Re: [time-nuts] syncronized clocks

[Can Altineller]

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  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 
> 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 

Re: [time-nuts] syncronized clocks

[Chris Caudle]

On Thu, September 17, 2015 8:26 pm, Can Altineller wrote:
> 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)

Any two sources which do not have a way to force synchronization to some
external reference at startup will always have a phase difference.
Did you really mean a phase difference (i.e. the oscillators did not start
oscillating at exactly the same time) or a change in the phase difference
over time (i.e. the oscillators are not at exactly the same frequency)?

The answer in both cases is yes, but the reasons are different.  You can
eliminate the starting phase difference by having a gate circuit in the
oscillators to allow holding the output off until a synchronizing signal
is received.  That will get them started together, but if the
synchronizing signal is a one time event then the two will drift apart
over time, either because of differences in the environment due to
influence of temperature, magnetic field, barometric pressure, etc. on the
oscillating mechanisms, or just imperfections in the mechanisms themselves
that prevent operating at exactly the theoretical absorption frequency of
rubidium.

To eliminate the drift in phase difference you have to discipline the
oscillator frequency with an external reference of some kind.  Of course
that is talking in absolutes, and there is probably a point where you no
longer care because the differences are very small.

The typical language used to describe the difference of one frequency
referenced to another is parts per million, meaning 1 Hz frequency error
of offset per MHz output, so 1Hz off for a 1MHz oscillator, or 10 Hz off
for a 10MHz oscillator, etc., sometimes written in scientific notation as
1 x 10^-6.
Similarly for parts per billion, 1 x 10^-9, and parts per trillion as
well, 1x10^-12, but it seems like the scientific notation is much more
common than the English language description when you get to those levels
(possibly due to differences in American and historical British definition
of billion and trillion, see Wikipedia references to long scale and short
scale).

-- 
Chris Caudle




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Re: [time-nuts] syncronized clocks

[Mike Cook]

> Le 18 sept. 2015 à 03:26, Can Altineller  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
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 

Re: [time-nuts] syncronized clocks

[Pete Stephenson]

On Fri, Sep 18, 2015 at 3:26 AM, Can Altineller  wrote:
> 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)

There's many DS3231 modules on DX. Which one did you order?

As for costs, DX sells a zillion of those modules and can buy the
components cheaply, likely from the local source in China. Comparing
single-unit pricing to bulk-quantity pricing is apples-to-oranges.

That said, the boards I see on DX *are* quite cheap. It's probably
good to be skeptical.

> 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.

Typically GPSDOs discipline a quartz oscillator as quartz usually has
better short-term stability than rubidium or GPS (at least out to a
few minutes). If GPS reception is lost for some reason, the GPSDO will
enter a "holdover" state where the quality of the local oscillator
determines the accuracy of the clock. If you expect holdovers to last
a long time or have very high precision requirements in holdover (such
as for telecom purposes), a rubidium oscillator will be useful.

A ovenized quartz oscillator can be much cheaper and provide quite
good performance -- the oscillator in the Trimble Thunderbolt is
specced to remain within +/- 1 microsecond after two hours in holdover
(within +/- 15 degrees C) and can perform even better with a more
stable environment and some judicious tuning.

<100ns performance has been possible for GPS receivers for a few
decades: the Motorola Oncore UT+ timing receiver ($20 on eBay) can
achieve this with ease. The PPS signal is only emitted on transitions
of the local clock, but the receiver sends data in the serial stream
so a system can correct for this (the "sawtooth correction"). In
general, though, the offset averages out. More modern receivers like
the NavSpark NS-T you mention below are similar, but have a smaller
error due to having a faster clock: "The programmable time pulse is
derived from 81.84MHz clock, it suffers from jitter due to
quantization or granularity of the clock."

> 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?

Sure. That's basically what GPSDOs do: the Thunderbolt, for example,
constantly monitors the frequency and phase offset of its ovenized
oscillator and steers the frequency of the oscillator to keep it in
sync with GPS. You can do it yourself with a PLL:
 or


> 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)

In theory, the two should be precisely in sync.

In practice, without external disciplining they will slowly drift
apart from each other. With many models you can make fine adjustments
so they drift apart more slowly, but they'll always drift a bit.

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

Since they receive external disciplining from GPS, GPS-disciplined
receivers should remain in sync so long as they have signal. There's
likely going to be some short-term jitter but that averages out.

For example, I connected the PPS outputs of a Thunderbolt, Oncore UT+,
Resolution-T, and Garmin GPS 18x LVC to my oscilloscope, with the
scope set to trigger on the Thunderbolt's PPS signal. The pulse of the
UT+ bounced around within a range of about 100 nanoseconds, the
Resolution-T bounced around within ~30 nanoseconds, and the GPS 18x
was within about 300 nanoseconds, all within spec.

> 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.

As for the external clock, it 

Re: [time-nuts] syncronized clocks

[timeok]

Dear CA,

In my document describe my Frequency and Time standard you can find some info 
about the difference and limit of the two solution.
GPSDO and Rubidium frequency standard can be alternative or complementary, 
depends on your goals.

see: 
http://www.timeok.it/wp/wp-content/uploads/2015/09/Timeok-Time-and-Frequency-House-Standard-v-2.11.pdf

Luciano



On Thu 17/09/15 02:00 , Can Altineller  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/ [1] 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-Disciplin
> ed-Clock-Antenna-power-/181810679481?hash=item2a54c2ceb9[2]
> or
> http://www.ebay.com/itm/Trimble-GPS-Receiver-GPSDO-10MHz-1PPS-GPS-Disciplin
> ed-Clock-with-rs232-port-/261997391557?hash=item3d0042eec5[3]
> 
> 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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> and follow the instructions there.
> 
> 
> 
> Links:
> --
> [1]
> http://webmail.timeok.it/parse.php?redirect=http://navspark.mybigcommerce.c
> om/navspark-mini-6pcs-pack/[2]
> http://webmail.timeok.it/parse.php?redirect=http://www.ebay.com/itm/Trimble
> -GPS-Receiver-GPSDO-10MHz-1PPS-GPS-Disciplined-Clock-Antenna-power-/1818106
> 79481%3Fhash%3Ditem2a54c2ceb9[3]
> http://webmail.timeok.it/parse.php?redirect=http://www.ebay.com/itm/Trimble
> -GPS-Receiver-GPSDO-10MHz-1PPS-GPS-Disciplined-Clock-with-rs232-port-/26199
> 7391557%3Fhash%3Ditem3d0042eec5[4]
> http://webmail.timeok.it/parse.php?redirect=https://www.febo.com/cgi-bin/ma
> ilman/listinfo/time-nuts
> 
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[time-nuts] syncronized clocks

[Mike Cook]

> Le 17 sept. 2015 à 02:00, Can Altineller  a écrit :
> 
> 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.

This is within the specs of the chip, so they may not be fakes. The data sheet 
indicates +/-2ppm. 

> 
> 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.

I have have Navsparc receivers and they are very good. +/- 10ns on the PPS of 
those chips should be good enough for most timing investigations. 

> 
> 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.

Hmm. Gets complicated.
The ones you give links to are GPS disciplined quartz oscillators You can have 
GPS disciplined rubidium oscillators too. 
A rubidium reference is a rubidium disciplined quartz oscillator. So a GPS 
disciplined rubidium reference. is a GPS disciplined rubidium disciplined 
quartz oscillator. 

Basically a rubidium clock will have better long term stability as the drift in 
the rubidium physics package is less than for a  quartz oscillator.
However for short ( <1s ) measurement intervals good pure quartz is better. 
There are rubidium clocks with very good quartz though for the best of both 
worlds.
Also, disciplining the oscillator in a rubidium ref. tends to introduce jitter 
in the output as you are dynamically modifying the oscillator frequency , so 
for short measurement periods pure quartz is generally better than rubidium. 
Discipling either with GPS 1PPS will increase  long term stability plus 
accuracy of GPS.

Check out tvb’s site leap second.com.  

> 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.

Remember that a man with two clocks doesn't know what the time is.

