Re: [time-nuts] Z3816 Oscillator Saga
On 05/01/14 08:51, Peter Schmelcher wrote: Chris, This hi res pic might help you. I modified my Z3816A to improve the performance. I changed the grounding and added a few extra capacitors. This pic is close to the start so very few changes. -Peter Peter, Thanks very much for sending that. The damage is in fact to the underside of the board, not the top side. I was using a solder sucker to remove the solder around each oscillator pin, which was then free enough of solder to use pliers to wiggle each lead side to side to free up. Unfortunately, the pliers slipped and sheared off ~3 parts from the underside of the board. It's not clear if all the parts are resistors or caps, though one is a cap for sure. It's too much to expect you to strip your board out just to send a pic, but if you ever have to do that, a jpg would be appreciated. It's really difficult to find info on these units, and haven't been able to find a schematic. It's not clear what all the pins of the header are for, though some may be to flash the cpu with firmware. There may also be added factory commands for things like replacing the oscillator, though one would expect that to be an auto function within the firmware. The fact that the efc always returns to the most recent value at power up and not do a sweep through the whole range might suggest this is not the case. Anyway, powered up the original 260-0558 oscillator on the bench today, with a helipot of the efc input and get the following: EFC Frequency, MHz -- 0v 5.000 003 54 1v 5,000 002 59 2v 5.000 001 57 3v 5.000 000 55 4v 4.999 999 53 5v 4.999 998 51 6v 4.999 997 35 Voltage for exactly 5Mhz output = 3.5413v So the oscillator looks good and the fact that the Z3816 doesn't lock with that in place suggests that some of those damaged parts are needed :-(. Have another of Ebay fluke seller's MTI 260-0664 ordered. The original is in the Z3816 and don't want to take it out again until i'm sure that it's necessary. At ~$18 each, it good to have a spare and will characterise that on arrival to see what the differences are between that and the 260-0558. Knowing my luck, probably find the efc is the opposite sign, or negative, but time will tell. Have posted this to the list as well, in case the info is useful... Regards, Chris ___ 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.
Re: [time-nuts] Z3816 Oscillator Saga
Hi Chris I've got some results to hand for three samples of the fluke.l MTI-260-0624-C oscillator, if that's the one you mean?, and for 5 MHz output they all require an EFC input on pin 1 between 2.8 and 3.1 Volts. That's with a 12 Volt supply on pin 5, which gives approx 6 Volts out on the Ref Volt Out pin, pin 2. So it would seem, in this respect at least, that they are quite close to your original oscillator, although I don't see any figures for EFC sensitivity in those particular results, but I wonder if there's any possibility of an initial adjustment in the Z3816, either hardware or software, that might limit the EFC range to more closely match individual oscillators? Regards Nigel GM8PZR In a message dated 02/05/2014 13:31:41 GMT Daylight Time, syseng.greenfi...@btconnect.com writes: On 05/01/14 08:51, Peter Schmelcher wrote: Chris, This hi res pic might help you. I modified my Z3816A to improve the performance. I changed the grounding and added a few extra capacitors. This pic is close to the start so very few changes. -Peter Peter, Thanks very much for sending that. The damage is in fact to the underside of the board, not the top side. I was using a solder sucker to remove the solder around each oscillator pin, which was then free enough of solder to use pliers to wiggle each lead side to side to free up. Unfortunately, the pliers slipped and sheared off ~3 parts from the underside of the board. It's not clear if all the parts are resistors or caps, though one is a cap for sure. It's too much to expect you to strip your board out just to send a pic, but if you ever have to do that, a jpg would be appreciated. It's really difficult to find info on these units, and haven't been able to find a schematic. It's not clear what all the pins of the header are for, though some may be to flash the cpu with firmware. There may also be added factory commands for things like replacing the oscillator, though one would expect that to be an auto function within the firmware. The fact that the efc always returns to the most recent value at power up and not do a sweep through the whole range might suggest this is not the case. Anyway, powered up the original 260-0558 oscillator on the bench today, with a helipot of the efc input and get the following: EFC Frequency, MHz -- 0v5.000 003 54 1v5,000 002 59 2v5.000 001 57 3v 5.000 000 55 4v4.999 999 53 5v4.999 998 51 6v4.999 997 35 Voltage for exactly 5Mhz output = 3.5413v So the oscillator looks good and the fact that the Z3816 doesn't lock with that in place suggests that some of those damaged parts are needed :-(. Have another of Ebay fluke seller's MTI 260-0664 ordered. The original is in the Z3816 and don't want to take it out again until i'm sure that it's necessary. At ~$18 each, it good to have a spare and will characterise that on arrival to see what the differences are between that and the 260-0558. Knowing my luck, probably find the efc is the opposite sign, or negative, but time will tell. Have posted this to the list as well, in case the info is useful... Regards, Chris ___ 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. ___ 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.
