Hi,
I have been using U-blox modules with Neo-M8N GPS which can be configured with the “U-centre” application via an FTDI interface, but they are poor quality and have a small amount of F.M. (3 to 5 Hz) on the PPS output when set to 1MHz.
I have been looking at the SparkFun GNSS Receiver Breakout - MAX-M10S module but it appears to only retain configuration data via battery backup.
I have searched the web for any information regarding adding something like a 24C32A but can’t find any reference to being able to do this?
So.
Is it possible to add FLASH memory to a MAX-M10S and will it program in the same way using the U-Centre as I have been doing with cheap Neo-M8N modules?
Finally is the PPS output from the MAX-M10S jitter free and devoid of low frequency F.M.?
Thanks in anticipation
Dave
I don’t think it exports enough of the right PIO to attach an SPI FLASH externally.
Beitian, and others, have modules that do contain a FLASH memory, the BE-126 is the equivalent to a NEO-M10 type design.
There’s always going to be some modulation, due to the NCO methods employed.
The M8 was using a 48 MHz timing clock, and the M10 is using a 64 MHz one
The NEO-F10N breakout board does contain FLASH. Has a 64 MHz timing clock for the NCO / TIMEPULSE
On the MAX-M10S I’d use OTP to make permanent configuration changes.
HI Clive,
Oh that’s a BIG problem as I was driving a PLL to GPS lock a 1.3GHz receiver.
I was experiencing Frequency Modulation on the resultant carrier which manifests itself as a wavering (in frequency) audio tone when heterodyned with a known ‘pure’ RF signal.
I assumed it was the (maybe fake) U-blox modules on cheap imported GPS modules and decided to ‘bite the bullet’ and buy from a ‘genuine’ source?
So are you saying all the U-blox modules will exhibit this FM anomaly, I’m sure some of the Precision GPS reference clocks have u-Blox GPS engines in them?
I’m confused.
what am I missing?
Dave
It’s a clock that has it’s performance quantified, not disciplined.
The NCO is like a DDS / Phase Accumulator, it clocks at 48 MHz on the M8, resulting in 21ns in jitter as it adds/drops cycles. You can see this if you trigger the scope and leave it in persistence mode. This is a ZED-F9T where the timing clock is 128 MHz
Now GNSSDO typically use a PLL and VCXO to generate a clean frequency output. Often using the 1PPS source, filter that over a longer term, or account for the reported quantization error.
The LEA-M8F uses a 30.72 MHz VCTCXO which it disciplines.
Hi Clive,
Firstly thank you for engaging with me on this, clearly I didn’t think I needed to know about GPS engines in such depth when starting this ‘5 minute’ project..LOL!
I’m still confused about where such a low frequency F.M. is coming from, the nSec ‘jitter’ you have pictured doesn’t seem to tie in with what I am experiencing?
For example when using the M8 GPS module I was aware that one had to choose a PPS output that is an EXACT division of the 48MHz internal clock. I was following a design that had a chart showing the ‘SAFE - NO JITTER’ output frequencies that one should use.
I therefore chose 1MHz to lock my PLL to.
My design does what you recommend i.e. I use a 96MHz VCXO (sine output) divided down to 1MHz and then I EXOR this to the 1MHz output from the M8 GPS module. I therefore didn’t expect that when I take the 96MHz from the VCXO and ‘beat’ it against a ‘pure’ ~96MHz signal I hear a wavering audio heterodyne. (Frequency modulated at somewhere around 3 to 5 Hz). If I take a second pure 1MHz signal and feed it into the EXOR instead of the GPS, then the heterodyne is steady, so the problem is coming from the GPS engine.
I don’t understand how the nS ‘jitter’ you describe manifests in this behaviour?
Thanks
Dave