> It seems to
> me getting an exact measurement of time is really challenging and
> interesting as a hobby.
> 

How stable , ppm/ppb/ppt, do you want the clocks and how close do you want the 
« sync » to be? micro/nano/pico secs …? . Unfortunately, once you have the bug 
, the exponent quickly drift out of your bank balances ability to satisfy your 
desires.

If your 2, no 3, better 5,  clocks are your Navsparc models you can be pretty 
sure (baring firmware problems - they can and do happen ), that you have better 
than 2x10-8s accuracy with a very good long term stability , so measuring time 
intervals from a second  out to a couple of years or so at that resolution 
should be no sweat. How you label the intervals becomes problematic a bit for 
longer intervals if you want to reference UTC.
If you want your clocks to beat at less than 1 sec and still have the same long 
term stability, you will have to discipline a faster oscillator with  the GPS 
PPS, i.e. That can be straight quartz, or rubidium or something better. 

> Any ideas/help/recomendations appreciated
> 
> Best Regards,
> C.A.
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Re: [time-nuts] syncronized clocks

[Pete Stephenson]

On Thu, Sep 17, 2015 at 2:00 AM, Can Altineller  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.

Out of curiosity, what caused you to think the RTCs were fake?

> 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.

How stable are the oscillators on the microcontrollers? It's certainly
possible that the microcontroller oscillators differ a bit, which may
affect the results.

The specs for the DS3231 says it's accurate to within +/- 2.0ppm. If
my understanding is correct, 1-2 microseconds per second would be
within spec.

> 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.

That's not a surprise: most non-PPS GPS modules don't make any claims
as to the precision of the serial data stream other than it typically
is emitted after the start of the second described in the string. On
one unit I've seen the start time of each serial data burst differs by
several tens of milliseconds, compared to the PPS signal from a GPSDO.

> 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.

I can't comment on that particular receiver, but I've been quite
satisfied with my Trimble Thunderbolt. Unfortunately, the prices for
used Thunderbolts on eBay have increased over the last year or two, to
the point where it may be better to get a new Thunderbolt-E from
Trimble (about $650) with the original factory warranty and get the
antenna separately rather than pay $400 for a used Thunderbolt kit
(including antenna, power supply, etc.). GPS receivers are nice
because they are continuously adjusted to track GPS time and so
inherit GPS' long-term stability.

Used rubidium standards have also gone up in price recently and
there's a wide range in quality. Rubidium lamps also have a finite
(but relatively long) lifetime. GPS receivers typically last
indefinitely modulo electronics failures, though old receivers like
the original Thunderbolt may have issues with GPS week rollovers (this
generally doesn't affect the 1PPS or 10 MHz output, but may affect the
date/time reported over the serial stream).

As a personal choice, I'd prefer the GPS receiver option unless your
devices will be operating in areas where GPS signal will not be
available (e.g. underground, in a room with no view of the sky, etc.).
Your mileage may vary.

That said, before you buy anything, consider your objective: What
precision is necessary? What is your budget?

TCXOs are cheap and widely available as easy-to-integrate modules. 2
ppm drift over a year is only about a minute or so.

If you're within range of the WWVB (North America) or DCF77 (Europe)
longwave time signals, simple low-cost receivers exist and can keep
clocks synced so long as they can receive signal.

As the next step up, timing-grade GPS receivers like the Trimble
Resolution-T (used) or various uBlox modules (new) are available
relatively cheaply (<$50, typically) on eBay or from other vendors. If
all you need is a PPS source that's linked to a global standard, that
might be a good option. They typically interface well with TTL logic
circuits.

Integrated GPSDOs like the Thunderbolt are larger, use several watts
of power, but output both 1PPS and 10 MHz signals. This is useful if
you need a frequency standard in addition to 1PPS. However, they tend
to be intended for telecom use and so require a little bit of tweaking
to work 

Re: [time-nuts] syncronized clocks

[Tim Shoppa]

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 
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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[time-nuts] syncronized clocks

[Mark Sims]

A friend of mine is the clock chip guru at one of the major semiconductor 
manufacturers.   He said that the jitter in clock chip oscillators is rather 
horrendous.  Getting an oscillator to reliably start and run smoothly on 
nanowatts of power is the major design issue for clock chips.   Jitter takes a 
back seat to power consumption in the clock chip market.
   
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