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SX1262 lots of CRC errors in SF10

Hi

We have observed some strange behaviour of the SX1262 in SF10 LoRa mode (other SF’s work just fine).
Small packets up to ~80 bytes can be transmitted without any issues, but as soon as the packet size reaches ~90 bytes, virtually every packet arrives with a CRC error.
Interestingly, the CRC errors vanish (!) when reducing the TX power from +14dBm to e.g. +1dBm, which is counter-intuitive. We have tested on several radio frequencies and with various distances between receiver and transmitter from ~10m to 2km, always with the same result.

Our settings: bandwidth 0 (125kHz), code rate 1, preamble 10, +14dBm TX power, explicit mode, CRC enabled

Any idea what could be happening here?

Hi Keepcoding,
We had a similar problem and it took us 3 month to find the reason!
At the end we found out, it was the XTAL. There is a very limited number of XTALs that are certified!
See App note: AN1200.59 Page 11!
Most of these Types are hard to find in the net. Try it with the Murata-Types. The special NDK-Type is especially made for LoRa. Don`t try a similar NDK-Type! You will have the same problem.

Good luck!

Yes, this is a known phenomena which is cause by heat dissipation of SX126x.Please refer to AN1200.37- “Recommendations for Best Performance”

AN1200.37

Thanks for your replies.

@ralflobenstein
Could be, but the crystal we chose (NX3225SA-32MHZ-EXS00A-CS02368) has better specs than the one suggested by Semtech. Maybe it is safer then to directly switch to a TCXO.

@hqiu
Yes, we have studied this document before we designed our board and paid special attention to the thermal design.

But I agree that this could be a possible explanation (at least partially). It doesn’t fully explain however why it only happens in SF10.