Cheap 10KHz output GPS module for frequency standard? - Page 1
Forum section: electronics / projects

author: SingedFingers
author_rank: Frequent Contributor
timestamp: January 21, 2017, 09:20:50 pm
content: Does anyone know of any cheap GPS modules that have a 10KHz output instead of the standard PPS output? This is to be used to discipline a VXO+HC4046 PLL based 10MHz frequency source to use as a frequency standard to cal my frequency counters against. I've looked at the Navman Jupiter-T but it's way out of budget.

author: Gyro
author_rank: Super Contributor
timestamp: January 21, 2017, 09:39:28 pm
content: The Ublox LEA-6T is a good candidate. It has two outputs which can be programmed anywhere up to several MHz (though you have to be careful to get an integer divide of it's TCXO at MHz frequencies). I did some experements (still need to box it!), the thread also has a link to a teardown I did of one of these modules contained in a cheap ebay GPS receiver...https://www.eevblog.com/forum/projects/my-u-blox-lea-6t-based-gpsdo-(very-scruffy-initial-breadboard-stage)/

author: SingedFingers
author_rank: Frequent Contributor
timestamp: January 22, 2017, 10:00:03 am
content: Thanks for the reference. Will have a read through that today. Much appreciated.It was either that or the MSF carrier frequency in the UK. GPS looks easier and more stable.

author: Gyro
author_rank: Super Contributor
timestamp: January 22, 2017, 10:36:20 am
content: Hope it helps.In pre-GPS days I built an MSF based standard, the problem is that MSF uses 100% carrier modulation (it's either on or off) so the pll needs to be able to cope with the carrier being missing half the time! I had to use an analogue switch to float the loop filter during the absences. It did work, but was jittery as hell.The alternative is DCF 77.5kHz from Germany which uses 75% carrier modulation, you can pull receivers out of lots of cheap radio controlled clocks but it's difficult to extract the carrier rather than the demodulated output. A high impedance buffer tapped onto the crystalmightwork but I've never tried it. It's also prone to interference - most radio controlled clocks work on the basis of getting a fix sometimes. Jitter in recovering MSF and DCF is not good either, time has moved on.Yes, GPS is definitely a better bet these days.

author: Bryan
author_rank: Frequent Contributor
timestamp: January 22, 2017, 11:15:14 am
content: Quote from: SingedFingers on January 21, 2017, 09:20:50 pmThere is quite a few GPS receivers that can output a accurate 1PPS,although haven't seen a lot of projects that use 1PPS to discipline a 10Mhz OCXO, not really sure why.

author: jpb
author_rank: Super Contributor
timestamp: January 22, 2017, 01:17:14 pm
content: Quote from: Bryan on January 22, 2017, 11:15:14 amA lot of GPSDO projects use 1PPS starting with Brooks Shera:http://www.a-aengineering.com/QST_GPS.pdfAnd there have been quite a few others based on a similar design.But James Miller's very simple design using the 10kHz output of a GPS works very well and is simple to build so I guess DIY people prefer it:http://www.jrmiller.demon.co.uk/projects/ministd/frqstd.htmMy understanding is that though higher frequency output (e.g. 10kHz or even higher from Ublox's Lea-6t) make life easier they don't gain you anything in that the higher frequencies aren't updated any faster than the 1PPS - this was according to something I read on time-nuts.

author: Gyro
author_rank: Super Contributor
timestamp: January 22, 2017, 01:48:46 pm
content: QuoteNot sure about that one... All output frequencies are derived from the on board 48MHz TCXO. On all outputs, including the 1pps where the output will be synchronised to the nearest edge of the TCXO output. All outputs will show this 'sawtooth jitter'. As I mentioned previously, jitter at higher output frequencies is also related to whether the output is an interger divide of the TCXO frequency. It's worth noting that there are significant internal firmware differences (optimisations) between the non-T (navigation) and the T (timing) suffix modules. I don't think there's anything that would cause corrections to be restricted to only once per second (I haven't seen any evidence of it anyway).There's a useful "GPS-based Timing Application Note" for the LEA-6T here:https://www.u-blox.com/sites/default/files/products/documents/Timing_AppNote_%28GPS.G6-X-11007%29.pdfP.S. Uncle Bob would probably be able to give a more definitive answer on this one.P.P.S. Come to think, it might have been the case on the Jupiter-T as it is much older and used a much lower frequency internal clock - I have a vague memory of reading something too.

