Is anyone using the uBlox M8T? What are you doing with it, how is it working?

[WPXS472]

I recently got a uBlox M8T module, the one that works off USB and has the SMA antenna connector. Neat little module. I was intrigued by the timing accuracy specs. and wanted to compare the timepulse set to 10 MHz against my Trimble Thunderbolt. I hooked some tiny coax across the timepulse LED, and after some frustration, got the timepulse set to 10 MHz. When I compared it to the Tbolt 10 MHz, there was so much sawtooth error, I couldn't really tell much. Changed to 1 Hz, and still the sawtooth error made meaningful observation useless. Has anyone else tried suing one of these as a GPS based source? Did you encounter the same thing I did? How did you solve it, or did you solve it? I am looking at using a Motorola M12+ to see if it is any better. I can't get 10 MHz out of it, but I can look at the 1PPS. I know the M12+ is old and doesn't see all the satellites that the uBlox does, but it has a good reputation. I found that although it has menu items for Bei Dou and Galileo, it won't let you select them. t does see Glonass birds though. Guess I'll have to wait on the highly anticipated ZED9.

[DaJMasta]

The M8T's frequency output options aren't really comparable to a full GPSDO like the thunderbolt - it has an onboard TCXO and some hardware to get various frequencies out of it, but I don't believe it has something like a PLL that is configured to the output frequency or even an analog drive frequency tuning adjustment for a VCO, it's more of a fixed oscillator for the board and then a digital divider which deals with the error in a way that shows you that noise you're seeing.  I believe the M8T will acquire GPS quicker and at lower levels than the trimble, as it has a much more modern receiver, but without the OCXO and disciplining mechanism, it doesn't have nearly the frequency stability or holdover.

I've done a fair bit of testing of PPS output accuracy, and the M12+ is actually one of the better ones[EDIT] Was thinking of the Resolution SMT, the M12+ is reasonable but does show its age[/EDIT], if I remember right (my data's on a different PC, I need to get around to actually posting it), in terms of raw timing error on a given pulse, but all of the uBlox modules perform almost identically for PPS accuracy back several generations (I think I tested an LEA 5T or 6T, several 7 (non T) variants, the M8T, and another?), and while it does stay rock solid in the long term, I think the adjustment step for the output timing is something like 20ns, so while it's always centered around 1s, the noise pattern around it is digital looking and never extremely tight.

If you want really stable frequency output from a GPS based system, you really want that PPS or other frequency output disciplining a VCO of some description, the extra layer of smoothing goes a LONG way in terms of stability and it seems like GPS modules that don't advertise themselves as GPSDOs never include it, even if they're made for "timing".

[WPXS472]

The error I am seeing with the M8T is definitely sawtooth error. It starts at about 0 nS, increases, I forget the maximum I see, it goes by pretty quick, then it suddenly jumps back to zero and starts back up again. Does the same thing with 1 PPS. I have heard good things about the M12+, and I have one. I also have a Resolution T as well as the Resolution SMT. They are both on the list to try. I also have a couple of the old Rockwell boards that put out 10 KHz. I plan on building one of those James Miller boards that divides down 10 MHz from an OCXO and compares it to the 10 KHz. He integrates the phase detector output to reduce the sawtooth error. I understand that the M12+ actually outputs what the expected error of the next 1 PPS pulse will be so you can do a sawtooth correction with software, but I don't want to even think about getting that complicated. The M8T interests me because of the small size, low cost, and the uCenter software. That software lets you do all kinds of stuff. I need to learn a lot more about it. The M8T will do a self survey, but I have never been able to get it to complete one. I think I have the requirements for location too tight. It can plot the variations in location data and I wanted to use that to see if a choke ring antenna had any advantages for timing applications. So far, results have been highly inconsistent. I think a better way to do that might be to compare ADEV plots from Lady Heather for the different antennas.

[metrologist]

What version FW is on those Jupiter T units? I suspect the one I have will not work.

[WPXS472]

I really have no idea. I haven't read anything about them having to have any certain vintage firmware. I'll be the first to admit that my knowledge of the Rockwell units is pretty limited.

[metrologist]

It was the first or first few revs that they did not connect the 10kHz PLL. v1.03 seems to have some meaning to me - either that is my rev or the rev that starts to work...

I read that on the popular site that was selling these, but unsure.

[WPXS472]

One thing I have noticed is that it has an option to do a self survey. You can specify the number of fixes required as well as the mean deviation in meters required to consider the survey complete and go into position fixed mode. I have tried numerous times to do a survey and it never completes. I currently have the mean distance parameter set at 1 meter and it has gone something like 84,000 fixes and still going. Has anyone ever actually gotten a self survey to finish? What figures did you use? Once it is done, does going into position fixed mode actually improve anything?

[LapTop006]

There's also the M8F which is a more complete GPSDO, I've been playing with one for an upcoming project.

[iMo]

Is there any practical experience with the Neo-8 built-in DOSC/ESRC (it can discipline an external OCXO via 12/14/16bit DAC)?

[SoundTech-LG]

Spectracom has shipped newer Securesync, and 9483 time servers with the LEA-M8T module replacing the outdated / obsolete Trimble Resolution T.
There is no comparison folks. The U-blox blows the T away. So many sats, so little time and quite accurate time to boot!
Accept no substitute!

[DaJMasta]

I've found that the Resolution SMT still has somewhat more stable PPS output (better than the T, better than the M8 series), but there is no comparison to the new chipsets in terms of satellite acquisition.  These new modules can average 10-15dB less signal strength for acquisition and can keep track of several times the number of satellites.

[edpalmer42]

Any bare GPS receiver is limited in what it can put out.  The GPS signal has to punch through the ionosphere which stretches, crunches, folds, spindles, and generally mutilates the signal.  The receiver does the best it can to bang out the worst of the dents and does a pretty good job considering what it's got to work with.

A timing receiver does have a few extra tricks up its sleeve.  As mentioned, it can do a self-survey to determine its position with higher accuracy than a navigation receiver.  It can also report the sawtooth error so that it can be corrected later.  There's also TRAIM (Timing Receiver Autonomous Integrity Monitoring) which helps the receiver pick which satellites to use.  All of these help produce a better output.  But about the best you can expect out of ANY GPS receiver is short-term stability of ~1e-8 over 1 second and many (particularly older units) won't be able to reach that.  Over the long term they will still produce an average frequency that is correct i.e. no drift.

Ed

[SoundTech-LG]

Here's a shot of a Resolution T type form factor board from Huawei (off the bay)with it originally utilizing the LEA-M5T, it just now got swapped out with the M8T for installation to one of my Spectracom Securesync / 9483 units. Also a shot of a factory installed Spectracom Ublox.
Spectacular upgrade, even considering the very expensive M8T module from Digikey. In order to do this upgrade to a unit with the old Res T installed, a Firmware upgrade to the EEPROM to recognize it is required.
Getting that from Spectracom was nearly impossible. Lots of whining, and complaining about going inside their equipment, and ruining any warranty.They only want this done with their parts, in their repair dept. Anyone know a cheap source for the M8T