Re: [time-nuts] Z3816 Oscillator Saga
Chris, The Z3816 has a few design short comings. In the circuit board pic the MTI oscillator circuit board grounding is improved. Subsequently I opened and modified the oven internally and added an extra oven feedthrough pin. The issue is the oven heater current modulates the ground pin voltage and that effects the EFC value (poor layout). I finally connected the oven power pins to a separate floating PSU with the new internal oven EFC signal ground pin the only common connection point, while hunting for the source of a short transient EFC event. I would caution you that the circuit board traces and hole plating are very thin, and both are easily damaged by external forces. I used multiple irons for un soldering problem pins. The two tantalum capacitors with the black marker dots are not original. These two capacitors had the wrong voltage rating and became noisy. These might also be the source of your problem. It can cause unexpected jumps in the reported EFC value and are not an oscillator problem, just a dumb assembly mistake. FYI at the turn of the century cell phone sales had a surge and tantalum capacitors became impossible to get at 10x the price. I replaced the Z3816 oscillator temporarily with no problems but observed at power up the smart clock algorithm used a much higher gain in the EFC loop for about an hour then drops to a more normal value. I also adjusted the oven temperature turning point pot, however, I suspect the MTI internal oven circuitry self calibrates the crystal turning point. I never figured out how to stop the oven from self calibration and had intended to fix the oven circuit before I determined it was a feature. The pot adjustment seemed to have more effect at power up. As I recall, during the relatively short calibration event, the frequency changes by 0.008Hz with a maximum gps phase error of about 400ns. I had wondered if the two algorithms were interacting badly but I never verified this. Even with this quirk, the 260 is a great oscillator. -Peter ___ 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.
[time-nuts] Small IRIG display wanted
Fellow time-techies, I'm on the prowl for a small panel-mount IRIG-B display, similar to the Datum 9520 series (they made one for aircraft use, which just happens to be the one I'm after). The only critical things about it are: --Needs to be able to run directly off a 12 or 24VDC source. --Maximum display width would be 7 inches. Height is not a big issue. If you have something laying around which you think might work, please contact me off-list. Thanks much. Bruce Lane, ARS KC7GR kyr...@bluefeathertech.com The Road to Bell is Paved with Good Inventions... ___ 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.
