[time-nuts] Trimble Resolution SMT GG weirdness

Daniel Ginsburg dginsburg at gmail.com
Mon Apr 22 20:26:13 EDT 2013


On 23.04.2013, at 2:57, Azelio Boriani wrote:

> I agree with Hal and if you have a reference you can tell how much the
> crystal is off. My timing GPS receiver after 1 hour has a total uncorrected
> wander of 112ns. Never seen 400ns on a timing receiver (maybe I haven't
> waited long enough).
> 

No other reference except the PPS from the module I'm playing with, unfortunately.
I wonder, what kind of timing GPS gives 112ns wander? Is it when locked to the signal? If yes, what are the other conditions? If you know what happens to your receiver it might give a clue to what goes on in mine.

> 
> On Tue, Apr 23, 2013 at 12:49 AM, Azelio Boriani
> <azelio.boriani at screen.it>wrote:
> 
>> Have you any reference? A TBolt, Z3801 or similar?
>> 
>> 
>> On Mon, Apr 22, 2013 at 9:53 PM, Daniel Ginsburg <dginsburg at gmail.com>wrote:
>> 
>>> 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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>> 
>> 
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