[time-nuts] syncronized clocks

Can Altineller altineller at gmail.com
Fri Sep 18 01:26:38 UTC 2015


Hello All,

Thank you for your illustrious answers. I suspected that the RTC's were
fake when someone alerted me on a forum, 43oh, when I was trying to make
them work with energia, and a TI microcontroller. A forum member stated:

"What does have me curious, is how cheap that is--since the DS3231 chip
itself in single qty's from most retailers is about the price of that whole
board.  You should do some tests on its accuracy to see if it's a real
DS3231 or some knockoff."

I had gotten them < 5$ a piece, and even the 1-2 ppm is within range, it
has other problems, like the battery is not recharging, and it has a diode
making it impossible to use with 5V (even though the chip itself  is 3.3V)

On the other matters, currently I am interested in a. keeping as close as
possible to UTC, within 1-100 nanoseconds is just fine, and making this as
cheap as possible. b. be able to measure different RTC's and observe
precision and accuracy. For example when I receive my NavSpark boards, be
able to measure and verify the datasheet.

So from what I understood from your replies is that if a GPS disciplined
clock is on for a long time with good reception, it can be used to
calibrate a Rubidium frequency source, thus more accurate. But a Rubidium
source has less jitter or phase noise, which can be useful in some cases.

I have the following questions:

a. If I measure the 1PPS from GPS, and take averages, can the average value
be used for calibration somehow. In other words, if I am able to measure
variances in a 1PPS pulse, and average them, can I get a more accurate
reading?

b. If we put two rubidium sources, and have them warm up and stabilize, and
then hook up a 2ch scope and observe the pattern, will there be a phase
difference between the 10mhz signals? (without the scope doing any syncing)
-

c. If we put two gps disciplined sources after warm up will there be a
phase difference.

d. I use micro controllers to measure micros and perform my calculations,
Is there a better way? Can I make my micro controller run with an external
clock? Since I am not building logic circuits to do the counting, I think
it is imperative that I have a good cpu clock.

I also have found the following products:

http://www.jackson-labs.com/index.php/products/firefly_iia
http://www.jackson-labs.com/index.php/products/lc_1x1
http://www.jackson-labs.com/index.php/products/lc_xo

I have no idea about the prices, although I sent them a request. These seem
realy nice, but I dont know if they will be match for a desktop gpsdo.

There is also:

http://navspark.mybigcommerce.com/ns-t-precision-timing-mode-gps-receiver/

It is really cheap, and it claims 6 nsec (1-sigma) timing accuracy, but yet
I would still need a more expensive gear to test it.

This seems like a nice hobby, and it has lead to learning of many things
for example those in embedded systems development and writing algorithms.
For example, now I can write algorithms that are more time conscious.

I also want to try arithmetically disciplining a RTC, such as the DS1340
(15ppm) which has some calibration registers that will add to the 32khz
signal.

Again thank you very much for all your answers.

Best Regards,
C.Altineller













On Thu, Sep 17, 2015 at 4:04 PM, Tim Shoppa <tshoppa at gmail.com> wrote:

> Wow, you're all over the place here :-)
>
> The "1-2 microsecond drift every second" you observe for the two RTC's
> relative to each other, is entirely consistent with the few ppm spec for
> that device. If you start tweaking the trim frequency you should be able to
> get it trimmed to better than a ppm at room temperature.
>
> The 10ms second-to-second variation you see in the serial GPS message, is
> entirely consistent with expected serial port jitter. If you want to use
> your current i2c module to trim the RTC frequency, 10ms serial message
> jitter will get you to 0.1ppm if you wait a day.
>
> It's likely but not guaranteed that somewhere in the i2c module you already
> have, there is a PPS signal, it just doesn't come out on a header (or isn't
> quite documented on a header).
>
> Tim N3QE
>
> On Wed, Sep 16, 2015 at 8:00 PM, Can Altineller <altineller at gmail.com>
> wrote:
>
> > Hello,
> >
> > I started putting a test setup together when I suspected the DS3231
> RTC's I
> > got from dx.com were fake. So I put 2 mcus, each measuring microseconds
> > from each 1PPS output with an interrupt, for the unit itself, and another
> > interrupt to measure 1PPS time from another unit and display them on a
> > nokia LCD.
> >
> > I have found out the RTC's differ by 1-2microseconds each second, on of
> > them was falling behind 1microseconds each second, relative to other one.
> >
> > Then I put a i2c gps module, (which does not have 1pps output
> > unfortunately) and parse out the time string, and generate a pulse for
> > another interrupt.
> >
> > I found out that time from gps (arriving time of timestamp) varies within
> > 10 milli seconds.
> >
> > So I already ordered some gps modules with 1PPS output,
> > http://navspark.mybigcommerce.com/navspark-mini-6pcs-pack/ and I am
> > thinking they should be good enough.
> >
> > Here is picture of my test setup:
> >
> >
> > ​
> > So here is the reason I am writing to the list: I am also out to buy a
> > rubidium frequency standard, or a trimble gps disciplined clock.
> >
> > Like:
> >
> >
> http://www.ebay.com/itm/Trimble-GPS-Receiver-GPSDO-10MHz-1PPS-GPS-Disciplined-Clock-Antenna-power-/181810679481?hash=item2a54c2ceb9
> > or
> >
> >
> http://www.ebay.com/itm/Trimble-GPS-Receiver-GPSDO-10MHz-1PPS-GPS-Disciplined-Clock-with-rs232-port-/261997391557?hash=item3d0042eec5
> >
> > I have read somewhere that these newer GPS disciplined clocks are much
> > better than an rubidium based atomic standard, like datum modules, or
> some
> > other modules we can get on ebay, since they are based on much more
> > advanced atomic clocks that are on board gps satellites, and correct its
> > oscillator continously.
> >
> > Is this true? What would be your recomendation? A GPS disciplined unit,
> or
> > a rubidium standard? What are the differences between them. For right
> now I
> > am interested in making two clocks beat (having the same 1pps output, in
> > sync) but later on I might be interested in other measurements. It seems
> to
> > me getting an exact measurement of time is really challenging and
> > interesting as a hobby.
> >
> > Any ideas/help/recomendations appreciated
> >
> > Best Regards,
> > C.A.
> >
> > _______________________________________________
> > time-nuts mailing list -- time-nuts at febo.com
> > To unsubscribe, go to
> > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> > and follow the instructions there.
> >
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>



More information about the Time-nuts_lists.febo.com mailing list