Chris
There is a low cost solution and I have the input circuit perfect for GPS
on a $1 gate array I have boards and am presently using Shera original
version.  Would like to buy his version 402NE but have not been able to get a
response from him. Have repeatedly asked for help on this list for some one to
step forward to write the uproc. program. No one. The total material cost
would  be less than $ 25 PCB included  GPS receiver OCXO or RB would be
extra. If  the FE 5680A with RS232 would be used cost is less than $ 15. There
are now  PIC's out there that can also do the timing function reducing cost
even more but  that will take more smarts.
Bert Kehren

In a message dated 12/4/2012 9:06:26 P.M. Eastern Standard Time,
albertson.chris@gmail.com writes:

With the  price of T-Bolts now higher, does it make sense to build your  own
GPSDO?

What is the simplest phase detecter that could  work?  I think only that,
and then a duouble oven crystal from eBay, a  GPS and and Arduido.

Yes the Aruino is expensive compared to a bare uP  chip but using one, I
thin you could build a GPSDO without a PCB and the  Arduino's USB connection
could be usful for power and  logging/control.

If ther phase detector where simple enough it could be  build on a prototype
board the fits on top of the Arduino.

There are  some other designs but because programming a uP and making a PCB
seem to be  rare skills that job tends to fall on one person.  Anyone can
program  an Arduino and with out need of a PCB the entire design could be
puted on a  web page and the replicated with common parts.

On Tue, Dec 4, 2012  at 4:01 PM, Bob Camp lists@rtty.us wrote:

to

start

can

--

Chris  Albertson
Redondo Beach,  California

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.

Now you can see the problem with designs that require both a PCB and a
programmed uP.  Most people can't do either of these and those who can
typically are good at only one.  Then you find someone and after he looses
interest the project is dead and un-suportable.

So I was thinking of how to build a GPSDO that does not need a programmed
uP and would be so simple that a PCB would not be needed.  It shoud be
simple enough that after getting the parts could be built quickly by anyone.

The Arduino has a USB interface and both ADC and DAC and digital IO.  I
read about the concern about using USB power.  The Arduino can also be
powered by a 9V battery so it will continue to run if the USB power goes
away.  Or you can use a power cube (aka "wall wort")  Anyone can program an
Arduino even if you know nothing about uP.  It is VERY easy and the
software runs on Mac OS X, Linux and even Windows.

I would use a separate power supply for the OCXO as they take more power
and this needs to be cleaner than I'd expect USB power to be.

The question I have again is about a simple phase detector.

On Wed, Dec 5, 2012 at 3:28 AM, EWKehren@aol.com wrote:

Chris Albertson
Redondo Beach, California

Chris,

If you want to understand how to approach the issue, you need to study the Shera
controller system.  It does exactly what you and others are discussing doing.  It
is relatively simple, straight forward and the HEX file is available to program
the CPU with.  The circuit board is all ready made and available.

The only hard part is the D/A which may be a bit of a problem with respect to the
original part.  However, even that may be available from another vender.  If not
there are some similar replacements, but that may require making new boards to
account for the parts being surface mount types.

Do yourself a favor and look at the following URL and download the reprint of the
QST article.

http://www.rt66.com/~shera/index_fs.htm

Bill....WB6BNQ

Chris Albertson wrote:

Chris:

I did ask if the arduino interrupt ports could be used as a phase
detector; one on the GPS and one on the OXCO. Too much jitter? If the 12
MHz clock is too slow, would an 80 MHz clock ARM arduino style processor
work? I'm simply too new at this to decide.

Don

--
"Neither the voice of authority nor the weight of reason and argument
are as significant as experiment, for thence comes quiet to the mind."
De Erroribus Medicorum, R. Bacon, 13th century.
"If you don't know what it is, don't poke it."
Ghost in the Shell

Dr. Don Latham AJ7LL
Six Mile Systems LLP
17850 Six Mile Road
POB 134
Huson, MT, 59846
VOX 406-626-4304
www.lightningforensics.com
www.sixmilesystems.com

Hi

Simple answer - no, not in a precision part. They are neither high enough resolution or deterministic enough to give you very high resolution.

More complex answer - you can do just about anything if you are willing to limit the best possible outcome. With the normal integration times you probably will be in the 1x10^-9 to 1x10^-10 noise (ADEV) range.

You would do better with an input capture port. They would be more deterministic, but they still have limited resolution. If they are driven by a clock multiplier, they likely will have a jitter component on their clock. Since low jitter is not a money spec in these low end parts, there can be some issues there.

There are many cheap / simple ways to to the counter. Five dollars will easily solve the problem with change left over to pay for breakfast coffee.

Next up on the expanded list is a way to align the pps signals. If you tune your OCXO by 0.1 ppm to align them, it will take you 100 days to get to the first stage of lock. If you use a CPLD to do that part, you can toss much of the counter in with it. You still have change from your five dollars.

Bob

On Dec 6, 2012, at 4:59 AM, Don Latham djl@montana.com wrote:

You'd have to seriously divide down the output from the 10MHz OCXO if you
were going to use it as an interrupt.  Maybe to divide by 10,000? and even
at the higher clock rate you'd still have poor resolution.

I image each interrupt handler would sample some internal counter and the
background task would look at the delta between the two and adjust the DAC
to drive the OCXO to close the difference.  The resolution would be
(maybe?) a "handful" of clock cycles.  Given enough time, say a 1000
second period it might wrk well enough.  I can't know without doing a more
detailed design

On Thu, Dec 6, 2012 at 1:59 AM, Don Latham djl@montana.com wrote:

--

Chris Albertson
Redondo Beach, California

Hi

The math is pretty straightforward.

Let's say the clock is 10 MHz, that's 100 ns.

Say a handful is 5 +/- 3 (2 to 8)

Your measure will bounce up and down by 6x100 ns = 600 ns.

Over a 100 second period that's going to be 6.0 x10^-9 bounce in the data.

If you run a 100 second loop as well, that's the noise in the loop (just from one source).

Six times faster clock, you get 1.0x10^-9. Step up the "handful" for the faster clock's pipeline and you may be back at the same place.

Take a bunch of readings (you only get one a second) and average - things get better by square root of the samples. That's IF you have enough jitter / dither in the system to smooth things out. If you have "lumpy" noise (as is often the case with interrupts) you may get very little gain from averaging.

With a 100 second loop, a TBolt is doing sub 1x10^-11, so you are at least 100X worse.

Bob

On Dec 6, 2012, at 5:57 PM, Chris Albertson albertson.chris@gmail.com wrote:

You can use the ATmega328 16 bit timer/counter in input capture mode
to count the number of 10 MHz OCXO cycles per pulse per second period
to a resolution of 100ns but there are some problems:

The ATmega328 16 bit timer/counter external clock is limited to 1/4 of
the CPU frequency with an asynchronous source so the 10 MHz OCXO would
need to be divided down which would further limit performance and
require an external divider.  Modifying the Aruino board to use the 10
MHz OCXO in place of the CPU clock solves that problem.

Then operating the counter/timer in input capture mode with the GPS
pulse per second signal connected to the input capture pin would allow
almost Shera like performance.  The timing resolution would be 2.4
times lower (and not asynchronous) limiting performance over short
time spans.

On Thu, 6 Dec 2012 14:57:19 -0800, Chris Albertson
albertson.chris@gmail.com wrote:

Hi

Again, the math is pretty simple.

A 16 bit capture running at a 1/4 clock is not going to get you very near a Shera. It's even further from the more modern "enhanced Shera" designs.

Bob

On Dec 6, 2012, at 6:59 PM, David davidwhess@gmail.com wrote:

Hi

The solution to the problem is well known in several forms. Cost is below $5 for pretty much all of them. No need to re-invent the wheel. The gotcha is that you can't do it 100% with internal CPU peripherals. You will need some glue.

Bob

On Dec 6, 2012, at 7:43 PM, Chris Albertson albertson.chris@gmail.com wrote:

It is not a 16 bit capture and it does not run at 1/4 the clock rate.

