothing else, right now I've got regular dropouts for failsafe testing...

Thanks,

Michael

On 01/11/2017 10:22 AM, Bob kb8tq wrote: