Suitable GPS for GPSDO OCXO - Page 1
Forum section: electronics / beginners

author: metrologist
author_rank: Super Contributor
timestamp: July 17, 2017, 08:03:50 pm
content: I wanted to put a few OCXO's I have into a GPSDO build. I was just randomly searching for pps GPS units and came across NEO-7M. I found on the time-nuts mailing list someone mentioned the jitter of these units was ~5ns and perhaps 20ns max, and then it was mentioned they would not be suitable for GPSDO.I bought a few Trimble units that have spec'd 95ns jitter with some verbiage about taking 100 measurements with 100 samples over a 20 minute period to get the result. I don't if that's good or not, but those are the units I ordered.If they are not suitable for GPSDO then I can use them for other less sensitive timing.The basic idea I had for the GPSDO was to use a micro clocked by the 10MHz OCXO and count clock cycles between the 1pps signal, then do some fancy math over a period of time to slowly drive the control voltage with a ~12 bit dac. I was hoping this would accommodate for any jitter.I'm sure there are designs already out there, I see one on the instructables site and I briefly caught another using an atTiny.Any ideas on this or suggestions of where to go looking for more info? I'd need some help probably with the math programming and maybe some of the circuit design and component selection. I'd probably use an Atmega 328p.

author: Gyro
author_rank: Super Contributor
timestamp: July 17, 2017, 08:12:56 pm
content: You want a 'timing' receiver rather than a 'location' one. For Ublox that means an LEA-xT rather than a NEO-xM (don't think there's a NEO-xT). For Trimble, a Resolution-T (or -SMT), or whatever the later models are, etc.It's worth trawling the specs on the manufacturers' sites.Examples:http://www.ebay.co.uk/itm/Trimble-Resolution-T-GPS-Timing-Receiver-Module-1PPS-Output-15-nsec-accuracy-/162586564826?hash=item25daea18da:g:OogAAMXQY8JRgFFRhttp://www.ebay.co.uk/itm/Trimble-GPS-SMD-IC-Multi-GNSS-Timing-Module-Resolution-SMT-GG-P-N-89999-00-/262621138349?hash=item3d25708dad:g:cdIAAOSwNphWWqGthttps://www.ebay.co.uk/sch/i.html?_odkw=LEA-6T&_sop=15&_osacat=0&_from=R40&_trksid=p2045573.m570.l1313.TR0.TRC0.H0.TRS0&_nkw=LEA-6T&_sacat=0Edit: Note that the Resolution-SMT will be a 'challenge' to breadboard with.

author: MosherIV
author_rank: Super Contributor
timestamp: July 17, 2017, 08:39:22 pm
content: QuoteI have been thinking of doing it the same way.I do not think you need to worry about the maths, the instructables way is simple enough - it just averages the count over a very long period (1, 2, 4, 12, and 24 hours). This is enough to average out the jitter in the PPS. Since you sound like you are using a comercial OCXO, their drift is usually less than 20ppm/year so this technic should be fine.Basically, the count should be dead on 10MHz (+/- 1 count because of the PPS jitter - you cannot over come this unless you sychronise the 10MHz to the PPS but then you will end up adding jitter to the 10MHz, so in my opinion it is better to live with the 1 count error and have the stability of the OCXO) if the count is off, then you adjust the 10MHz. Whether you do this on the 1, 2, 4, 12 or 24hour is up to you.Do not forget to have some non volatile storage to save the current DAC output, for cold starts and when GPS lock is lost.Obviously, do not average when GPS lock is lost.As Gyro pointed out the M variant of the Ublox devices are the general purpose positioning devices. The T variant is the timing variant but just like you, I found they are more difficult to get hold of.The main difference from your point of view is the M variant does not have a TCXO so the temperature of the GPS module will have an effect on the stability of the PPS.Other differences are that it has less facility for timing eg does not provide 10KHz signal. (other GPSDO seem to be beased on PLL the 10MHz to the 10KHz because the 10KHz is derived off the timing from the satellite signals).I am curious, you say you already have a Trimble GPS. There are other project blogs on the web which use the Trimble. The Trimble has the 10KHz signal.I will follow your progress with interrest. Good luck and have fun

author: Gyro
author_rank: Super Contributor
timestamp: July 17, 2017, 08:52:22 pm
content: I'm not sure if you saw the ebay links I added to my reply. It's the first time in a while that I've seen a Resolution-T listed (note that itonlydoes 1pps though).The LEA-6Ts are still reasonably available in those Chinese "Ublox LEA-6T GPS Module w/Compass for APM2.5 APM2.6 Flight Controller Multirotor" modules that I did a teardown on.It depends on how fine the tuning range of the OCXO is of course but I have a gut feel that 12 bit resolutionmightnot be enough.

author: MosherIV
author_rank: Super Contributor
timestamp: July 17, 2017, 09:38:37 pm
content: Thanks Gyro, did know you could get those.Just quickly read your tear downGiven the choice.....go for the T variant. It is less affected by temp of the module.Is there a way to seperate the areial and add a smb connector for external ariel?

