> Le 17 sept. 2015 à 02:00, Can Altineller <altinel...@gmail.com> 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. > <unnamed.jpg>_______________________________________________ > 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. "The main function of a modern police force is filling in forms." _______________________________________________ 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.
