tcxo crystal should be improve the accuracy

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justinleeyang
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tcxo crystal should be improve the accuracy

Post by justinleeyang » Thu Mar 26, 2020 3:07 pm

hi,

I noticed that dwm1000 uses a normal crystal, if I use a TCXO crystal, I think it can improve some accuracy



marcus
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Re: tcxo crystal should be improve the accuracy

Post by marcus » Fri Mar 27, 2020 9:04 am

Hi!

I looked into this a while ago and from what I understood it's not so easy or useful (depending on the use-case).

The first thing is that TCXOs seems to change frequency in discrete steps which can effect the measurements badly unless you really know how the crystal works and your algorithms take this into account. I've read some posts on the Decawave list where people have tried to model the crystal behavior to be able to get around this. So it seems to be harder than what I initially thought.

The second thing is our use-case for the loco positioning system, where the nodes are powered for long periods of time. For the first minute or so the node temperature will change as the PCB heats up, but after that the temperature is fairly stable from what we've seen. So if you're interested in powering on/off the system and directly doing measurements the temperature will probably effect it but once it's warmed up probably not so much.

The third thing is the algorithm that's being used. For our TDoA3 the clock drift is removed in the calculations, but for other algorithms this might not be the case.

It would be interesting to hear your thoughts on the above points, maybe some of the assumptions are wrong? If anyone has experience in this matter it would be really interesting to hear about it since I'm not at all an expert on this :D

justinleeyang
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Re: tcxo crystal should be improve the accuracy

Post by justinleeyang » Fri Mar 27, 2020 12:13 pm

hi, marcus:

Yeah, from the tdoa sync , we find that when power on the chip, the tcxo warm up about 1min . tdoa2 don't any question.
my doubt is tdoa 3 send a packet about random 30ms, the packet record the tx timestamp, and other anhcor receive and record the rx timestamp, badly, if the two packet time interval is 30ms, when you calc the distance diff with two anchor, I think the clock correction error is large, because the normal crystal, if we use the tcxo, it must also be synchronized once in 100 ms. so I want to konw how to correction use the normal crystal with tdoa3

justinleeyang
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Re: tcxo crystal should be improve the accuracy

Post by justinleeyang » Fri Mar 27, 2020 12:20 pm

hi,

and tcxo is indeed very stable. For example, the distance measurement has a stable jitter, which is generally less than 10 cm. In addition, there will also be a stable absolute deviation. We only need to do a linear correction. If an ordinary crystal is used, a non-linear correction is required. And the correction between each two devices may be different, so the consistency is poor.
the Decawave list where people have tried to model the crystal behavior
please show the list, I will check it. thanks.

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