Ublox LEA-M8T module to discipline an external oscillator - Page 1
Forum section: electronics / projects

author: cncjerry
author_rank: Unknown
timestamp: October 19, 2022, 05:22:06 am
content: I was looking at the Ublox LEA-M8T documentation as I bought a few of these modules a while back. I came across the attached picture, actually there are several examples of how to use the module. If I am reading the doc correctly, you can attach a DAC to the Ublox module pins, then configure the module as to the type of DAC, and it will then drive the DAC to discipline an external oscillator through the oscillator's EFC. Note in the picture that PPS is also coming back into the module. So it looks like the module can do a complete GPSDO in itself with very few components. Or am I missing something? I have to do more reading as I know the module supports fixed timing mode, has sawtooth correction, etc. I also read that instead of PPS1 output it looks like you can configure it to output a signal up to 10Mhz, but again, it will take more reading to understand this completely.Has anyone built a complete GPSDO with this module using only its internal functions? I've seen others that use it with external timing, PID loops etc.ThanksJerry

author: nctnico
author_rank: Super Contributor
timestamp: October 19, 2022, 08:18:26 am
content: You can build a self contained GPSDO using this module, an external DAC and OCXO. Been there, done that. It did take me a bit of time to get the settings right. Using the stationary mode helps to achieve better stability but you'll need to run 'location' survey and keep the data as the survey may take a couple of hours to complete.Higher update rates (> once per second) don't seem to work. Also you have to set phase tracking mode. Frequency tracking (which would be the obvious choice to me) doesn't give good results.If you want to develop this, then you'll need to have at least a 'known good' GPSDO to test against.I don't see why you'd want to use an external PID loop with this particular module. You can adjust the control loop for the OCXO you are using for optimal results.

author: cncjerry
author_rank: Unknown
timestamp: October 19, 2022, 04:17:52 pm
content: I would have thought there would be dozens of GPSDOs around using this module. I have three DOs, two lucent and then another based on Lars' by Jim Harmon. I just really need something that has 1pps as Lady Heather will set the tuning word for my Rb unit, was going to build something using this module to hang on the Rb board and then saw the pictures.ThanksJerry

author: vk3jpk
author_rank: Newbie
timestamp: October 21, 2022, 09:59:55 pm
content: Hi Jerry,I have played with LEA-M8T modules and unfortunately the configuration you show in your post is NOT supported by that particular module - instead you need an LEA-M8F module.Ublox documentation isn't very good at making it clear what functionality is supported on what modules until you dig into the messages needed to implement things. If you look at the UBX messages required to implement the configuration with the DAC controlling an external oscillator, the documentation says it requires a "Time & Frequency Sync" product, and unfortunately the LEA-M8T is classified by Ublox as only a "Time Sync" product.James

author: cncjerry
author_rank: Unknown
timestamp: November 15, 2022, 06:55:12 pm
content: Thanks, I had tried sending it messages to set the higher frequency on pps line without luck. I didn't go much further and glad you posted otherwise it would have taken me a while to find that out.