Anyone familiar with LEA-M8F? - Page 1
Forum section: electronics / projects

author: mojoe
author_rank: Frequent Contributor
timestamp: February 29, 2024, 02:36:46 am
content: I just got one of these modules on a board. It seems to work with u-center and Lady Heather. I'm still reading through the documents that I downloaded, but my understanding is that this module is essentially a mini-GPSDO, with a reference output of 30.72 MHz that is frequency locked to GPS. The internal TCXO is also supposed to be locked to GPS, eliminating sawtooth jitter. There was a previous GPSDO that locked it's internal oscillator to GPS, but I forget which one it was.BTW, that oddball frequency is used for certain cellular systems. It has something to do with audio modulation. In fact, you can divide it down nicely for 96 KHz, 32-bit, 2-channel audio. I guess if you're an audiofool, you could GPS lock your stereoIn u-center, it seems to work very much like an M8T. One curious thing, when I set the TP2 to various frequencies, the ones that look stable, without lots of jitter are the same frequencies that are integer divisors of 24 MHz, not 30.72 MHz. The maximum TP2 frequency I can set is 24 MHz. So, is the M8F using the same TCXO as the M8T, and somehow still locking that oddball 30.72 MHz to GPS? I'm confused.In LH, the M8F looks pretty much like an M8T. However, the device name is shown as "Ublox timing", and not a specific module. The sawtooth error shows as exactly 0.000000 nS. The accuracy is exactly 6.000000 ns (it started out as something over 1000, then dropped to 9 ns, then 6 ns). It just went up to 8 ns, then dropped to 7 ns (I'm indoors, so perhaps that's why it's jumping). Since LH reports sawtooth error from other u-blox receivers, can I assume that this M8F actually has no sawtooth error?One anomaly regarding the reference output: it's supposed to be 30.72 MHz, but I'm reading 32-34 MHz, and it's not stable.Does anyone have experience with the M8F, and can provide some insight? I'll continue to read the documentation.The M8F is also supposed to allow disciplining an external oscillator, with the addition of an external I2C DAC. This is the reason I got this module. If it works, this should make a dandy GPSDO with an external 10 MHz OCXO.

author: mojoe
author_rank: Frequent Contributor
timestamp: February 29, 2024, 03:01:19 am
content: I solved the reference output anomaly. It doesn't like 50 Ohms. When I put a x1 scope probe on it, I got 30.72 MHz. The waveform isn't exactly a sine wave. It's sort of a cross between a sine and a triangular wave.The TP2 output worked just fine, feeding 50 Ohms. Pretty decent looking square waves.

author: SCSKITS
author_rank: Regular Contributor
timestamp: March 24, 2024, 03:32:59 pm
content: Have a few of these parts and tried to use the DAC with success on one of the M8F modules and no results on the other.I am concerned about counterfeits, but the EBAY part worked and the part I received from a 3rd party as new did not.I did not find the DAC controlled OCXO performing as well as I expected, but I did not dig into it any further at this time.The board that worked has an M8F with the DAC. The OCXO was on a second board.The board that did not work had both the M8F, DAC, and the OCXO on the same board.I have to find time to look into the two boards some more.I attached a short plot of the M8F 30.72MHz output.I will try to get a longer plot in the next few days..eded

author: SCSKITS
author_rank: Regular Contributor
timestamp: March 25, 2024, 05:15:19 am
content: Latest plot of the LEA-M8F 30.72MHz output.ed

author: ch_scr
author_rank: Frequent Contributor
timestamp: March 26, 2024, 12:36:06 pm
content: I also got drawn in by the "Sirens call" of these modules. See the kind of "eval board" I've made to mess around with it (DAC still missing in that picture). Came so far to hook the extIN1 to a 10MHz OCXO and the Vtune DAC through it's LPF directly in parallel onto the OCXO's Vtune potentiometer wiper. Some poking around in the settings, but eventually it "pulled it". Some observation: it only started disciplining once I've manually set the "is calibrated" flag. I've also let it do the cal cycle and watched the vtune sweep. It then correctly set the "Hz pro Volt" sensitivity very low (as the pot in parallel keeps it close to center), it took about 15 minutes IIRC. Haven't yet messed with switching the module over properly to the OCXO as it's "disciplining source" or telling it to survey in and then setting it to stationary. Also have yet to used it to measure another OCXO on it's other input. Pretty good fun so far though!

author: SCSKITS
author_rank: Regular Contributor
timestamp: March 26, 2024, 01:08:50 pm
content: I got the bug as well and made a couple boards for testing. Used JLCPCB, only takes about a week to 10 days.The smaller board is just the M8F with the DAC, no OCXO, the DAC has since been removed and placed on the larger board.The small board worked with an off board OCXO but I did not have time to properly characterize it.The large board does not control the DAC properly and seems to be different in configuration than the M8F on the small board.Again ran out of time to investigate the issues. Wondering if the M8F is a counterfeit.ed

author: ch_scr
author_rank: Frequent Contributor
timestamp: May 05, 2024, 01:53:37 pm
content: After a long while, I came back to this, mainly to try out my measuring system.I measured two OCXO's free running (one was still warming up, hence the drift) and then disciplined the lower noise one with the Ublox M8F.Two measurements again, one in "normal" mode and one in "stationary" mode after a quick survey-in.I'm using a "normal" gps puck antenna on the outside windowsill, likely performance would be better if half the sky wasn't blocked by the building.But the survey-in seems to have helped a bit nonetheless. Now I sure hope (and think) my scaling factors are correct... but take this with a grain of salt

author: SCSKITS
author_rank: Regular Contributor
timestamp: May 05, 2024, 02:36:26 pm
content: I have attached plots from a NEO-M9N with a PLL controlling a 5V Piezo Technik OCXO P/N PTOC32227.The NEO-M9N is not a timing module, it is just one of the regular models and I did not do a survey in prior to taking data.I suspect that the result could be a bit better using the LEA-M8F, but I am near the limit of my ability to properly test with the NEO-M9N.I posted a frequency difference plot earlier of the 30.72MHz from the LEA-M8F which was about +/-7E-10.That is much better than the NEO-M9N output (at 8MHz). I use 10KHz output on the PLL board.I am not sure that the extra cost of the LEA-M8F ($105 at Mouser) vs the NEO-M9N ($27 at Mouser) is worth it if an external PLL is used.ed

author: ch_scr
author_rank: Frequent Contributor
timestamp: May 05, 2024, 07:02:30 pm
content: Quote from: SCSKITS on May 05, 2024, 02:36:26 pmHi ed,not sure the latest plot got uploaded? I guess the appeal of the M8F is that it's almost a completely integrated solution, only add DAC and OCXO.The external TCXO/OCXO doesn't have to be 10MHz, either. Regarding the price, there are reasonably priced desoldered modules on Aliexpress, for now.I suspect when they run dry so will the hobbyist interest in these specific modules...

author: SCSKITS
author_rank: Regular Contributor
timestamp: May 05, 2024, 07:35:52 pm
content: Here are the plots

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: May 06, 2024, 09:41:59 am
content: I have some experience of using an M8T vs an M6N, the frequency stability when using these in a GPSDO referenced to a rubidium standard is that using the M8T results in half the short term frequency wandering, compared to using an M6N with the same loop time constants. You can see the kind of stability I am able to obtain at:https://www.qsl.net/g4aon/gpsdo/Beware of "marking your homework" with Timelab, the results can look impressive but I am not sure it is really the best way to measure stability. The better method is to compare the frequency of your oscillator with a high stability reference, such as a rubidium. Using a phase frequency analyser, I can obtain repeatable results day after day.Many seem obsessed with the long term accuracy measured over hours in some cases... the more relevant stability is short term, in the case of being used for frequency control jitter is a better measure, or perhaps measurements taken over a period of 10 seconds to 5 minutes if used as means of calibrating test equipment.Incidentally, the Ublox part number suffix gives a clue as to the intended use, F = frequency, N = navigation and T = timing, although all will do basically a similar job.There are fake modules on eBay, I have a couple of fakes that were cheap and are good enough for playing around with. A much better source of assembled modules is the GNSS Store:https://gnss.store/SJ

author: SCSKITS
author_rank: Regular Contributor
timestamp: May 06, 2024, 11:35:12 am
content: SJ:The plots were made against an HP Z3801 GPSDO.I also have a couple Rubidium oscillators, but have to permanently install them so they can run for a while before use.For my purposes, the PLL type GPSDO is adequate.What do you use for a Phase/frequency analyzer?ed

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: May 06, 2024, 11:51:06 am
content: I use a TinyPFA phase frequency analyser, you can also load alternative firmware into a NanoVNA H4 and use it as a PFA (and swap back again to VNA firmware).I have three GPSDOs, 2 home made (Lars Walenius and G3RUH type), and a Leo Bodnar GPSDO (uses a TCXO and navigation grade GNSS module). My rubidium is an Efratom FRS-C that is ex cellular use, it was a gift from an electronic surplus dealer.Of my four "standards", the poorest stability one is the Leo Bodnar, but oddly enough it is the one that I use the most as it is almost instantly stable within a few seconds from switch-on and can output any frequency up to 800 MHz. It can also be powered from USB, so consumes very little power.SJ

author: SCSKITS
author_rank: Regular Contributor
timestamp: May 06, 2024, 12:25:40 pm
content: SJ:I have a TinyPFA coming in later this week. Looking forward to seeing how it performs.My 3Ghz spectrum analyzer's bandwidth only goes to 10Hz so not low enough for phase noise measurements.I have access to an MXA analyzer but have not had time to try it.If the MXA has the phase noise measurement I will try to compare to the TinyPFA. I am sure this has already been done..I had a Leo Bodnar two output GPSDO for a few weeks last year for a project, it is a nice unit. Unfortunately had to return it when done.It uses a MAX-M8Q with a SKYWORKS SI5328C jitter correction chip.The Si5328C has loop filters from 0.05Hz to 6Hz so it can clean up the UBLOX output.I have been experimenting with the SI5328C to see if I can get similar results. but not quite there yet.ed

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: May 06, 2024, 12:34:27 pm
content: Quote from: SCSKITS on May 06, 2024, 12:25:40 pmEd, I have the dual BNC Leo Bodnar unit. The level of frequency jitter I measured is around +/- 4 parts in 10^10, which doesn't sound much, but is around ten times worse than my home made Lars Walenius unit. Both measured at 10 MHz. I tried powering the LB from both a wall wart phone charger (USB) and from a linear power supply to the DC input socket, both produced the same level of jitter.When you run out of things to test with the PFA, you can have endless fun measuring inductors, capacitors and filters with it configured as a VNASJ

author: Marc_H
author_rank: Newbie
timestamp: January 03, 2025, 04:48:30 pm
content: Hello everyone,very amazing projects! I played around with some ublox gps receivers like the LEA-M8T and wanted all the time to build a gpsdo but never finished the project.But today i found the LEA-M8F-0-00 on aliexpress for under 20€https://de.aliexpress.com/item/1005006898887014.htmland bought 2.i have a used rubidium cell tower standard here but this is far too good to wear it out by permanent use.Hope the LEA-M8F-0-00 modules arrive in the next two weeks then i can tell if its a fraud or a good deal.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: January 03, 2025, 05:02:32 pm
content: From the Ublox datasheet, the attached are what real ones look like. They are 5 x the cost of the Chinese ones ($105) on AliexpressSJ

author: SCSKITS
author_rank: Regular Contributor
timestamp: January 03, 2025, 07:46:42 pm
content: The LEA-M8F is somewhat unique for the UBLOX modules. It's time pulse output is from 0.5Hz to 2Hz where others can exceed 10MHz.. It also has the ability to drive an external 12 bit or 16 bit DAC to control an OCXO. The OCXO output is connected to one of the EXT-IN pins to complete the loop. I tried the LEA-M8F a while back but I did not find it to be significantly better than others I have used. Now using the NEO-M8N, NEO-M8T, and LEA-M8T that are available on EBAY for under $30.

author: Marc_H
author_rank: Newbie
timestamp: January 04, 2025, 12:54:06 pm
content: Quote from: Solder_Junkie on January 03, 2025, 05:02:32 pmYes the product page is all ready offlineThe missing logo is a good hint to but i thought it was Photoshop because the upper one had the Trade Mark R on it on the right position.Let see ... i have a tracking-number ...I bought my first M8T even on aliexpress, sold as new but was a unsoldered one. it works quiet good up today.Quote from: SCSKITS on January 03, 2025, 07:46:42 pmThe direct control of the 10/26 MHz OCXO via DAC is what i am looking for. Before i bought the first M8T i hoped (Clear case of RTFM) that it was able to control the OCXO.The frequency output of the M8F is not a problem for me i just want the the stable 10MHz from the OCXO.What DAC do you use? My plan is to use the 16bit DAC8571 as told in the datasheet and possibly combine it with the ADR1001AEZ to have my voltage and frequency "standard" in one box.

author: SCSKITS
author_rank: Regular Contributor
timestamp: January 04, 2025, 02:42:32 pm
content: I did the same, used the DAC8571 called out in the data sheet.

author: G0HZU
author_rank: Super Contributor
timestamp: January 04, 2025, 03:11:03 pm
content: I'm probably going to have to get a GPSDO this year because the local BBC 198 kHz transmitter at Droitwich is going to be turned off soon.Over 30 years ago (!) I designed and built an offair standard that locks a 10 MHz oscillator to this transmission.A while back I upgraded the discrete crystal oscillator to a commercial OCXO to give an improvement in short term stability. The plot below shows the lock time from a cold start. A lot of this is the OCXO warming up. The offair standard gives really good performance but it can be prone to increased errors if used at dusk or dawn. It works best during the day and at night. However, the addition of the OCXO seems to have largely smoothed out the issues at dusk or dawn.Before I fitted the OCXO it used to lock in just over a minute with the discretely designed crystal oscillator. Now it takes just over 10 minutes but the short term stability is improved by about an order of magnitude now.I have an old Jackson Labs Firefly 1A GPSDO here on loan and used it as the reference in the test below. However, I'm not convinced that it is much better than the offair standard and I think that some of the short term fluctuations will be from the Firefly GPSDO. It only has a tiny OCXO in a DIL 14 package.How does this performance compare to the low cost GPSDO types being discussed? I'd want something that warms up fairly quickly and reliably. I'm not looking to run a GPSDO 24/7. The scaling in the plot below is 0.01 Hz per division.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: January 04, 2025, 03:36:54 pm
content: The fast warmup Leo Bodnar GPSDOs, especially the new small one, are hard to beat in terms of versatility and value for money. Cost is around 130 GBP.If you don’t mind a fixed frequency and want (need?) much better short term stability than 4 parts in 10^10, and can wait up to 30 mins for your home made GPSDO to fully stabilise, then it is great fun and relatively straightforward to make a pretty good GPSDO. Including a genuine timing GPS module, a nice case, and a half decent oven, will push the cost well over that of a ready made Leo Bodnar unit.As I have mentioned previously, my LB unit is used every day… I cannot remember the last time either of my home made units were turned on.SJ

author: G0HZU
author_rank: Super Contributor
timestamp: January 04, 2025, 04:02:35 pm
content: Thanks. I noticed with the JL Firefly 1A that I have to constantly check it via RS-232 to make sure it still thinks that it is locked. I'm using an outdoor active patch antenna and get C/N of just over 40dB on at least 4 satellites. It can usually detect several more at >30dB C/N. However, I think it has some issues because it sometimes reports itself as unlocked.This may be due to the location of the outdoor antenna as it doesn't quite have a full all round view of the sky.I'm not into Allan variance plots or achieving the very best in stability, but I do want something I can trust to deliver good results when averaged over a couple of minutes if necessary. The Firefly 1A sometimes loses lock and drifts up to 0.03 Hz which is a bit more than I'd like.I only use the offair standard a few times a year to check and trim various OCXOs in my RF test gear. Sometimes I won't use it for a year. But I do want something equally as reliable once the 198 kHz transmitter gets shut down. The OCXOs typically drift anything up to 0.1 Hz a year but some are much better than this.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: January 04, 2025, 04:17:31 pm
content: Andy, G4JNT, had a couple of articles in RadCom showing how to make a 60 KHz (MSF) frequency standard. Two of us built them and they work reasonably well. However, the short term stability is only around 1 part in 10^8, we only used ordinary crystal oscillators, maybe they would be better with at least a TCXO.The articles are on his pages:http://g4jnt.com/I have no problem with GPS lock using a magnetic mount “hockey puck” antenna in an east facing window, it is the one supplied with the Leo Bodnar units. Cheaper eBay antennas are not as good. I also have an outdoor antenna from Leo Bodnar (cost around 50 GBP from memory).SJ

author: G0HZU
author_rank: Super Contributor
timestamp: January 04, 2025, 05:30:14 pm
content: Thanks again for the info. I think the antenna itself is OK because I'm using the patch antenna that came bundled with an official Ublox EVK-M8N eval kit. I've also got another commercial patch antenna that works about the same.https://www.u-blox.com/en/product/evk-8evk-m8I haven't done much with the EVK-M8N-0-01 kit yet although I'm more interested in the performance of the 1 PPS reference it produces (for a different project).I think the EVK-M8N has a TCXO inside but I haven't played with it much yet. I've made an interface connector for it and confirmed that it produces a 1 PPS signal. I also have another Ublox module here and I think it is based on the TIM-LF module. I've got this for 1 PPS research as well.

author: SCSKITS
author_rank: Regular Contributor
timestamp: January 04, 2025, 05:51:58 pm
content: If you are using a patch antenna outdoors, you should upgrade to one of the many cone shaped GPS antennas available on EBAY. They also have a built in LNA. I switched to one years ago and it works much better than any patch I had. I just mounted it off of the deck and it has a good view.