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Precision clock - Sync, but second of and to slow
reckter Posted: 14 Jul 2022, 11:28 AM
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Hey,

My clock can get a GPS fix, and then shows the correct time but it doesn't seem to have an active PPS signal. (no blinking of the PPS led, nor the LED's at the front.)
With that it runs a bit slow, and then suddenly jumps 1 s forward.
(compared to time.is).

It's always a bit "in the future", and slowly drifting back, only to jump a second forward.
I now tried the standard and an active Car antenna, both with the same result. (getting a fix inside my home seems to be a bit problematic, but even on the balcony it doesn't improve).


Sometimes it doesn't have the drifting and instead the second jump around like crazy (here's a slomo shot: https://www.youtube.com/watch?v=RVF_52rJ_JE ).

I checked the connection to the GPS module, had some ground pin problems but even after correcting that, the issues continue

Last edit by reckter at 14 Jul 2022, 11:38 AM

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mit Posted: 14 Jul 2022, 01:40 PM
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yeah whatever

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Hello,

If the PPS isn't blinking, and the colons on the front aren't blinking, then it does not yet have a GPS fix.

The module will output the approximate time as soon as it picks up a single satellite, but the time will be off by a few seconds until it gets a fix.

The reason the time jumps around is because until the PPS starts pulsing, the oscillator on the attiny is undisciplined, and from power-up will be only approximate. Once the PPS starts pulsing, the firmware adjusts the oscillator to track the GPS so that the clock ticks perfectly.

I test every GPS module before shipping, so even with the stock antenna it should be able to get a fix, at least outside, within a few minutes.

I recently changed the firmware so that it only blinks the colons if it has a full 3D fix (data valid). Under some conditions, it can have a PPS pulse before then, so you might see the PPS blinking before the colons blink. (This is specifically to check for the leap seconds offset, so it only blinks the colons if it is certain the time is correct.)

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reckter Posted: 14 Jul 2022, 04:25 PM
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Then I will have to do some (literal) field testing and go out into a field and test there. It might just be that the House I am in has too much concrete in it.

Will report back, thanks clarifying the PPS interaction!

Reckter

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reckter Posted: 14 Jul 2022, 05:46 PM
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Turns out: Concrete with steel in it is bad for GPS.
I put the antenna over the balcony and now it can get a fix in seconds.

I need to find a new place to put it now, but at least it all works!
Thanks for the help :D

Really happy with the clock otherwise, great write up here!

Which my building had better reception, oh well. (One Idea would be, to get a really long antenna kable (like idk 10 - 15m) and try that), I fear that the distance would throw it of though, maybe I try that one day :D


cheers, Reckter

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mit Posted: 15 Jul 2022, 11:51 AM
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yeah whatever

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Good to hear it works, it is annoying how some buildings are really opaque to GPS.

I have had good success with a 5m SMA extension cable. There will be attenuation with a longer cable, but the stronger signal from having the antenna outdoors normally balances it out. The cable needs a 50ohm impedance, but I've heard that getting a physically thicker cable (with larger diameter) will have lower losses.

For a very long extension, it makes more sense to move the GPS module, extending the 5-pin header instead of the antenna.

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