The Shera uses a 12 bit counter to capture the phase difference
between the OCXO frequency divided by 16 and GPS pulse per second
output to a resolution of about 42ns.

What I suggested effectively captures the same phase difference but
divides the OCXO by 65536 and only has a resolution of 100ns.  The
OCXO division is just conceptual though as it comes from the 16 bit
timer/counter overflow internal to the microcontroller so in practice,
the timer/counter is arbitrary length with an interrupt about every
6.5ms doing the housekeeping for the extra bits.  It directly measures
the pulse per second period to a resolution of 100ns using the OCXO as
a clock.  If a 20 MHz OCXO was used (limited by the maximum clock
frequency of the microcontroller), then the resolution would be 50ns.

That is not how I plan on designing my own GPSDO which has taken a
significant turn from what I posted about here some time ago but I
have stopped discussing that until I have some results to share.

On Thu, 6 Dec 2012 19:34:08 -0500, Bob Camp lists@rtty.us wrote:

There are lots of sampling ADCs which will support that type of
operation directly or you can easily design and build a sampling phase
detector but that all involves significant extra circuitry outside of
the microcontroller.

Take a look at the Racal Dana 1992 reference frequency multiplier
option (the schematic is on page 7-33 of the service manual) for an
example of a sampling phase detector used in a similar application.

On Thu, 6 Dec 2012 16:43:26 -0800, Chris Albertson
albertson.chris@gmail.com wrote:

Sorry.  The Shera counter is 16 bits and not 12 bits but that does not
change what I posted.

On Thu, 06 Dec 2012 19:17:48 -0600, David davidwhess@gmail.com
wrote:

Chris: yes, dividing would have to be done. Doesn't TVB have a simple
divider block?
You don't really have to "close" the difference, just maintain it?
Don L
Chris Albertson

--
"Neither the voice of authority nor the weight of reason and argument
are as significant as experiment, for thence comes quiet to the mind."
De Erroribus Medicorum, R. Bacon, 13th century.
"If you don't know what it is, don't poke it."
Ghost in the Shell

Dr. Don Latham AJ7LL
Six Mile Systems LLP
17850 Six Mile Road
POB 134
Huson, MT, 59846
VOX 406-626-4304
www.lightningforensics.com
www.sixmilesystems.com

Hi

The Shera counter is not running in the same fashion you would be running an input capture pin.

Bob

On Dec 6, 2012, at 8:45 PM, David davidwhess@gmail.com wrote:

Hi

Unless you really want to go crazy with measuring very long delays, you do indeed want to align the pps from your OCXO with the pps from your GPS.

Bob

On Dec 6, 2012, at 8:48 PM, "Don Latham" djl@montana.com wrote:

Hi

If all you want is a "something" locked to a GPS:

Take the pps from the GPS and hook it to an AD9548. You probably will need a 50 cent CPU to set up the registers.  No muss, no fuss,  nothing to invent or design.

Weather it does what you need to do is an entirely different question. Without a defined objective / need / performance goal this could go on for a couple hundred years…..

Bob

On Dec 6, 2012, at 7:43 PM, Chris Albertson albertson.chris@gmail.com wrote:

Good thought, Bob. AD9548 $27, eval board a whopping $250, get a
thunderbolt :-). The eval board has a lot of SMA's on it...
Don L
Bob Camp

--
"Neither the voice of authority nor the weight of reason and argument
are as significant as experiment, for thence comes quiet to the mind."
De Erroribus Medicorum, R. Bacon, 13th century.
"If you don't know what it is, don't poke it."
Ghost in the Shell

Dr. Don Latham AJ7LL
Six Mile Systems LLP
17850 Six Mile Road
POB 134
Huson, MT, 59846
VOX 406-626-4304
www.lightningforensics.com
www.sixmilesystems.com

Hi

In this case it's very much a "you get what you pay for" sort of thing. You are indeed comparing an hourglass to a cesium standard.

Bob

On Dec 7, 2012, at 2:20 AM, "Don Latham" djl@montana.com wrote: