[time-nuts] Trimble Resolution SMT GG weirdness

Daniel Ginsburg dginsburg at gmail.com
Mon Apr 22 19:53:05 UTC 2013


Hi all,

I have bought a Resolution SMT GG for me to play with, and build a very simple breakout board.

The receiver appears to be working (in a sense). That is, it powers up, it emits valid TSIP, it was able to survey it's position - a sane one. Trimble GPS Studio is able to recognize it, and extract validly looking data from it. In other words, I don't believe I have botched the module while hand-soldering it.

Now I have hooked it up to a stm32f4discovery board to sync with PPS. That's were the weirdness begins. 

First, the frequency offset of the microcontroller. I use a built-in counting timer in the uC which runs at 84MHz to measure the duration between 2 PPS. What I get is ~84008000 timer ticks between two pulses, which corresponds to about 95ppm offset. While the crystal on the board is of the cheapest variety, I think 95ppm is just too much. Is that correct, and 95ppm offset even for a the most crappy oscillator is not a reasonable value?

Second, the GPS sawtooth. The PPS does exhibit sawtooth, which is expected. But the sawtooth is HUGE. While the spec claims +-15ns, I see more that +-400ns with period about 30 seconds. This is not sane. I must have done something totally wrong.

Something has to be terribly wrong with my measurements. I'd really appreciate any hints on how to find out the cause of the described weirdness.


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