[time-nuts] Low cost GPS module for 100ns timestamping error
Hi, I'm new here so please be gentle! I'm considering designing and building some dataloggers, probably ARM Cortex based (eg. STM32F4xx), which record the time of infrequent events, preferably to better than 100ns and if possible better than 50nS. The data loggers will be continuously powered, in fixed locations and should have reasonably good views of the sky so the use of a low cost GPS module should be feasible. I believe it shouldn't be too difficult to resolve the PPS timing to +/- 5ns or better with a 100MHz+ microcontroller clock, but obviously jitter would add to the error requiring the GPS to be better than perhaps 90ns or so worst case. Inevitably cost and power constraints apply - ideally the GPS would cost less than $20 (in quantities of 100), and $15 would be good, but it doesn't seem easy to find very lost cost receivers with timing outputs that are properly specified, presumably because of the relative market volumes. The power consumption of most timing receivers also seem to be higher than navigation units - eg. the Trimble SMT-x spec is 100mA compared to the ADAfruit MTK3339-based module which draws 20mA (but they are a bit too expensive at $24 apiece). There are several cheap modules that have PPS outputs but no accuracy specification; it's possible that these could be used with sufficient averaging/filtering of the PPS output. Actually repeatability is the important requirement rather than accuracy as they could be calibrated. Perhaps even a PPS o/p is not absolutely necessary - could the NEMA output timing be used given enough averaging and a sufficiently stable oscillator? Compromising the timing accuracy requirement a bit to say 150ns may be acceptable if the GPS device is cheap enough. I understand that the PPS outputs of some cheap modules sometimes become ill-behaved, but in this application the time stamp can be adjusted (or anomalous clocks ignored) post-event if necessary to correct for temporary disturbances. This also raises questions about the short term stability of the microcontroller oscillator required to maintain sufficient accuracy when GPS timing is temporarily lost for some reason - but how long would that need to be? 30s? 5 minutes? 30 minutes? An OCXO or a Stratum-3 TXCO would be too expensive, but oscillator manufacturers don't seem to publish short term frequency stability specifications for low cost/low power oscillators, and finding such information isn't easy. Can anyone point to figures for a typical non-TXCO low cost oscillator, 10 or 16MHz? I did find this study, http://tf.nist.gov/general/pdf/2276.pdf, measuring the stability of some low cost quartz wristwatches which gives some interesting data of 20 to 65ppb Allan deviation over 100s. That, but a 32kHz oscillator might give rise to jitter problems when multiplied up to a suitable frequency. Some oscillator datasheets specify Allan deviation values, but I guess what I need for estimating worst case timestamp error during holdover periods are actually MTIE values. Is there any way to estimate the latter from Allan deviations specs? Would an ADev of 65 x 10^-9 over 100s imply up to 6.5us of error after 100s? Any thoughts? Thanks, Tony H ___ 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.
Re: [time-nuts] Low cost GPS module for 100ns timestamping error
tn...@toneh.demon.co.uk said: Can anyone point to figures for a typical non-TXCO low cost oscillator, 10 or 16MHz? In general, low cost oscillators make pretty good thermometers. I think you have a much better chance of getting good results if you are willing to post-process the data. I suggest you get one of the GPS units you are considering and collect some data. That will answer many of your questions and probably raise a few more. -- These are my opinions. I hate spam. ___ 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.
Re: [time-nuts] Low cost GPS module for 100ns timestamping error
Welcome to the nuts Tony You are not specifying exactly how accurate time has to be but in my book and based on tests the most reasonable priced GPS with 1 pps is a Ublox 6M that you can get with antenna for less than $ 22 antenna included from _www.DX.com_ (http://www.DX.com) . They have volume discount. Shipping is very slow but included. They seem to be presently out of the 1 pps version but all ublox units have a 1 pps output and I use with and without and all I do is solder a wire to pin 3. Bert Kehren In a message dated 5/2/2014 7:02:57 P.M. Eastern Daylight Time, tn...