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UAVTracking / UAVTrackingProtocol

An open radio protocol for transmitting UAV flight data in real time.
https://xkcd.com/927/
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Frequency stability should be specified #21

Open pjalocha opened 6 years ago

pjalocha commented 6 years ago

I copy from my "list of thoughts" :-)

Cheap crystals can have up to 100ppm frequency offset and indeed such offsets are observed with the cheap RTL-SDR USB receivers. This is not a major problem, as long as the offset is known and properly corrected in software. From the experience of running the OGN network, one could say the reasonable requirement for the transmitter frequency error would be 10-20kHz. Larger tolerance on the receiver side needs more computational power. As observed most FLARM units and the OGN trackers (RFM69 modules) fit within +/-10kHz without any specific frequency correction. For the RFM69 module the frequency can be corrected in very fine steps, but can not be so for the nRF905 module. For PowerFLARM units a systematic offset of 10kHz has been observed.

optimaltracking commented 6 years ago

Great. What do you suggest to xrite in the final document?

pjalocha commented 6 years ago

I would say 10ppm or 20ppm. Can somebody with knoledge of electronics part say what are the "standard" crystal grades ?

Saying 20ppm is maybe safe, so that if the manufacturer choses a 10ppm grade crystal and takes account for some other inaccuracies in the electronics circuit like tolerances for the capacitors ?

For the more modern RF chips he could use even lower grade crystals but compensate in software for the frequency offset.

20ppm fits well in the OGN SDR receiver which normally has +/-25kHz margin for frequency offset. At 868MHz 20ppm is 17.4kHz frequency error.