author: Gyro
author_rank: Super Contributor
timestamp: July 17, 2017, 09:56:06 pm
content: You're welcome. Yes, definitely go for the T version, it's not just the TCXO, the internal firmware is also optimized for timing duty. The LEA also gives you two programmable frequency outputs versus one in the NEO.Well it's easy to strip the existing antenna, LNA and related components and substitute a coax, which could have a bulkhead mount SMB on the end. I did it out of necessity when I 'lost' the LNA (). The whole saga is documented on page 2 of the teardown thread...https://www.eevblog.com/forum/projects/ebay-u-blox-lea-6t-gps-module-teardown-and-initial-test/for reference.Here's what it looked like, ignore the little inductor - that's only there because I killed the internal one! The consolation is that it works better with a cheap external active antenna than it did with the existing patch one. It also keeps the frequency outputs inside the GPSDO box rather than on the far end of a cable.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: July 17, 2017, 10:01:31 pm
content: Quote from: MosherIV on July 17, 2017, 08:39:22 pmNo need for synchronization if you code it right. The frequency error will accumulate until you get to an extra count. Eg: if the OCXO is putting out 10,000,000.1 Hz, you'll get one extra count every 10 seconds, which will go into the averaging and affect the tuning.

author: metrologist
author_rank: Super Contributor
timestamp: July 17, 2017, 10:25:24 pm
content: I already bought one of the Trimble based GPSDO systems and wanted to build a few more of my own since I have several good quality OCXO's. I'm not sure if the GPSDO I bought is an upgraded version of the ThunderBolt (newer gen?), or probably just something built for the telecom industry. It is the system on ebay that is similar to the BG7TBL, with the sine and square wave 10 MHz and 1PPS outputs.I bought a couple of these GPS modules just because they were $3 (just the GPS module):ftp://ftp.trimble.com/pub/sct/embedded/bin/Manuals/Lassen_LP.pdfI figured I could average out the jitter and only slowly steer the OCXO with the micro to keep the long term drift in line, maybe just run a calculation a couple or few times per day and slowly adjust the dac between those intervals. I do not know how the commercial GPSDO's control their oscillators. I'm thinking that would be a better solution than trying a PLL and introducing jitter.If I understand how to apply my 10 MHz OCXO spec, it is 5E-10/day aging. I think that is 5 mHz/day, or about 2 Hz/year? After typing that, it seems that the micro would not be able to count an offset until it saw one cycle out, which would take half a year? Maybe I'd have to use a phase detector and apply the same approach. Is that even possible? It would seem a better than 0.5ns resolution phase detector would be needed to even see something after a day.I have so much learn and relearn about this.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: July 17, 2017, 10:57:04 pm
content: Quote from: metrologist on July 17, 2017, 10:25:24 pm5e-10/day aging is 5 mHz/day. So after 1 day, the frequency will be off by 5 mHz. At a 5 mHz frequency offset, it would take 200 seconds to accumulate an extra cycle.I think you're worried about the wrong thing though, aging isn't really the problem, it's frequency stability over temperature. Most OCXOs only have a temperature stability of around 1e-7 over their operational temperature range. Unless you're putting this thing in a VERY tightly controlled environment, things like A/C or a heater kicking on and changing the room temperature by 1-2 deg C is going to have a much larger effect than the per day aging. Your MCU needs to be able to react and tune the OCXO much faster than a few times a day, I'd say once a minute is probably the slowest you'll want to do it. I can't speak for all commercial GPSDOs, but the few I've tested update every second.

author: metrologist
author_rank: Super Contributor
timestamp: July 17, 2017, 11:24:51 pm
content: Quote from: suicidaleggroll on July 17, 2017, 10:57:04 pmOK, that should have been obvious...I think that would even need to be integrated over time (where are my calculus books?).The temp spec is 2E-10, 0C to 55C. Would it help to have it in a well insulated box?Short term stability is 5E-12/sNow I am more motivated againI think I would want 100 averages at least to deal with this GPS's 95ns jitter.

author: metrologist
author_rank: Super Contributor
timestamp: July 18, 2017, 12:42:38 am
content: He took my offer for two. He has about 70 units.http://www.ebay.com/itm/Trimble-39263-Lassen-Low-Power-LP-GPS-Interface-Module-Timing-1PPS-MCX-M-SMA-F-/401314512604

author: MosherIV
author_rank: Super Contributor
timestamp: July 18, 2017, 07:36:57 am
content: QuoteI would hope that a ovened crystal is better than that ! Otherwise why would they have bothered to build a crystal into an oven !Typically the oven temp is 45 to 50'C, well above ambient so that ambient should not affect it.If the OCXO had to be adjusted every minute, the GPSDO would not be that stable.