@toneh.demon.co.uk writes: Hi, I'm new here so please be gentle! I'm considering designing and building some dataloggers, probably ARM Cortex based (eg. STM32F4xx), which record the time of infrequent events, preferably to better than 100ns and if possible better than 50nS. The data loggers will be continuously powered, in fixed locations and should have reasonably good views of the sky so the use of a low cost GPS module should be feasible. I believe it shouldn't be too difficult to resolve the PPS timing to +/- 5ns or better with a 100MHz+ microcontroller clock, but obviously jitter would add to the error requiring the GPS to be better than perhaps 90ns or so worst case. Inevitably cost and power constraints apply - ideally the GPS would cost less than $20 (in quantities of 100), and $15 would be good, but it doesn't seem easy to find very lost cost receivers with timing outputs that are properly specified, presumably because of the relative market volumes. The power consumption of most timing receivers also seem to be higher than navigation units - eg. the Trimble SMT-x spec is 100mA compared to the ADAfruit MTK3339-based module which draws 20mA (but they are a bit too expensive at $24 apiece). There are several cheap modules that have PPS outputs but no accuracy specification; it's possible that these could be used with sufficient averaging/filtering of the PPS output. Actually repeatability is the important requirement rather than accuracy as they could be calibrated. Perhaps even a PPS o/p is not absolutely necessary - could the NEMA output timing be used given enough averaging and a sufficiently stable oscillator? Compromising the timing accuracy requirement a bit to say 150ns may be acceptable if the GPS device is cheap enough. I understand that the PPS outputs of some cheap modules sometimes become ill-behaved, but in this application the time stamp can be adjusted (or anomalous clocks ignored) post-event if necessary to correct for temporary disturbances. This also raises questions about the short term stability of the microcontroller oscillator required to maintain sufficient accuracy when GPS timing is temporarily lost for some reason - but how long would that need to be? 30s? 5 minutes? 30 minutes? An OCXO or a Stratum-3 TXCO would be too expensive, but oscillator manufacturers don't seem to publish short term frequency stability specifications for low cost/low power oscillators, and finding such information isn't easy. Can anyone point to figures for a typical non-TXCO low cost oscillator, 10 or 16MHz? I did find this study, http://tf.nist.gov/general/pdf/2276.pdf, measuring the stability of some low cost quartz wristwatches which gives some interesting data of 20 to 65ppb Allan deviation over 100s. That, but a 32kHz oscillator might give rise to jitter problems when multiplied up to a suitable frequency. Some oscillator datasheets specify Allan deviation values, but I guess what I need for estimating worst case timestamp error during holdover periods are actually MTIE values. Is there any way to estimate the latter from Allan deviations specs? Would an ADev of 65 x 10^-9 over 100s imply up to 6.5us of error after 100s? Any thoughts? Thanks, Tony H ___ 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. ___ 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.
Re: [time-nuts] Low cost GPS module for 100ns timestamping error
Welcome! Take a look at NavSpark from SkyTraq (http://www.skytraq.com.tw/). They had an Indiegogo (https://www.indiegogo.com/projects/navspark-arduino-compatible-with-gps-gnss-receiver) campaign recently and should deliver real soon now. The NavSpark chip has an trigger pin for time capture, a feature suggested by a fellow time-nut and a 100 MHz clock. Edésio On Fri, May 02, 2014 at 07:59:14PM -0400, ewkeh...@aol.com wrote: Welcome to the nuts Tony You are not specifying exactly how accurate time has to be but in my book and based on tests the most reasonable priced GPS with 1 pps is a Ublox 6M that you can get with antenna for less than $ 22 antenna included from _www.DX.com_ (http://www.DX.com) . They have volume discount. Shipping is very slow but included. They seem to be presently out of the 1 pps version but all ublox units have a 1 pps output and I use with and without and all I do is solder a wire to pin 3. Bert Kehren In a message dated 5/2/2014 7:02:57 P.M. Eastern Daylight Time, tn...@toneh.demon.co.uk writes: Hi, I'm new here so please be gentle! I'm considering designing and building some dataloggers, probably ARM Cortex based (eg. STM32F4xx), which record the time of infrequent events, preferably to better than 100ns and if possible better than 50nS. The data loggers will be continuously powered, in fixed locations and should have reasonably good views of the sky so the use of a low cost GPS module should be feasible. I believe it shouldn't be too difficult to resolve the PPS timing to +/- 5ns or better with a 100MHz+ microcontroller clock, but obviously jitter would add to the error requiring the GPS to be better than perhaps 90ns or so worst case. Inevitably cost and power constraints apply - ideally the GPS would cost less than $20 (in quantities of 100), and $15 would be good, but it doesn't seem easy to find very lost cost receivers with timing outputs that are properly specified, presumably