Anyone familiar with LEA-M8F?

[mojoe]

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 stereo 

In 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.

[mojoe]

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.

[SCSKITS]

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..

ed

ed

[SCSKITS]

Latest plot of the LEA-M8F 30.72MHz output.

ed

[ch_scr]

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!

[SCSKITS]

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