[time-nuts] Trimble Resolution SMT GG weirdness

[Azelio Boriani]

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).


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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>
>