author: CJay
author_rank: Super Contributor
timestamp: July 18, 2017, 10:04:37 am
content: Quote from: metrologist on July 18, 2017, 12:42:38 amDamn shame he won't ship outside the US, that's a *really* good price

author: edpalmer42
author_rank: Super Contributor
timestamp: July 18, 2017, 03:46:00 pm
content: Quote from: MosherIV on July 18, 2017, 07:36:57 amAlthough a temperature of 45 to 50C is possible, it would be unusually low. A more typical range would be 60 to 80C for both AT and SC cut crystals. The idea is to set the temperature to the point where the change of frequency with temperature is zero. For optimum performance, each oven has to be adjusted seperately because each crystal is unique.Ed

author: suicidaleggroll
author_rank: Super Contributor
timestamp: July 18, 2017, 04:11:59 pm
content: Quote from: MosherIV on July 18, 2017, 07:36:57 amBecause without the oven it would be even worse, by several orders of magnitude.

author: metrologist
author_rank: Super Contributor
timestamp: July 18, 2017, 05:34:20 pm
content: Those receivers are on sale today, less than I paid. Now I paid too much! I will buy a couple more when I get home if they are still on sale...

author: CJay
author_rank: Super Contributor
timestamp: July 18, 2017, 05:45:52 pm
content: Quote from: metrologist on July 18, 2017, 05:34:20 pmWell if you fancy buying a couple for me and shipping them...

author: metrologist
author_rank: Super Contributor
timestamp: July 18, 2017, 07:47:37 pm
content: I've never shipped anything out of the country. I did just buy 3 more (he wasn't as nice on the shipping this time, but for $2.19 I wasn't going to haggle on the price) and could see how they turn out. I really do not need 5 of themI wonder if they have a chance at receiving using a passive antenna? I wanted to try building a quadrifilar helical antenna after my failure at a simple helix wire wrapped around a plastic form with a disc ground plane.These plans for a stationary install, and I think I'd need a 1nF DC blocking cap:http://www.i1wqrlinkradio.com/antype/ch30/chiave1743.htmIf it does not work then I'd just buy a few more Chinese active puck antennas...

author: cdev
author_rank: Super Contributor
timestamp: July 18, 2017, 08:02:44 pm
content: A turnstile antenna would be easy to make and tiny. Also you could make a "pinwheel" type.

author: metrologist
author_rank: Super Contributor
timestamp: July 18, 2017, 08:36:01 pm
content: I'm also looking at this online calculator, but I do not understand the total compensated lengths.http://www.jcoppens.com/ant/qfh/calc.en.phpI don't even understand the first example I posted because if you add up each segment, the length does not come out...For example, in the first reference, I will add 30 + 55 + pi15 = 132mm (and that is not accounting for the small radius at the bends), but the length specified is only 105mm.The online calculator is giving me a corrected length of 211.8 and 201.5 with approximate diameter of 26.7. It's longer and narrower (I used 0.4 ratio), but still does not add up to anything close.I had good luck building a 23cm band bi-quad reflector with lips and successfully used it with a C-band TV satellite dish in place of the original horn LNAs. I measured ~10dB gain relative to a dipole just for the bi-quad reflector. I also tried a double bi-quad for the 2m band. It was fun. We'll see how it goes, maybe tonight if I find some time and materials laying around.

author: cdev
author_rank: Super Contributor
timestamp: July 18, 2017, 08:49:05 pm
content: A small QFH would be the best but not so easy to get right if you're making it out of wire. Copper tape around a low loss form of exactly the right size would be better. Also you'll likely need to feed it using semi rigid coax.Take a look at the ones used in radiosondes.The passive ceramic patch antennas from Taoglas work quite well and aren't that expensive.Only a small subset of GPSs are really suitable for use in a GPSDO.

author: cdev
author_rank: Super Contributor
timestamp: July 18, 2017, 08:52:38 pm
content: I have a cheap timing antenna and it is a QFH. I paid around $20 for it on ebay. Its the one that looks a bit like a soft freeze ice cream cone. Sold under Lucent, MaxRad and a few others.Quote from: evb149 onTodayat 14:00:00

author: metrologist
author_rank: Super Contributor
timestamp: July 18, 2017, 09:01:17 pm
content: I'll give it a wing. I can also get a smith plot at work as we have a few VNA's.I do have a commercial helix, about the size of my thumb with a recessed SMA male that will screw right on to that pigtail, as long as I remember to decouple the DC. I'll have to make up an adapter.

author: edpalmer42
author_rank: Super Contributor
timestamp: July 18, 2017, 10:25:39 pm
content: There are a few things to keep in mind about antennas.Most timing receivers monitor the current fed to the antenna and get upset if the current is too low or too high. If you build your own antenna, you'll have to include a fake load. Check the data sheet for your receiver.If your receiver is a bit older, it may be a bit deaf. Without the gain provided by an active antenna it might have trouble.Timing grade antennas don't have to be expensive. Looks like they start in the $20 - $30 range.If you buy an antenna, make sure that the voltage the GPS receiver puts on the antenna lead is compatible with the antenna. Really old or weird receivers used +12V. Most older ones use +5V. The newer ones use 3V3. Lots of antennas are +3V3 and +5V compatible.Ed

