Lars DIY GPSDO with Arduino and 1ns resolution TIC

[W3AXL]

Thanks for the advice everyone! I actually tweaked my original design and added in a wilkinson divider for the dual outputs. Looks like I get ~2.4 Vpp into 50 ohms which should be more than enough.

I do have another question though. I logged the output of my GPSDO over 12 hours and plotted the TIC value, DAC value, and diff value:



To me, it looks like the control loop isn't doing a good regulating the TIC value. Quite a few excursions above +/- 100ns. There's quite a bit of noise and swing too, which might explain the MDEV plots I was getting earlier. Notice that the diff_ns value never changes by very much in response to the quite large TIC jumps. Seems like maybe some increased gain is needed? Right now I'm using G=26 and a time constant of 32.

Additionally, I checked the logs and the only PPS losses occurred at 15:45 and 17:21 which probably explains the big jumps at those times. But it doesn't explain the overall lack of stability.

[MIS42N]

Aditionally, I checked the logs and the only PPS losses occurred at 15:45 and 17:21 which probably explains the big jumps at those times. But it doesn't explain the overall lack of stability.

Why are you getting PPS losses at all? Have you connected your GPS module to a survey program to see signal strength, sky view, etc. If you are getting deviations above +-30ns in PPS it hints you may have a poor signal and GIGO principle applies.

I have had some success with a GPS program on a mobile phone, just wander around looking for good signals. In the end I mounted the Gps antenna on the roof.

[W3AXL]

I'm probably not in the best location - I live in an apartment with a balcony that faces roughly east-northeast. Decent view of all of that part of the sky, but a large building blocking my view west.

I've been using a cheap magnetic GPS puck as my outdoor antenna, and it seemed to be working well enough. Just now I replaced my original outdoor antenna with a (theoretically) better one, so we'll see if that helps things out at all. But I've looked over most of my logs and very rarely do I loose PPS. Not sure how much better I can make things in my current location without moving.

Attached is a U-center plot from the old small antenna. Doesn't look super bad, but not amazing either. I need to work in the GPS serial passthrough into my current code so I can get data for the new installation.

EDIT

One other thing - I've noticed with my 32u4 setup that the code execution will hang whenever I disconnect the serial console. I've never seen that before and I have no clue what would be causing something like that to happen. Anyone ever experienced any strange issues like that? I'm using the Sparkfun Pro Micro bootloader if that helps. It's probably some goofy quirk of the USB serial implementation not working totally right with Lars' original code.

[MIS42N]

Attached is a U-center plot from the old small antenna. Doesn't look super bad, but not amazing either. I need to work in the GPS serial passthrough into my current code so I can get data for the new installation.

That looks OK to me. The dB plot shows plenty of satellites over 30dB and that's regarded as adequate. Your PDOP is a little high (Positional Dilution of Precision). I wonder if you are getting reflected signals, I don't know how you would find out (or what to do about it). Doesn't explain why you get lost PPS. My GPSDO design keeps stats, can go a week without losing a PPS. And the code looks at the $GPRMC status field which is more reliable than the PPS. Many GPS units keep pumping out PPS signals even though they lost fix. The $GPRMC status is more reliable.

Can't help with the Arduino problem. My last was an ATmega328P.

[AndrewBCN]

Thanks for the great chart!

It's difficult to correlate the performance of Lars' DIY GPSDO with GPS reception because the design does not have the AVR MCU connected to the GPS serial TX and RX at all. What you can do to circumvent this limitation is to simultaneously connect the GPSDO and GPS module to a PC using two separate USB serial line interfaces.

There are two parameters that are reported by the u-blox receiver every second, that are easy to monitor and give you a very good idea of the jitter in the PPS* : no. of satellites being tracked and HDOP. My anecdotal experience is that in my STM32 GPSDO with 11 or more satellites tracked and an HDOP<0.9, I get a low enough jitter to stabilize my OCXO within +/- 1ppb (10E-9) or better. IOW under these conditions I achieve my target OCXO accuracy/stability.

Ideally you would plot the no. of satellites tracked and the HDOP in the chart that you posted and it would definitely give you a clear idea of how Lars' PLL loop performs.

Note that you may find that the 32u4 and similarly the original 328P 8-bit, 16MHz MCUs simply do not have enough flash and/or processing power to simultaneously monitor the GPS and run Lars' GPSDO code, using for example the TinyGPS++ library.

*: the jitter in the PPS is really what we are interested in, because it's the noise "floor" that the PLL has to deal with in any PLL-based GPSDO such as Lars' GPSDO.

[W3AXL]

Eventual plans for my version of the code is to have a toggleable "passthrough" mode or something along those lines that would let me run bidirectional serial to and from the UBlox module for programming and monitoring. But you're right in that it may cause issues with the limited program and flash in the MCU.

However for now I can probably hack in a listener for the GPGGA message that spits out # of satellites and HDOP to the running log.

EDIT

Alright, I now have sats and HDOP being logged. Looks like I'm averaging 6-7 sats and an HDOP of ~ 1.2 with this current antenna setup. Probably can't ask for much better than that with my current location.

[AndrewBCN]

...

However for now I can probably hack in a listener for the GPGGA message that spits out # of satellites and HDOP to the running log.

EDIT

Alright, I now have sats and HDOP being logged. Looks like I'm averaging 6-7 sats and an HDOP of ~ 1.2 with this current antenna setup. Probably can't ask for much better than that with my current location.

Well done! 

[Johnny B Good]

@axel

 Assuming your GPS module is a navigation type such as the u-blox M8N that many in this thread are using, the most cost effective upgrade would be an M8T based module (or any of u-blox's earlier timing modules). It doesn't sound like you have the option of installing a rooftop GPS antenna which would likely be an even costlier upgrade than blowing a 100 dollars or more on an M8T based module.

 Your situation is eloquently described as being in an "Urban Canyon" where not only do you have a limited sky view but also the problem of signals being bounced off adjacent buildings which will only serve to reduce the stability of the time pulse. The C/No reports might look good enough but a strong delayed signal is worse than no signal at all.

 You can reduce this interference from reflected signals by increasing the elevation angle filter from its 5 deg default but this will likely cut down the number of good signals to below the 3D fix threshold of 4 required by navigation modules to stay locked to the PPS time pulse long before it can filter out these rogue reflections.

 A timing module can maintain a valid PPS pulse with only a single SV in view once it has been surveyed in and running in 'over-determined mode'. In theory you can raise the elevation filter value to maybe as high as 65 deg but this risks having only a single out of service SV being visible above this elevation angle, leaving the receiver unable to provide a locked PPS pulse until the next working SV comes into view maybe as much as an hour or two later.

 I'm in the privileged position (finally!) of being able to raise my mag mount patch antenna a few inches above the ridge tiles for a full 360 degree view of the horizon where I'd very often observe SVs falling off the edge of the sky plot at elevation angles of -1 degree (on rare occasions, even -2 deg!). I'm using an M8T module and this allows me to increase the elevation angle to 35 degrees (I don't have unwanted signal reflections to contend with) to guard against as many as three out of service SVs becoming the only ones above the elevation filter angle -an  important consideration with a basic hardware only PLL based GPSDO design.

 This, of course, is the very situation where an mcu based design can really shine since it can ride out such short term outages that may well repeat two or more times a day depending on how much of the GPS constellation is in the out of service state. It's not unusual to see the odd GPS SV remain out of service for days to weeks at a time, so if your mcu based GPSDO can routinely cope with this situation, all the better for you.

 Although you can mitigate this problem by using additional constellations (Galileo and GLONAS for example) this does compromise the stability of the timing pulse (GLONAS being the worst offender in this case) so either way, you're going to have problems, less so if your mcu based GPSDO provides good holdover performance.

 Since you're working on an mcu based GPSDO, upgrading to a timing module in your situation would seem to be your best solution imo.

 I've attached some screenshots for you to compare. I don't often bother to monitor the GPS with u-centre since I have to trail a 5 metre USB lead across my hobby room floor to reach the test bench behind me from the PC desk. They represent the past 2 1/4 hours u-centre run time.

[W3AXL]

Good info!

Here's a question - are the NEO-M8T modules pin-compatible with the NEO-6Ms? I'm using the little 5 pin breakout boards with 6M modules, and I'd love to keep that form factor since I've kinda designed the whole board around them. From the datasheets it looks like almost all the pins are the same. Only differences I see are some extra functions on the M8T that are reserved on the 6M, and the SPI & UART are combined. But otherwise I think I might be able to get away with a chip swap, assuming the little modules do well with a hot air gun.

I can get used M8T modules from China on cutout boards for not too much, so I'd like to go that route if possible since I'm trying to stick with the "barebones budget" approach.

[thinkfat]

Are you sure those are NEO? I know one eBay vendor who has LEA-M8T modules on cut-outs and I have in fact bought quite a big number of them in the past, but they are LEA and will not fit a breakout board for NEO modules. I haven't found anyone selling NEO-M8T modules in quite a while.

EDIT: Okay, I found the vendor of the NEO-M8T modules, it's the same one I bought the LEA-M8T from. Hint: try haggling for the price, especially if you plan to buy in bulk

[AndrewBCN]

In a DIY GPSDO such as Lars', upgrading to a timing module is simply a waste of time and money.

Please check this very interesting article which compares the performance of various u-blox receiver modules.

https://hamsci.org/sites/default/files/publications/2020_TAPR_DCC/N8UR_GPS_Evaluation_August2020.pdf by John Ackermann N8UR

What the data in the article shows is that all the tested u-blox GPS receiver modules perform more or less the same, when they are used to provide a 1PPS signal in a stationary GPSDO application.

Under ideal conditions (very good rooftop antenna), the PPS from the timing modules have one extra bit of resolution compared to the non-timing modules, and I doubt one extra bit would make any difference at all in a Lars' GPSDO. As far as I can tell Lars himself did not see the need for a timing module.

[thinkfat]

I agree, mostly.

If you're not doing "sawtooth correction", the noise from the GPS is anyway mostly getting drowned by the aliasing, and if you have a perfect view of the sky, little can be gained from using a timing receiver. But, it doesn't hurt, either. So if you have an opportunity to get a timing receiver like an M8T, go for it.

[thinkfat]

Good info!

Here's a question - are the NEO-M8T modules pin-compatible with the NEO-6Ms? I'm using the little 5 pin breakout boards with 6M modules, and I'd love to keep that form factor since I've kinda designed the whole board around them. From the datasheets it looks like almost all the pins are the same. Only differences I see are some extra functions on the M8T that are reserved on the 6M, and the SPI & UART are combined. But otherwise I think I might be able to get away with a chip swap, assuming the little modules do well with a hot air gun.

I can get used M8T modules from China on cutout boards for not too much, so I'd like to go that route if possible since I'm trying to stick with the "barebones budget" approach.

The hardware integration manual for the NEO-M8 series has a migration guide (chapter 3.2)

https://www.u-blox.com/sites/default/files/NEO-8Q-NEO-M8-FW3_HIM_UBX-15029985.pdf

[Johnny B Good]

@axel

 I'm afraid I can't help you with that question. I did briefly look into that aspect (footprint compatibility) between the navigation and timing variants about two years ago when I was searching for an economically priced timing module, preferably on a daughter board rather than just a bare module. I wasn't up to speed with designing and laying out smd boards back then. I'm still not. I've only just recently bought myself a cheap hot air rework station as a prelude to taking this next step.

 After some 6 to 12 months of only finding 2nd hand overpriced boards blessed with timing modules being offered for sale on Amazon and Ebay, I chanced upon an Amazon dealer who happened to be offering a "GY-GPSV3-M8T NEO-M8T GLONASS GNSS GPS Antenna Module" exactly like the one pictured here:

https://www.amazon.co.uk/Antenna-Module-GY-GPSV3-M8T-NEO-M8T-GLONASS/dp/B07VWLMS2R/ref=sr_1_3?keywords=GY-GPSV3-M8T&qid=1636382803&sr=8-3

https://tinyurl.com/yk639d2p

 Obviously a stock photo since the actual module I bought had a later version and unique serial number.

 What had caught my attention, especially considering I was looking at a dealer on Amazon, was the very low price of just over 41 quid for a complete ready to go module. Since the dealer had fully described its timing feature set, I took a chance and ordered one. I'd only ever paid a maximum of 22 quid for a genuine NEO M8N Arduino/RPi module so this represented a considerable investment in what might turn out to be yet another fake.