author: SingedFingers
author_rank: Frequent Contributor
timestamp: January 22, 2017, 03:48:18 pm
content: Thanks everyone. I wanted to avoid the PPS output as I'm impatient and it takes a shorter amount of time for the PLL to lock on a higher frequency signal from my slightly shoddy experiments with the 74hc4046.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: January 22, 2017, 03:55:04 pm
content: Quote from: SingedFingers on January 21, 2017, 09:20:50 pmCome and join the GPSDO party. There are a number of ex-mobile phone mast units on the recycled market e.g.http://www.ebay.com/itm/262401923665or a very nice lookinghttp://www.ebay.com/itm/252610556561Or if you fancy being a bit left of field...http://www.ebay.com/itm/290709958177

author: jpb
author_rank: Super Contributor
timestamp: January 22, 2017, 04:46:50 pm
content: Quote from: Gyro on January 22, 2017, 01:48:46 pmHere is the thread I was thinking of :http://time-nuts.febo.narkive.com/fgHmbYDE/gps-w-10khzTo quote from it:Quote

author: Gyro
author_rank: Super Contributor
timestamp: January 22, 2017, 05:29:29 pm
content: Quote from: jpb on January 22, 2017, 04:46:50 pmAh ok, I was interpreting what you said as the higher frequencies getting corrected only once per second. Sure, you won't get any higher accuracy than you do from the 1pps output, no reason you would, [Edit: no worse though,] the only advantage is that the pll divide ratio is much smaller, so you get a much faster lock time. You can turn it on and have a useable (for most purposes) output within a few minutes rather than hours. Obviously it is still going to take hours to reach its unltimate stability.Quote from: NivagSwerdna on January 22, 2017, 03:55:04 pmPah! Too easy.Actually, those Trimble ones don't look bad - they look as if they've at least been tested (if the photo is accurate) so hopefully less chance of parts being ripped off the board, as has been experienced with other salvaged GPSDOs.

author: jpb
author_rank: Super Contributor
timestamp: January 22, 2017, 06:45:54 pm
content: Quote from: Gyro on January 22, 2017, 05:29:29 pmMy long term project is designing and building a GPSDO, but the fact that you can buy them cheaply on ebay makes it more an interesting learning exercise rather than a good means of getting one.

author: David Hess
author_rank: Super Contributor
timestamp: January 22, 2017, 08:36:22 pm
content: It is no easier to design an accurate GPSDO oscillator with a 1pps source or a higher frequency source because almost all GPS units which provide the high frequency source just poorly phase lock it to the internally generated 1pps source so no extra information is available. If this could work, then you could use the original high frequency output and not bother to discipline an external oscillator. In other words, the pulse to pulse jitter in the original 1pps signal just gets transformed into long period jitter in the high frequency output which will shift at every 1pps update. This allows for an easy PLL design but the short time constant to support the high frequency reference output will follow the jitter in the original 1pps signal defeating the whole purpose of creating a GPSDO.The simple solution is to do what Brooks Shera did by dividing the 10MHz frequency source down to a slightly lower frequency to prevent cycle slipping and then compare the phase to the 1pps signal. The fact that loop updates only occur every second is irrelevant because that is all of the information available whether the GPS output is 1pps or a higher frequency like 10kHz.Some GPS units will provide fully computed 5Hz or 10Hz outputs which helps but the same objection applies; a long time constant is still needed or the oscillator is just going to follow the high jitter GPS output.

author: texaspyro
author_rank: Super Contributor
timestamp: January 23, 2017, 05:50:48 am
content: Quote from: SingedFingers on January 21, 2017, 09:20:50 pmRDR-Electronics sells the Jupiter-T Pico on Ebay for $15. They are nice 3.3 volt receivers and have a 10 kHz output. The main problem with using them is the 0.5mm pitch on the connector. RDR includes the mating connector and the listing points out where you can get a board that breaks out the connector to a DIP pinout. I managed to solder some 30 ga wire wrap wires to the connector... but it took around an hour to do.

author: Gyro
author_rank: Super Contributor
timestamp: January 23, 2017, 02:07:28 pm
content: Quote from: David Hess on January 22, 2017, 08:36:22 pmDavid, you might want to follow the link to my breadboard experement... and to the module teardown which includes a lot of discussion. I say shorter lock time, but the filter time constant is still long, it just has more edges to work on in a given time. It is in fact possible to use the original high frequency direct for visual comparison of a an OCXO on a dual trace scope - sure you have to ignore the edge jitter (which is inconveniently large at 10MHz due to non -interger divide) and watch the underlying slow roll of one waveform versus the other but perfectly doable. BTW no 1 second cyclic shift visible in any experiments. Yes, of course the pll filter time constant needs to be a good number of seconds to avoid the gps short term jitter.The scheme could certainly be improved to reject longer term GPS variations with averylong filter time constant, but it comes down to what is good enough... 10e-10, e-11, e-12??? versus convenience. Brooks Shera certainly developed anexcellentstability continuous operation GPSDO but it is almost certainly overkill for non-timenut level calibration purposes.(Wanders away, head down, hands in pockets, muttering "I am not a timenut, I am not a timenut"......)

author: David Hess
author_rank: Super Contributor
timestamp: January 23, 2017, 04:13:15 pm
content: Quote from: Gyro on January 23, 2017, 02:07:28 pmYou identified the exact problem inyour discussion thread:I think the problem here is that, although there isn't non integer divide jitter at 1MHZ, there is still the characteristic GPS sawtooth superimposed on the signal as it isn't possible to sawtooth correct at higher than 1Hz and with data correction.If you use the short time constant which the synthesized high frequency output allows, then the PLL tracks the GPS solution and timing jitter (just the timing jitter is 1/48MHz or 20.8ns on the u-blox LEA-6T). A longer time constant will make your oscillator track the GPS's XO or TCXO oscillator. A longer time constant yet will make your oscillator track the GPS atomic clock.In theory a timing GPS with a DDS synthesized (sine) high frequency output (1) could produce a good result with a relatively short time constant but offhand I do not know of any that have this.Note that on the u-blox LEA-6T, there is no effective integer division of the 48 MHz clock no matter what the output frequency is because the 48 MHz clock is not phase locked to the GPS solution; it will always produce "hanging bridges". It is actually better to use a division factor which results in a minimum of synchronizing between the output frequency and the clock so you can take maximum advantage of dithering.So what I am getting at is that while it makes the circuit implementation easier, phase locking to a higher frequency source provides no advantage in performance when that source is only updated at 1 second intervals and not phased locked to the GPS reference itself.(1) Or a receiver timing clock phase locked to the GPS solution but GPS receivers that do this are both rare and expensive. Offhand I do not know of any even though this seems like an obvious, inexpensive, and easy solution. The ones I have heard about are effectively custom designed GPS receivers.

author: Gyro
author_rank: Super Contributor
timestamp: January 23, 2017, 08:10:30 pm
content: QuoteAh, well it just goes to show how quickly my mind and memory are turning to mush doesn't it!Well spotted.Thanks for the detailed explanation, understand what you mean now. There are clearly a lot of stages of tradeoff between straight GPS output and all singing, all dancing, actively sawtooth corrected solution.Without active sawtooth correction, as you say it comes down to the loop filter time constant either tracking the GPS TCXO or being long enough to achieve 'full' accuracy and inversely, worstening the startup time.QuoteThat's an interesting thought, it sounds like an easy direction for some additional experimentation without significantly complicating things.I guess personally I have reached a 'logical' solution in being (1) Higher accuracy that my needs and (2) Beyond my means of evaluating any improvement, oh and (3) Cost (not planning to shell out for a better double oven OCXO). I suppose this is the point that every solution will ultimately reach, at whatever level. The phase locked receiver timing clock or DDS synthesised clock either in purchase cost or development time, but as you say, it doesn't necessarily need to be in the right circumstances.At this point is sounds as if the OP may need to consider what level of accuracy he is hoping (needing) to achieve too.Thanks again for clarifying.

author: voltsandjolts
author_rank: Unknown
timestamp: January 23, 2017, 08:38:00 pm
content: To the OP:The LEA-6T timepulse outputs have an accuracy better than 20 ppb (==0.02 ppm) when averaged over a 10 second gatetime.If this is good enough for calibrating your frequency counter then I have good news - this is a $30 solution.Search ebay.com for LEA-6T and modify like Gyro did, or like I did in my post here:https://www.eevblog.com/forum/testgear/economical-option-for-precision-frequency-reference/msg754025/#msg754025(you might need to click this link twice to cache images which mess up page alignment when they load)