author: cdev
author_rank: Super Contributor
timestamp: July 18, 2017, 11:43:19 pm
content: Is your helix a Sarantel antenna? They do a size reduced GPS QFH thats pretty cool.A QFH is avery goodall around antenna with a uniquely omnidirectional pattern that is not effected practically at all by its surroundings, and maintains the right hand circular polarization, nomatter what direction the signal is going to, coming from. It may actually be the best antenna if you are not using multiple constellations. (more than just GPS L1) - the problem is a QFH will work within its own band, and likely also has a built in SAW filter so isnt so great for signals from other GNSS systems than its designed for.A QFH is certainly an antenna that will do excellently for GPS timing.Quote from: metrologist onTodayat 15:01:17

author: cdev
author_rank: Super Contributor
timestamp: July 19, 2017, 04:10:55 am
content: The inherent bandwidth of a ceramic patch antenna is significantly more narrow than its equivalent in air. Also the ceramic antenna is substantially smaller.Coverage of other constellations varies a lot from antenna to antenna. Ceramic antennas are designed to be narrow banded, they have a specific asymmetrical shape which is how they implement circular polarization. The machines that make them have to trim them individually while measuring their RF characteristics.That makes them much more frequency specific. Also size does matter. The bigger a patch antenna is the more likely it is to "work" on, say Glonass, but is it a good antenna for surveying or precise measurement of position? That will be dependent on a number of things, specifically phase center variation and rejection of multipath reflections which tend to be reversed polarization. Ideally the electrical center of the antenna should be the same irregardless of frequency.Good broadband antennas often are definable simply by their geometry, not their size.

author: cdev
author_rank: Super Contributor
timestamp: July 19, 2017, 04:31:09 am
content: The best circular polarization antenna designs leverage the circular polarization to extract an extra 3 db of gain out of the signals, and also give you an additional 3 db rejection of the opposite sense signal (reflections) which matters the most at low angles.. (where the patch antennas tend to lose their circular polarization)Rejection of multipath reflections can really make a huge difference with a GPS signal.In order to improve the crappy pattern at low angles, you can set up what amounts to a trap for signals that they go into and never come out of. But this trap which looks a bit like a hollowed out wedding cake with stacked rings, can only work on one narrow band of frequencies.You were talking about a quadrifilar helix. Most QFHs we see are fed at the top but they can also be fed at the bottom and use a 90 degree hybrid to create the needed phase shift. The best GPS antennas have four or more arms to make the pattern completely symmetrical. They look a bit like a pinwheel. You can also implement interesting antennas that wrap the arms around a sphere. Those antennas should be able to reject LHCP even at low angles. But they are big.

author: cdev
author_rank: Super Contributor
timestamp: July 19, 2017, 04:55:22 am
content: RTKlib is a good way to learn about GPS antennas. Its a very rich toolkit that utilizes a substantial number of different technologies, really a marvel of engineering and science.

author: cdev
author_rank: Super Contributor
timestamp: July 19, 2017, 05:00:56 am
content: Look at Vaisala radiosondes

author: metrologist
author_rank: Super Contributor
timestamp: July 19, 2017, 06:51:47 am
content: I'll have to followup with the posts later. This is what I was doing tonight...

author: CJay
author_rank: Super Contributor
timestamp: July 19, 2017, 06:55:45 am
content: Quote from: cdev on July 19, 2017, 05:00:56 amThose Vaisala turnstile antennae also have a nice little LNA on the board, I highly recommend powering one up before tearing it down so measurements can be made and the LNA can be used too, they're realy rather useful.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: July 19, 2017, 12:30:27 pm
content: Quote from: evb149 on July 19, 2017, 05:42:03 amNo those are too close, but even if they weren't it wouldn't matter. To make a real difference the two frequencies need to be coming from the same satellite, not two different satellites.

author: cdev
author_rank: Super Contributor
timestamp: July 19, 2017, 02:51:02 pm
content: The transmitting/receiving antennas on GPS and remote sensing satellites that use GPS are usually QFH or QFH-like multi-arm antennas.Example in the image belowIt should be mentioned that for precise measurement, antenna's performance has to be characterized by calibration.Professional antennas go to great lengths to minimize multipath. Once you do that then the problem becomes phase center variations. Basically the speed of light in different directions is modified by the structure of the antenna such that it performs differently depending on the angle a signal is being received from.Professional antennas for geodesy are marked so you can position the antenna such that the calibration information - which is not that complicated, is accurate.People who want high levels of accuracy end up spending a lot more money on their antenna than you would for a non-calibrated GNSS antenna.If you are making an antenna commercially in the US you may be able to get help with calibration.Multi-constellation adds a new element into the mix and substantially increases the bandwidth required of an antenna for example, optimal GLONASS performance requires an antenna maintain its resistance to multipath reflections (which have inverted polarization) at low angles at 1600 MHz.New GNSS systems that are in the pipeline will increase that bandwidth requirement still more.

author: metrologist
author_rank: Super Contributor
timestamp: July 19, 2017, 03:05:22 pm
content: This is what I got. I think the decoupling capacitor cracked. It resonates at 1585MHz.

author: cdev
author_rank: Super Contributor
timestamp: July 19, 2017, 03:20:39 pm
content: The two loops in a QFH antenna that is built (non-optimally) without a 90 degree hybrid resonate at two frequencies, one is above the range its supposed to work at, the other below.I don't own any kind of network analyzer so I have no idea how this would resolve on a network analyzer. I suspect you would see a combination of the two, but the signals in the two individual loops could be analyzed separately to determine their sizes, one a bit too big, one a bit too small.

author: cdev
author_rank: Super Contributor
timestamp: July 19, 2017, 03:24:21 pm
content: Do the cheap Trimble units have any signals, such as 10 KHz, 1 MHz, etc. that might be useful for a GPSDO (besides the timing pulse)?

author: metrologist
author_rank: Super Contributor
timestamp: July 19, 2017, 03:43:50 pm
content: I do not think there is any other timing signals from the cheap Trimble units.Here is the GPS spectrum with my antenna sitting inside the middle of a typical office building. At least it hears something. I'm a little hopeful it will work, but I may build another. I cut all of the elements a little short, about 0.5mm. The disc sander I was using to trim the ends nice and square got a little hungry

author: Gyro
author_rank: Super Contributor
timestamp: July 19, 2017, 03:56:15 pm
content: Quote from: cdev on July 19, 2017, 03:24:21 pmAccording to the product flyer it's an 8 channel receiver with 1pps (+/-95ns) only. Pretty jittery by modern standards, it's not intended as a timing receiver.http://www.dpie.com/datasheets/gps/11547e%20Lassen%20LP%200205.pdfP.S. The manual, 1999 vintage:http://www.dpie.com/manuals/gps/lassenlpman.pdf

author: metrologist
author_rank: Super Contributor
timestamp: July 19, 2017, 06:07:21 pm
content: Quote from: cdev on July 18, 2017, 11:43:19 pmIt might be. It does look like S1203A from google images. The antenna is unmarked, however. It measures as an open circuit though, so maybe it is no good or is already decoupled. I thought I once measured it as a short, but now it is definitely DC open.

author: metrologist
author_rank: Super Contributor
timestamp: July 19, 2017, 09:26:45 pm
content: Do we think this receiver will be relatively deaf? I know my Jupiter struggles with a puck antenna in the window. The receivers I can use to compare are old Garmin portable units, an 12XL (and why are those still so expensive on ebay?) and a GPS60. These work fine outdoors with clear sky, but not indoors very well. The puck antenna t the window works well with them.So, I am not to expect this receiver to work well enough with my OCXO's? I'd be happy if I could keep it limited to it's hour or so drift and keep it in cycle. Isn't that GPSDO's do anyway and are ultimately limited by the oscillator for their short term stability? I'm not quite clear why such a highly stable pps clock reference is needed? OK, 250ns is more than two clock cycles, but wouldn't we be able to average that out over minutes?

author: MosherIV
author_rank: Super Contributor
timestamp: July 19, 2017, 09:48:16 pm
content: QuoteI did not think OCXO drifted that badly over such a short term. I would expect OCXO to take hours before they even drift by 1 cycle. Xtals are suppose to have better stability in the short term than the PPS of GPS, I suppose that is if they are kept in stable temp.Bear in mind that they need to warm up to full operating temp first, usually takes an hour. They need to be housed in some case which further insulates them. They need to be left on.Have you run any experiments with your OCXO to verify what the drift is?QuoteWith such a bad jitter in the PPS, it may take 10s of hours possibly even days to average out.You would really be better off investing in a better GPS. Even a position one with lower jitter would be better. From the sound of it, a newer more sensitve one may even help with your antena problems.

author: cdev
author_rank: Super Contributor
timestamp: July 19, 2017, 10:18:27 pm
content: Its not just simply a matter of getting four satellites so you get a 3D fix, the quality of the satellite signals will determine the stability of the timing output.

author: cdev
author_rank: Super Contributor
timestamp: July 19, 2017, 10:35:18 pm
content: You can buy the newer Ublox modules on ebay for less than $10 now. You can use them, or use the Navspark Mini (also very inexpensive)or any decent GPS with a reasonably precise 1PPS to discipline the clock of a raspberry pi using NTP and use the RPI GPIOs to generate an arbitrary frequency. Hams do it all the time using "WSPrry Pi"https://github.com/JamesP6000/WsprryPi. Note that I am just pointing to that as an example of GPIO PI use.. Unless you are a ham, your emissions need to be confined to the insides of coaxial cable to be legal.

author: metrologist
author_rank: Super Contributor
timestamp: July 19, 2017, 11:21:36 pm
content: Quote from: MosherIV on July 19, 2017, 09:48:16 pmI have no way to verify my oscillators, but are spec'd < 5mHz/day. This GPS timing accuracy is stated as +/- 100ns in the manual and accuracy is spec'd +/-95ns. Their 100ns figure is the 1 sigma value of 10 sets of 100 samples over 20 minutes. They say "not allowing for SA" but I'm not sure if that is with SA on or off.

author: cdev
author_rank: Super Contributor
timestamp: July 20, 2017, 12:25:12 am
content: That would be fine for a stratum 1 NTP server. When a GPS produces other signals that are locked to the GPS, they typically arent stable enough to be used as a frequency standard unless their output is averaged over a long period of time in various ways to remove the jitter.

author: metrologist
author_rank: Super Contributor
timestamp: July 20, 2017, 02:34:36 pm
content: QuoteThat means with SA turned off, the way it is now?My QFH antenna seems to work well on my Jupiter receiver sitting on my desk in the house. I'll compare that with these Trimble receivers when they arrive.

author: metrologist
author_rank: Super Contributor
timestamp: July 20, 2017, 06:32:51 pm
content: Quote from: evb149 on July 20, 2017, 05:32:25 pmThanks for the insights about the Trimble. I was wondering how as well.No LNA on the antenna. About 1 foot of RG174 to an MCX connector. I should remeasure it because the decoupling capacitor was definitely cracked.I could not get a lock with the commercial QFH antenna but should give it another try as I might have had a coax issue.

author: edpalmer42
author_rank: Super Contributor
timestamp: July 20, 2017, 07:03:33 pm
content: Quote from: evb149 on July 20, 2017, 05:47:45 pmHere's the manual for a commercial unit called a GPSTCXO. On page 24 it shows the Allan Deviation. Better than a bare GPS receiver, but maybe not as good as an OCXO, although it would be a lot cheaper to buy if you were comparing prices for new units from the manufacturers. I actually have one of these. It's performance is pretty much identical to the graph.http://www.jackson-labs.com/assets/uploads/main/gpstcxo_manual.pdfNote that the part of the Allan Deviation graph that slopes down and to the right is the 'signature' of the performance limit of GPS. I call it the 'GPS Line'. Every GPS receiver and GPSDO will more or less follow that line. On a bare GPS receiver, the line continues in a straight line up and to the left. The OCXO or TCXO comes in from the left and intersects the line at some point, producing a graph similar to the one shown in the manual.Ed

author: suicidaleggroll
author_rank: Super Contributor
timestamp: July 20, 2017, 07:41:17 pm
content: Quote from: evb149 on July 20, 2017, 05:47:45 pmSee the attached figure. This is from a GPS unit that uses a 10 MHz Vectron TCXO to run the clock. The TCXO is NOT disciplined, it's free-running, what you see in the graph is the frequency error of the clock in Hz as a function of time for a day. You can ignore the large spikes every few hours, those are caused by the GPS doing a clock fixup which gives erroneous results for the clock velocity when it happens. This TCXO isn't temperature controlled, but it is in an enclosure with a relatively constant temperature, inside a room with a relatively constant temperature.

author: metrologist
author_rank: Super Contributor
timestamp: July 20, 2017, 08:08:40 pm
content: I wish I had taken notes when I was calibrating my HP5314A counter with the high stability TCXO oscillator. I sort of remember it fluctuating through 0.8Hz each day for the week I had it set up at work monitoring the GPS 10MHz reference. Its oscillator spec is 1E7/month and 1E6 0C to 50C, but the room was temperature controlled. I could be remembering monitoring something else with that counter though. Can't wait for my GPSDO to come in.

author: cdev
author_rank: Super Contributor
timestamp: July 20, 2017, 11:56:43 pm
content: I really had never looked into this much until recently and it appears to me now that there really is no silver bullet that allows you to turn a piece of cheap silicon (a cheap DDS, for example) into a stable dial in your frequency VFO or frequency standard, yet.

author: edpalmer42
author_rank: Super Contributor
timestamp: July 21, 2017, 04:12:09 am
content: Here are a few links that talk about input signal conditioning, amongst other things.http://www.wenzel.com/library1.htmandhttp://www.techlib.com/. Wenzel makes some of the lowest phase noise oscillators available. Charles Wenzel likes to tinker with stuff and actually tell you about it! techlib.com is his personal tinkerer site. He even has a GPSDO. No performance data, unfortunately.http://www.ko4bb.com/getsimple/index.php?id=timing. On a good day I can understand maybe 30% of the stuff Bruce Griffiths shares on Time-Nuts.while going downhillwith a tailwind.http://www.ham-radio.com/sbms/LPRO-101.pdf. This document includes a section on interfacing the LPRO-101 Rb Standard sine wave output to digital systems. They include schematics and phase noise test results.Ed

author: pigrew
author_rank: Frequent Contributor
timestamp: July 24, 2017, 03:01:45 am
content: Quote from: cdev on July 19, 2017, 03:24:21 pmI was playing with the Trimble Lassen LP this afternoon and managed to implement reasonable sawtooth correction! The trick is to figure out its oscillator bias (ppm). The value can be calculated from the clock bias rate returned by the 0x56 or 0x54 packets. Dividing that value by the speed of light (defined as 299792458) will get the oscillator frequency bias away from 12.504 MHz.Knowing that, I reverse-engineered the quantization algorithm and was able to remove the sawtoothness (it'll probably eventually have a tooth, but I've not seen one in half an hour of data). Another interesting thing was that it outputs only on every fourth clock cycle (quantized to around 320ns periods).The board can be run in over-determined/stationary mode, though I'm not sure if that'll help so much.I was able to hand-discipline my Racal 1992 counter (with 4E oven) using the Lassen LP module (and a signal generator doing clock division) to be about +1ppb above 10 MHz. Too bad the oscillator's tempco is 0.6ppb/C, so it will drift.I'm using one of the GPS-TMG-26N antennas from eBay (the manual suggested about +26 dB gain) from the upstairs of my house. It has a lock on 5-6 satellites.But, I'm not sure that I'll end up making a GPSDO with it. Currently, I have no need for better than 0.5 ppm frequency accuracy (to tune BlueTooth transceivers). Also, for the few dollars more that a timing-quality board costs, the timing board would be worth it.Another caveat for these boards is that the header is 2mm pitch (not 0.1in), so I'm having to get new jumper cables for it. I'm using the "Trimble VTS 2.3.12" software to configure the board.

author: metrologist
author_rank: Super Contributor
timestamp: July 25, 2017, 06:56:38 am
content: Thanks for sharing. Hopefully I can eventually understand some of that. I got my boards today and have tested three of them for basic communication and a lock.I am interfacing with a ch340G serial-USB (3.3v) and could not get PuTTy to decipher the data. I launched Lady Heather and it recognized a TSIP receiver, with is listed as a Trimble, but it too does not decipher any of the data.I noticed the serial data is inverted so I guess I need to invert that, butit's puzzling how Lady Heather identified the receiver type but does not detect serial stream.I finally found Trimble VTS and that is communicating, tracking 3 SVs.

author: pigrew
author_rank: Frequent Contributor
timestamp: July 25, 2017, 05:09:21 pm
content: Quote from: metrologist on July 25, 2017, 06:56:38 amI've only tested one of the boards I got. It doesn't make sense for me to have multiple boards in operation (as I want different instruments frequency locked to eachother), and the cost of extra GPS antennae or splitters/LNA. I bought five boards, but it only came with three MCX->SMA pigtails.I'm using a "FT232RL" (probably a counterfeit) USB serial converter from Amazon, using its default EEPROM configuration (and setting 9600-8N1). By default, the board outputs in binary TSIP, so you putty won't help you unless you change its output format to ASCII or NMEA. I also found that Lady Heather could detect that it was TSIP, but wouldn't display anything on the screen. It looks like one can apply to be able to download Trimble's SDK, but instead of that I implemented a parser in C# (and used that for my sawtooth correction).After a few days, mycounterSDG2042X is about 5 ppb offset. I'm attaching some figures. The sawtooth correction works for at least three hours without creating any teeth. (I did set a maximum offset for the correction, so that the corrected value can't drift too far from reality). EDIT: Ooops, my SDG2042X had defaulted to its internal reference, so the reference clock is the TCXO in it, instead of the counter's OCXO.

author: metrologist
author_rank: Super Contributor
timestamp: July 25, 2017, 06:04:45 pm
content: I was fortunate to find a 12 pin dual line to single header connector cable. I will probably cut it in half so I can wire up two units. I just used wire stuck into the other end to attach to my breadboard. I did not get the satellite data to show like yours. Will try to post a pic later tonight.5 boards one pigtail

author: cdev
author_rank: Super Contributor
timestamp: July 25, 2017, 09:06:38 pm
content: Not GPSDO related.. but interesting.. the longer it churns in one place, the tighter the circle of dots gets.. This is not my expensive GPS, this is my $6 GPS.It's got a stable fix and its circle of uncertainty is only a few centimeters. This is not RTK, its regular GPS with the benefit of two sats from Beidou.Using a passive antenna.

author: cdev
author_rank: Super Contributor
timestamp: July 26, 2017, 04:12:31 am
content: Most GPSs have different presets that go all the way from what timing GPSs do, position pinning, all the way to what you want on an RTK GPS, which is no pinning at all. This GPS is currently in "auto" mode which means that once the circle of solutions gets very small it seems to pick one location and stay there. But if I move the gps an inch or two it registers the change. For example, I had the GPS sitting in front of my monitor with its plank that I have it and my el-cheapo USB logic analyzer and a small USB hub taped to. So in order to have some room I moved it to the bench near the window. Roughly a meter away, and my arm described an arc with the GPS as I was moving it. The track faithfully captured that arc and accurately estimated the difference between the two points. How much did the track say I moved it away? 140 cm actually, looking back at the log. Measuring it thats what the distance is. This is indoors with no special equipment, using its default settings, (except that I turned off power saving mode) There is an old CD taped to the board under the GPS antenna to act as a ground plane.

author: cdev
author_rank: Super Contributor
timestamp: July 26, 2017, 04:25:54 am
content: The PPS also has to be emitted on a clock cycle. In the case of Skytraq its the Leon 3 CPU clock cycle. But its internal clock is much higher than, say an Arduino. Its I think 80 or 100 MHz.By discipling a Raspberry Pi computers clock thats running NTP it also gives the computer an accurate enough idea of time to make an accurate frequency generator possible with its GPIO pins.