 It duly turned up and proved itself to be exactly as advertised, making it the bargain of the past two years. However, that accolade swiftly shifted to the next two M8T modules a couple of weeks later when that very same dealer decided to reduce the price to just over 24 quid each.

 Despite some slight misgivings I'd had over a possible minor firmware bug in the original (a weird issue with the FTD232 module I was using in my GPSDO as it turned out), I immediately snapped up another two before the price soared into the stratosphere (as indeed it did a fortnight later when he started asking 61 quid each). Just for once, I had touched lucky in my search for a cost effective M8T module, made all the more amazing by the fact that it had come from a seller on Amazon, famed more for exorbitant rather than bargain pricing.

 Unfortunately for most of us hobbyists who like to build 'on the cheap', the pricing of anything with a GPS timing module has gone the way of those 20 dollar drone navigation units sporting LEA6T modules for some unfathomable reason about 5 or 6 years ago once the sellers discovered the real reason why they were selling so much better than the later 30 dollar "superior" M8N versions.

 In jewellery shop terms, it would have been the discovery that they'd been selling silver plated gold bars for their weight in silver, hence the dramatic 100 dollar price increase on the older "inferior" (from a navigation performance PoV) LEA6T versions.

 If it weren't for the fact that I'd lucked out some 13 months back and was only now looking for a 'cheap' timing module, I'd snap up that 90 quid module without batting an eyelid. It's amazing how much two years of upgrading to better T&M kit and some 3 or 4 grand's worth of investment can effect such a radical "attitude adjustment" with regard to the cost of such components. 

 Given your situation, investing in a timing module, however you implement it, seems a no-brainer option in this case.

 Just to satisfy anyone's curiosity, I've attached another three u-centre screenshots to reveal an almost complete sky plot survey.

[MIS42N]

I have been using VisualGPS to view NMEA data. I'm looking at the u-center plots and wondering if they give more or less information. The presentation is more flashy, but that's not necessarily a good thing.

[W3AXL]

I've now got a 26 hour plot of a whole bunch of parameters, including new HDOP and # of sats.

Looks like the moral of the story is, with my current location I can't expect amazing regulation of the TIC. Seems like most of the time it's within +/- 50ns but occasionally gets out to +/- 150ns. The spikes in TIC value seem to correspond pretty well to the lowest number of satellites in view, so I'm guessing that's definitely the root cause.

I think this will probably be "good enough" for now. I don't expect to be in an apartment much longer (fingers crossed) but this should get the test equipment I've got today reasonably locked.

[Johnny B Good]

@MIS42N

 If you're using a u-blox gps module, you can try u-centre for yourself (I think it only works with u-blox receivers BICBW).

 What VisualGPS calls the survey window, u-centre calls its version the deviation map. Right now, since my M8T has long since been 'surveyed in' all it shows is a rather boring single dot in the centre which is why I no longer include it. Back in the days before I acquired my M8T modules, it offered a bit of idle entertainment that I referred to as "The Dance Floor" (effectively a 5 by 5 metre patch of ground).

 Since it included my location co-ordinates, I either arranged it so the C/No reports window occluded this information or else edit the screen capture image to blank it out so I don't miss it one little bit since it saves me having to deal with keeping my co-ordinates hidden when attaching these images to any postings I might make here.

 The sky map plots show the same data but in different ways, I prefer u-centre's colour patchwork scheme but I guess it's what you get comfortable with. There are plenty more display options including histograms and so on but I'm only showing the more informative and visually appealing such as the last 20 seconds signal C/No history on the right hand side with the clock showing the date and time (using the illogical yankee date format, I've just noticed BTW ).

 The windows I'm showing are all I need to see the status of the SVs at a glance. Incidentally, I'm running an ancient version of u-centre since the latest versions as of a couple of years ago aren't compatible with winXP (running in a Vbox VM under Linux Mint) and, although later versions had retained winXP compatibility, u-blox (bless their cotton socks) don't archive these previous versions... at all!  and I wasn't able to track down alternative archive sources to get my hands on the last winXP compatible version they'd created so I'm stuck with version 8.16 for the time being.

 The only problem with v8.16 is that it makes it very difficult or impossible to flash update the NEO M8T's firmware (I wasn't able to figure out which was the case). Luckily for me, I have no urgent need to reflash my M8Ts right now but that could all change at some point in the future. I suppose I could always install a later version of windows in a VM in the event that such a requirement ever arises, so I'm not overly concerned of what may or may not happen in the future.

 I've just searched for older versions of u-centre and came across a request for version 8.25 in a u-blox customer forum where he'd been given a link to https://www.u-blox.com/en/product/u-center#tab-documentation-resources and advised to scroll down the list until he found what he wanted.

 I followed the link and scrolled down the list until I saw a "show legacy documents" button which revealed a long list of vista to win 10 compatible versions v19.02 through to v21.09 with this last one having been revised as recently as Oct 2021 with version 8.16 identified as the last winXP compatible version. This is at odds with the implied existence of a v 8.25 and my own vague recollection that the winXP versions had extend all the way to v18.xx.

 I do have an installation CD for the later version of Vista which wasn't the omnishables of the initial release (either that or the original and a service pack update that made it tolerable enough to use) so I suppose I could try installing it into a VM for a giggle. I've already downloaded all those versions for possible future use.

 It just seems rather odd that u-blox seem to have removed all winXP compatible versions later than 8.16. With that request for version 8.25, I feel pretty sure now that I had actually seen mention of even later winXP compatible versions rather than just imagined I had. At the end of the day, if push comes to shove, I suppose I can always try setting up a Vista VM to get round this issue.

 I've added the last 7 screenshots. The first shows a completed sky survey plot with the rest showing the various combinations of data that can be applied to the sky plot.

PS I almost forgot to mention that the first image shows in the history window (RHS) GPS13 reporting an elevation angle of -1 just a few seconds before it dropped off the edge of the plot. This isn't unusual and If I care to observe such stats on SVs about to drop off with a -1 deg elevation, I'll sometimes see a final report of -2 deg just seconds before it disappears completely.

[thinkfat]

I've now got a 26 hour plot of a whole bunch of parameters, including new HDOP and # of sats.

Looks like the moral of the story is, with my current location I can't expect amazing regulation of the TIC. Seems like most of the time it's within +/- 50ns but occasionally gets out to +/- 150ns. The spikes in TIC value seem to correspond pretty well to the lowest number of satellites in view, so I'm guessing that's definitely the root cause.

I think this will probably be "good enough" for now. I don't expect to be in an apartment much longer (fingers crossed) but this should get the test equipment I've got today reasonably locked.

This is where a timing receiver can be beneficial, since it can derive time even from one visible satellite after you have given it the position of the antenna.

[MIS42N]

I've now got a 26 hour plot of a whole bunch of parameters, including new HDOP and # of sats.

Looks like the moral of the story is, with my current location I can't expect amazing regulation of the TIC. Seems like most of the time it's within +/- 50ns but occasionally gets out to +/- 150ns. The spikes in TIC value seem to correspond pretty well to the lowest number of satellites in view, so I'm guessing that's definitely the root cause.

I think this will probably be "good enough" for now. I don't expect to be in an apartment much longer (fingers crossed) but this should get the test equipment I've got today reasonably locked.

I find your plot a bit mystifying. The HDOP figures are not good, anything over 5 is regarded as very marginal. Yet the bad HDOP doesn't correspond to minimum satellites or to the larger tic figures. I think there's other factors here, probably reflections. Not worth investigating if you are moving soon, if it works don't fix it.

[AndrewBCN]

I've now got a 26 hour plot of a whole bunch of parameters, including new HDOP and # of sats.
Looks like the moral of the story is, with my current location I can't expect amazing regulation of the TIC.
...

All DIY GPSDO designers/builders/users eventually reach a similar conclusion that a GPSDO's performance essentially depends on good, stable GPS signal reception, which in turn essentially depends on antenna placement and a clear view of the sky.

...
I think there's other factors here, probably reflections. Not worth investigating if you are moving soon, if it works don't fix it.

Reflected signals will definitely throw off the GPS receiver calculations and translate into higher HDOP figures and much higher PPS jitter. And of course the best way to avoid reflected signals is to have a proper antenna placement with a clear view of the sky, so that the GPS receiver can prioritize the stronger signals from the satellites at a higher elevation to reach a more accurate timing solution for the PPS.

[Johnny B Good]

I've now got a 26 hour plot of a whole bunch of parameters, including new HDOP and # of sats.
Looks like the moral of the story is, with my current location I can't expect amazing regulation of the TIC.
...

All DIY GPSDO designers/builders/users eventually reach a similar conclusion that a GPSDO's performance essentially depends on good, stable GPS signal reception, which in turn essentially depends on antenna placement and a clear view of the sky.

...
I think there's other factors here, probably reflections. Not worth investigating if you are moving soon, if it works don't fix it.

Reflected signals will definitely throw off the GPS receiver calculations and translate into higher HDOP figures and much higher PPS jitter. And of course the best way to avoid reflected signals is to have a proper antenna placement with a clear view of the sky, so that the GPS receiver can prioritize the stronger signals from the satellites at a higher elevation to reach a more accurate timing solution for the PPS.

 Which was the whole point of my suggestion to use a timing grade receiver such as the NEO M8T to achieve this goal with minimal risk of total loss of timing lock.

 The problem with using a navigation only grade receiver in cases like this is that if you increase the elevation filter angle too much, it will lose the timing signal as soon as the SV count drops below 4. The timing grade receivers will carry on generating a valid PPS right down to the moment the SV count drops to zero, hence the suggestion in this case, that this would be a relatively cost effective solution.

 Even assuming axel could get permission to install a rooftop GPS antenna (or install such by stealth) I'd be very surprised if the costs alone of the feeder and antenna and mounting hardware (and possibly a line powered LNA module to boot) would come to less than the price of a NEO M8T.

 Anyway, regardless of my opinion on this, I followed the link you gave a little earlier to that N8UR article and had a quick skim through it before downloading it only to discover that I'd already downloaded it back at the end of August. However, whilst looking in that folder of useful GPSDO related articles, I spotted an earlier thesis paper I'd download back in February by Joseph Paul Gauthier, entitled "Achieving Synchronization and Syntonization Using GPS Receivers" which covers this issue of GPS reception in more detail. After a DDG search I managed to locate a downloadable copy here:

http://unsworks.unsw.edu.au/fapi/datastream/unsworks:11967/SOURCE02?view=true

 So, yet another pdf worth looking at for anyone who hasn't already managed to add it to their GPS and GPSDO related document collection. 

[W3AXL]

Two NEO-M8T modules are on the way, slow boat from China. Knowing my luck they'll show up a few days before I'm moving out...

[Johnny B Good]

Two NEO-M8T modules are on the way, slow boat from China. Knowing my luck they'll show up a few days before I'm moving out...

 You can rest assured that they won't go to waste. They'll still offer an improvement over the M8N even where the antenna has a clear all round view of the horizon. Besides which, if that's how it pans out, consider it a minor investment to have Sod's Law work in your favour for once. I can't imagine you'd want to delay your move any more than you can help it.

[MIS42N]

Even assuming axel could get permission to install a rooftop GPS antenna (or install such by stealth) I'd be very surprised if the costs alone of the feeder and antenna and mounting hardware (and possibly a line powered LNA module to boot) would come to less than the price of a NEO M8T.

I put the GPS receiver on the roof, with a line driver into Ethernet cable. Sending the 1pps and NMEA data down the cable with power going up. Only a handful of parts, about $5, plus Ethernet cable. I haven't measured the distance, about 12 meters I'd guess. I've tested this with a 20 meter cable and it works. I am worried it will get fritzed by lightning but so far no problem after several storms in a period of over 6 months.

To be precise, the antenna is on the roof and the receiver just under. I recently bought a (probably fake) neo7 with a patch antenna, with the intent of mounting in a waterproof enclosure (probably plastic drainpipe) to see if I can save the cost of an active antenna. Previous tests swapping modules with patch antenna (a 5 year old neo6) with an active antenna setup showed a difference in signal strength but no noticeable change in performance.

[W3AXL]

Got the laser cut end panels made for the GPSDO. Enjoy some photos!





Plus a bonus 28ish hour plot. I upped the gain to 100 which seems to have pulled in the regulation of the TIC even better.