[time-nuts] syncronized clocks

Fri Sep 18 09:32:30 UTC 2015

> Le 18 sept. 2015 à 03:26, Can Altineller <altineller at gmail.com> a écrit :
> 
> 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.

I think the word calibration means to alter or adjust rather than to measure, though you have to measure to make meaningful adjustments.
So - NO , I don’t think long term GPS measurements are the right tool for calibration. They would certainly be useful in verifying any adjustments based on a more stable short term reference.

> But a Rubidium
> source has less jitter or phase noise, which can be useful in some cases.

Less than GPS 1PPS by maybe 3 orders of magnitude.

> 
> 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?

Hmmm. No and yes, maybe and sort of. It all depends on the objective. If you look at GPS 1PPS pulses they are just as likely to be +ve or -ve offset from 0. It is determined by the GPS’s local oscillator and called quantization error. Basically the GPS puts the pulse out on the nearest local clock tick. The better models give you the delta in a message which allows you to adjust for it by hardware or software.
So if you average the pulses over a long period the result will be a 0 offset, but this is not much use to you as the next tick (your reading) will still be +/- the quantization error. 

> 
> 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)

 YES. 

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

There will most like be a little, at least I have never managed to get 0,0.

> 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.

 Yes. This is often done by nuts among this lists contributors. 

> 
> 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

Very good stuff I believe. Though I have none my self.

> 
> 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.

One of the bugs of this affliction. A good interval counter will help . Look up PICTIC2 in the archives. Not expensive to put together with a resolution of 250ps IIRC. 

I have a couple of these - excellent and in spec. If I can find them, I could send you some measurements taken with a 53230.

> 
> 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.

Have fun. 

> 
> 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.
>>> 
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