author: pigrew
author_rank: Frequent Contributor
timestamp: July 26, 2017, 04:40:51 am
content: I ran the Lassen LP PPS input versus a Racal 4E OCXO for 7 hours, and have tried to plot the results.The deviation of the sawtooth-corrected signal and the raw data is less than 20 ns after 7 hours, so I have to say that sawtooth correction is very doable, even with non-"timing grade" ancient GPS units.I was playing around with the TimeLab software to look at the results. As sawtooth-ness is kinda-Gaussian, I think it'd make sense to look at the modified Allan deviations. I think that this implies you need to average over perhaps 6000 seconds to get rid of the sawtooth-ness without doing proper correction? Or should I look at the modified plot to get the averaging constant needed?I don't really know if this is a good or a bad result. It's at least not too-bad, especially for a non-temperature controlled residential room with air conditioning. ADDENDUM: The measured time intervals are only 1ns resolution, so that might cause the plots to look bad. Perhaps averaging data could help (but would remove the higher-frequency analysis)? Also, I expect the GPS PPS is what's causing the deviation, not so much the OCXO.Another aspect to building a GPSDO is temperature-correction. Especially during holdover, but I guess always, there should be some sort of temperature input, which can correct the oscillator before the GPS detects the change? Or not? I know that temperature-correction is often done during holdover, but personally I'm not so interested in accuracy during holdover, or maintaining time..Quote from: evb149 on July 26, 2017, 02:48:31 amThe Lassen LP has a static filter, that engages when it's not moving much.From the manual:The Position-Velocity (PV) Filter is the main filter and is used to “soften” the effect ofconstellation switches on position fixes. The filter has no effect on velocity output andthere is no lag due to vehicle dynamics. There may be a small increase in accuracyhowever.A feature of the PV filter is the “Static Filter” which engages when the receiver is movingvery slowly. This feature improves accuracy in the urban environment. The static filtershould be turned off for the following applications:• Slow-moving environments such as walking or drifting with the current• When rooftop testing of receivers for moving applicationsThe altitude filter is a simple averaging filter with a time constant of a few seconds. Itshould be left on in marine and land applications.

author: metrologist
author_rank: Super Contributor
timestamp: July 26, 2017, 12:36:52 pm
content: I'm going to need a better antenna and put it outside as it keeps loosing lock. It does OK for a few minutes, then corrects. The trace persists for 20 samples.

author: metrologist
author_rank: Super Contributor
timestamp: August 04, 2017, 12:26:51 pm
content: I haven't been able to configure Timing Receiver Mode (Failed to execute: Mode 0, State 5). I've had it set to auto 2D/3D, but setting to O-D Clk Timing Mode doesn't seem to matter much.Lassen is top trace, bottom is loose OCXO, middle is another GPS reference. Is that +/-100ns jitter or looks like hunting to me.

author: pigrew
author_rank: Frequent Contributor
timestamp: August 04, 2017, 04:42:26 pm
content: Quote from: metrologist on August 04, 2017, 12:26:51 pmThe timing mode is only available on the timing receivers (such as the Resolution T/SMT). It adds two main things, the first is an auto-survey mode where it averages a few thousand position fixes, and then puts the GPS in a special fixed-position mode. But, I think that this can be emulated by a micro with the non-timing receivers. The second is that it can output the quantization error of the PPS edge.I've been thinking about the uncertainty of the PPS signal. There's the +/- offset due to quantization, but the manual seems to say that there's still a ?=15 ns accuracy, which seems to be a similar level to the quantization of the timing receivers (Resolution T/SMT/uBlox). (Although, uBlox publisheddata on their LEA-6Twhich indicated that they were able to measure with a ?=6.7 ns. )Building a GPSDO, I'm anticipating using a microcontroller's counter unit for timing. Perhaps the Tiva TM4C123GXL or a Cypress PSoC 5 would do well. Both have 80 MHz clock frequencies, so they should be able to measure timing with ~15ns resolution. Perhaps this is good enough, since the PPS signal is likely about as accurate. The other option is to use a CPLD/FPGA, with the advantage that it could run ~250 MHz, for ~4 ns resolution. Yes, this is better, but I'm not sure it is really worth it due to the accuracy of the GPS timing itself.I think that it would be a big advantage to use uBlox GPS modules, do to their ability to output higher frequencies, which would allow quantization noise to be averaged out. I'm not sure exactly what the proper math is, but wanting an output accurate to approx 10^-13 (reasonable based on published Allan deviation plots?), we'd need to have (1.5e-9 / 10^-13 = 15000 sec = 4.2 hour) time constants? This seems too long (as I'd expect environmental factors to vary on the order of minutes). However, extrapolating the uBlox data, we'd only expect an Allan deviation of around 2*10^-12 at 15000 seconds.So, is this why we need a GPS-disciplined rubidium-disciplined crystal-oscillator?