Jeff,
I have been using a TrueTime XL-AK purchased on eBay a few years
ago. It has worked flawlessly for the last five years. It seems to
work well with most amplified antennas that I have tried. Supposedly
accurate to better than 10 -12 but I have nothing to compare it to. It
will supposedly control an external oscillator. Mine has five 10 mhz.
sine wave outputs as well as IRIG-B out.

I paid around $150 for mine, but it seems they are asking around

$500 these days. With patients they can be had for less.

Al, k9si

A GPSDO is not hard to make.  All you need is some way to compare the
phase of two signals, an XOR gate can do that.  Then a small $2
process moves the control voltage on the crystal.    I tried one to
build the simplest GPSDO that could still work.  Got the parts count
down to about four or five and the cost well under $10 plus the OXO
which was about $20.  The simplest dumb one I could make keeps about
e-10.  Not great but enough for many uses.  I compared to my
Thunderbolt and I could see the phase advance and retreat.  Just a
little most sophistication and I likely could do much better but my
goal was to prove to myself that a GPSDO could be build VERY simply
with cheap parts

On Fri, Mar 10, 2017 at 1:39 PM, al wolfe alw.k9si@gmail.com wrote:

--

Chris Albertson
Redondo Beach, California

On Fri, Mar 10, 2017 at 2:35 PM, Chris Albertson
albertson.chris@gmail.com wrote:

Hi Chris, that's good news that a GPSDO is that easy to make (at least
a basic one) as that is exactly my medium term plan ! The issue of
course is having something to test the newly built GPSDO against... I
got one of the rehoused Trimble UCCM-based GPSDOs off ebay a while ago
but haven't been super happy with it. It's quite a bit less sensitive
than more modern GPS receivers and it often struggles to get even 1
satellite with the indoor patch antenna. At one point both red alarm
LEDs came on and stayed on despite power cycles - I eventually fixed
that by taking it apart and finding and hitting a reset button on the
board. Currently although I can talk to the unit over serial and it
seems to respond, Lady Heather is not seeing any output from it.

Combined these things don't give me a great deal of confidence that
this unit will act as a stable master reference. I was wondering if a
second GPSDO like Russ linked to would work better (I have a ublox
LEA-6T GPS already which I plan to use as the basis of the homebuilt
GPSDO and it consistently sees many more satellites than the UCCM
with a similar indoor antenna)  or put the money to getting an outdoor
antenna mounted (don't feel happy drilling holes in the house myself)
by someone. Do 2 GPSDOs tell you much more or just that each is
different and you need a third to adjudicate ? (I can see a slippery
slope looming from here...)

Cheers,
Tim

Hi

Actually it’s much easier. Just put a DVM on the XOR once a week and
adjust your oscillator with a screwdriver. It saves lots of time and money.

Bob

Yes, that is a problem.  How to test a 10 MHz oscillator of any kind
(GPSDO or not) if you only have one oscillator.

I call it the "bootstrap problem"  and I think it's worth some
discussion.  The easy way is to buy a few known good clocks before you
set out to make a clock but that is the "easy way".  that

It is not hard to test its long term stability.  Simply count how may
cycles your new device makes in (say) 10 seconds.  Now all you need
is a way to measure 10 seconds.    GPS receivers are good at this.
For short term stability you have yo have some faith.  Buy a $25
oversized crystal oscillator that was made by the name brand with a
decent reputation and it will likely be reasonable

All the GPSDO does is adjust the electronic frequency control of that
$25 eBay oscillator until there are EXACTLY 10,000,000 counts between
every 1 Hz pulse.  The GPSDO is self testing.  Every second it
decides if that $25 crystal is running to fast or to slow and then
does something.  One of the things it should do it tell its owner if
the crystal is reusing much to fast or slow or if it is just right.
You will know the GPSDO has decent long term stability because the
green :just right" LED stays on all the time.

You can also verify the GPSDO is working correctly by looking at the
EFC voltage.  That is the control signal the controller sends to that
$25 eBay crystal.  You'd expect that the ESFC volts follows some kind
of theory and tracks the environment and you'd not expect a random

In short you can verify the correct operation of a home build GPSDO
because you can see inside of it.  When you see the inside parts are
working as expected you can have high confidence it is all working
well.    But for proof you need at least to more known-good
oscillators and you can measure beat frequencies between them.

On Tue, Mar 14, 2017 at 3:33 PM, Tim Lister listertim@gmail.com wrote:

--

Chris Albertson
Redondo Beach, California

On Tue, 14 Mar 2017 21:25:19 -0700
Chris Albertson albertson.chris@gmail.com wrote:

This depends very much on the oscillator used. If it's a good OCXO,
you will see hardly any influence from environment and the EFC will
be dominated by GPS noise at short time scales and aging of the OCXO
at long time scales. Have a look at what TvB reported in November[1,2]
on the measurement of a bunch of Thunderbolts(free running). Yes, there
is something that seems to be environmental effects for some oscillators
at some time scales. Figuring out where you actually see such effects
will need some serious statistical skills. IMHO it's much easier to just
get a known-good GPSDO and a rubidium for verification.

		Attila Kinali

[1] https://www.febo.com/pipermail/time-nuts/2016-November/101913.html
[2] https://www.febo.com/pipermail/time-nuts/2016-November/101939.html

--
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

On Wed, Mar 15, 2017 at 1:09 AM, Attila Kinali attila@kinali.ch wrote:

Yes, for sure it is easiest to buy something that already works.
But the question was how to bootstrap from scratch?  In other words,
if the only oscillator you own is one you built, how would you know
that it is working?

I actually did use your method.  I have a Rb and Thunderbolt, a pair
of freq. counters and so on.    But still I wanted to see if I could
build from scratch and verify proper operation and keep the budget
under say $50 for everything from antenna to power cord.    I think
it can be done but one can only verify longer term stability.

(Yes you were correct a GOOD oversized XO is not sensitive to the
environment.  But notice the above budget.)

--

Chris Albertson
Redondo Beach, California

On Thu, 16 Mar 2017 00:06:49 -0700
Chris Albertson albertson.chris@gmail.com wrote:

Hmm.. doing verification of self-built (or aquired) equipment from
scratch is a different game altogether. If you want to build things
yourself, you first have to form a model of what disturbs your system,
measure these parameters and verify the model against your measurements.
Then you start building systems that exhibit different distortions,
model these, measure and verify them. After you have built enough
systems with different environmental characteristics, you verify
them against eachother to make sure that your models faithully
model reality and contain all parameters up to the error bound of the model.

After a couple of decades of building and verification, you can be
reasonably sure your GPSDO works correctly ;-)

50$ for a complete GPSDO, batteries included, is quite a low price limit.
A decent OCXO already costs 20-30$ on ebay. 100-200$ is a more realistic
limit for a homebrew GPSDO with time-nuts like performance.
A non-ovenized XO will not come close to time-nuts standards ;-)

			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

I would say that the price for a home-built time-nuts quality GPSDO is going to be significantly greater than $200.  Yes, if you are making them for sale, eventually the unit cost will get down there.  But to build one single good GPSDO, you're going to throw away a lot of prototypes on the way to the one that works well enough to stand up to the scrutiny of this group.  The costs of those prototypes add up, and then there's your time engineering, building and testing.  It adds up to a lot.

Bob

  From: Attila Kinali <attila@kinali.ch>

To: Discussion of precise time and frequency measurement time-nuts@febo.com
Sent: Monday, March 20, 2017 2:01 PM
Subject: [time-nuts] time-nuts equipment verification from scratch (was: WTB: GPSDO)

On Thu, 16 Mar 2017 00:06:49 -0700
Chris Albertson albertson.chris@gmail.com wrote:

Hmm.. doing verification of self-built (or aquired) equipment from
scratch is a different game altogether. If you want to build things
yourself, you first have to form a model of what disturbs your system,
measure these parameters and verify the model against your measurements.
Then you start building systems that exhibit different distortions,
model these, measure and verify them. After you have built enough
systems with different environmental characteristics, you verify
them against eachother to make sure that your models faithully
model reality and contain all parameters up to the error bound of the model.

After a couple of decades of building and verification, you can be
reasonably sure your GPSDO works correctly ;-)

50$ for a complete GPSDO, batteries included, is quite a low price limit.
A decent OCXO already costs 20-30$ on ebay. 100-200$ is a more realistic
limit for a homebrew GPSDO with time-nuts like performance.
A non-ovenized XO will not come close to time-nuts standards ;-)

                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

time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Attilla,

You are talking about product design, development and optimization, not the
production of a one-off for home use. Since performance standards are
already well established, it is only necessary for the developer to test
the bench built instrument against published standards and determine if
performance is good enough to suit him. Given a sound understanding of the
role of various components in the system, it will be a great deal faster
and easier for the builder to tinker with the one-off then to go through an
extensive process of model development and verification.

I have spent a good deal of my career doing performance modeling,
verification, and validation in collaboration with other scientists and
also engineers. You describe the process correctly but I think it is
generous overkill for the topic under discussion here. Or have I missed
something in the discussion? Is the desired end result a device for
manufacture and sale? If so, then your approach is right on target.

Bill

On Monday, March 20, 2017, Attila Kinali <attila@kinali.ch
javascript:_e(%7B%7D,'cvml','attila@kinali.ch');> wrote:

--
William H Fite, PhD
Independent Consultant
Statistical Analysis & Research Methods