because of the relative market volumes. The power consumption of most timing receivers also seem to be higher than navigation units - eg. the Trimble SMT-x spec is 100mA compared to the ADAfruit MTK3339-based module which draws 20mA (but they are a bit too expensive at $24 apiece). There are several cheap modules that have PPS outputs but no accuracy specification; it's possible that these could be used with sufficient averaging/filtering of the PPS output. Actually repeatability is the important requirement rather than accuracy as they could be calibrated. Perhaps even a PPS o/p is not absolutely necessary - could the NEMA output timing be used given enough averaging and a sufficiently stable oscillator? Compromising the timing accuracy requirement a bit to say 150ns may be acceptable if the GPS device is cheap enough. I understand that the PPS outputs of some cheap modules sometimes become ill-behaved, but in this application the time stamp can be adjusted (or anomalous clocks ignored) post-event if necessary to correct for temporary disturbances. This also raises questions about the short term stability of the microcontroller oscillator required to maintain sufficient accuracy when GPS timing is temporarily lost for some reason - but how long would that need to be? 30s? 5 minutes? 30 minutes? An OCXO or a Stratum-3 TXCO would be too expensive, but oscillator manufacturers don't seem to publish short term frequency stability specifications for low cost/low power oscillators, and finding such information isn't easy. Can anyone point to figures for a typical non-TXCO low cost oscillator, 10 or 16MHz? I did find this study, http://tf.nist.gov/general/pdf/2276.pdf, measuring the stability of some low cost quartz wristwatches which gives some interesting data of 20 to 65ppb Allan deviation over 100s. That, but a 32kHz oscillator might give rise to jitter problems when multiplied up to a suitable frequency. Some oscillator datasheets specify Allan deviation values, but I guess what I need for estimating worst case timestamp error during holdover periods are actually MTIE values. Is there any way to estimate the latter from Allan deviations specs? Would an ADev of 65 x 10^-9 over 100s imply up to 6.5us of error after 100s? Any thoughts? Thanks, Tony H ___ 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. ___ 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. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to
Re: [time-nuts] Low cost GPS module for 100ns timestamping error
On 03/05/2014 00:48, Hal Murray wrote: tn...@toneh.demon.co.uk said: Can anyone point to figures for a typical non-TXCO low cost oscillator, 10 or 16MHz? In general, low cost oscillators make pretty good thermometers. True, but it's short term stability that matters here - over 10s of seconds the temperature shouldn't change much - especially if a bit of insulation is used around the oscillator. I think you have a much better chance of getting good results if you are willing to post-process the data. I'm not sure I understand this - I need to record the time that an event occurs so I need an accurate time reference. What can I store to post-process? The time reference drift and jitter relative to the local oscillator before and after the event? I suggest you get one of the GPS units you are considering and collect some data. That will answer many of your questions and probably raise a few more. Well yes, but that will cost time and money - I was hoping to get some recommendations/suggestions from people who have some experience with low cost GPS modules, so as to narrow the field down a bit before experimenting myself. Thanks, Tony ___ 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.
Re: [time-nuts] Low cost GPS module for 100ns timestamping error
On 03/05/2014 00:59, ewkeh...@aol.com wrote: Welcome to the nuts Tony Thanks, Bert. You are not specifying exactly how accurate time has to be but in my book and based on tests the most reasonable priced GPS with 1 pps is a Ublox 6M that you can get with antenna for less than $ 22 antenna included from _www.DX.com_ (http://www.DX.com) . They have volume discount. Shipping is very slow but included. They seem to be presently out of the 1 pps version but all ublox units have a 1 pps output and I use with and without and all I do is solder a wire to pin 3. Bert Kehren As I said in my first post I'd like to achieve an accuracy of better than 100ns - or 50ns if possible at reasonable cost. I had come across the Ublox 6M when I was looking earlier, but I misunderstood the data sheet and thought it was only the expensive ($135) LEA/NEO-6T versions which provided timing. Definitely worth a closer look - the NEO-6M is specced at 30ns RMS which is good enough. The power consumption is a little higher than I would have liked at 37mA/3V, but still rather less than others. Thanks, Tony ___ 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.
[time-nuts] Low cost GPS module for 100ns timestamping error
The Neo-6M based module (Crius CN-06) is available from HobbyKing for $20 (sometimes on sale for $16). You do have to add the wire to access the 1PPS signal. In my testing, I prefer it over the Adafruit Ultimate GPS. The Neo-6M seemed to a a little more sensitive (could get reliable lock and tracking sitting on the floor of a 2 story stucco house (stucco over wire mesh) away from any windows and did not have issues with reporting fixes of a rather stationary antenna - the Adafruit module would fake a fixed position if it was not moving). I did a little testing of the 1PPS output and it seemed to be quite good. My application was mainly driven by the desire to calculate the range and bearing between two units spaced 100 feet to a couple of miles (using an Xbee radio link). I could move a unit in a circle a couple of feet radius around another one and get good range/bearing info... not too shabby for a $20 hockey puck. ___ 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.
Re: [time-nuts] Low cost GPS module for 100ns timestamping error
here is what I'd do Get a decent OCXO (ovenized crystal oscillator and control it with your GPS. Don't worry if the GPS's PPS is 50ns or 5ns. You are going to be averaging these for a while.Basically you build a GPSDO.These have become very easy to make. I have one I made for about $8 and it has not gained or lost 100ns in weeks. Next get a second uP, not the one controlling the GPSDO. let your GPSDO 10MHz output drive this uP's counter and have the thing your are timing connect to the pin that will capture the counter. This is done in HARDWARE.The pin can cause the hardware counter to be captured to a register. So the software need not be real time The counter is ALSO captured by the 1 PPS from the gps.This way you capture both the one second tick and your event. You log the number of counts past the second. You can use ARM processors but I'm doing this with an Arduino cone I got on eBay for under $4. You do not need much CPU power as all the real time stuff is done in the uP's peripheral hardware. The uP only has to send the data over a link or log it so an SD memory card. even the 8-bit AVR is faster than I need for that. On Fri, May 2, 2014 at 3:54 PM, Tony tn...@toneh.demon.co.uk wrote: Hi, I'm new here so please be gentle! I'm considering designing and building some dataloggers, probably ARM Cortex based (eg. STM32F4xx), which record the time of infrequent events, preferably to better than 100ns and if possible better than 50nS. The data loggers will be continuously powered, in fixed locations and should have reasonably good views of the sky so the use of a low cost GPS module should be feasible. I believe it shouldn't be too difficult to resolve the PPS timing to +/- 5ns or better with a 100MHz+ microcontroller clock, but obviously jitter would add to the error requiring the GPS to be better than perhaps 90ns or so worst case. Inevitably cost and power constraints apply - ideally the GPS would cost less than $20 (in quantities of 100), and $15 would be good, but it doesn't seem easy to find very lost cost receivers with timing outputs that are properly specified, presumably because of the relative market volumes. The power consumption of most timing receivers also seem to be higher than navigation units - eg. the Trimble SMT-x spec is 100mA compared to the ADAfruit MTK3339-based module which draws 20mA (but they are a bit too expensive at $24 apiece). There are several cheap modules that have PPS outputs but no accuracy specification; it's possible that these could be used with sufficient averaging/filtering of the PPS output. Actually repeatability is the important requirement rather than accuracy as they could be calibrated. Perhaps even a PPS o/p is not absolutely necessary - could the NEMA output timing be used given enough averaging and a sufficiently stable oscillator? Compromising the timing accuracy requirement a bit to say 150ns may be acceptable if the GPS device is cheap enough. I understand that the PPS outputs of some cheap modules sometimes become ill-behaved, but in this application the time stamp can be adjusted (or anomalous clocks ignored) post-event if necessary to correct for temporary disturbances. This also raises questions about the short term stability of the microcontroller oscillator required to maintain sufficient accuracy when GPS timing is temporarily lost for some reason - but how long would that need to be? 30s? 5 minutes? 30 minutes? An OCXO or a Stratum-3 TXCO would be too expensive, but oscillator manufacturers don't seem to publish short term frequency stability specifications for low cost/low power oscillators, and finding such information isn't easy. Can anyone point to figures for a typical non-TXCO low cost oscillator, 10 or 16MHz? I did find this study, http://tf.nist.gov/general/pdf/2276.pdf, measuring the stability of some low cost quartz wristwatches which gives some interesting data of 20 to 65ppb Allan deviation over 100s. That, but a 32kHz oscillator might give rise to jitter problems when multiplied up to a suitable frequency. Some oscillator datasheets specify Allan deviation values, but I guess what I need for estimating worst case timestamp error during holdover periods are actually MTIE values. Is there any way to estimate the latter from Allan deviations specs? Would an ADev of 65 x 10^-9 over 100s imply up to 6.5us of error after 100s? Any thoughts? Thanks, Tony H ___ 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. -- 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.
Re: [time-nuts] Low cost GPS module for 100ns timestamping error
tn...@toneh.demon.co.uk said: In general, low cost oscillators make pretty good thermometers. True, but it's short term stability that matters here - over 10s of seconds the temperature shouldn't change much - especially if a bit of insulation is used around the oscillator. Ballpark is 1 PPM per degree C. That's 1 microsecond/second. You are interested in low ns, so you have to at least think about temperature changes. Will your box be outdoors in the sun? What happens when a big black cloud comes over? I'm not sure I understand this - I need to record the time that an event occurs so I need an accurate time reference. What can I store to post-process? The time reference drift and jitter relative to the local oscillator before and after the event? I was assuming that you would capture the time of your events and also the time of several/many PPS events surrounding your events. By time, I mean the counter value from the counter/timer module. I'm not familiar with the chip you are interested in, but the ARM SOC chips I've worked with have a switching matrix between the input pins and their collection of IO modules. I would try to set things up so the PPS signal can be used by both counters. (maybe using 2 pins) The idea is to switch the event counter to the PPS signal to find the offset between the counters. You also have to monitor the serial port so you can label the time of a PPS pulse and/or do sawtooth correction. You also need status info. How good is the fix? ... What are you going to do if/when the GPS fades out? (or gets confused by multipath) One of the reasons to collect some data is so you know what happens just before and/or just after your GPS unit decides it can't get a good fix. -- These are my opinions. I hate spam. ___ 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.
Re: [time-nuts] Low cost GPS module for 100ns timestamping error
-Original Message- From: Tony [] I'm considering designing and building some dataloggers, probably ARM Cortex based (eg. STM32F4xx), which record the time of infrequent events, preferably to better than 100ns and if possible better than 50nS. The data loggers will be continuously powered, in fixed locations and should have reasonably good views of the sky so the use of a low cost GPS module should be feasible. I believe it shouldn't be too difficult to resolve the PPS timing to +/- 5ns or better with a 100MHz+ microcontroller clock, but obviously jitter would add to the error requiring the GPS to be better than perhaps 90ns or so worst case. [] === The MAX-7Q has a TCXO and works well as a PPS source. https://www.u-blox.com/en/gps-modules/pvt-modules/max-7.html or perhaps the MAX-M8Q but I've not played with one of those. Cheers, David -- SatSignal Software - Quality software written to your requirements Web: http://www.satsignal.eu Email: david-tay...@blueyonder.co.uk ___ 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.
