Reply To: SX1276 in FSK mode: what AFC, AGC, RSSI and FEI do, and why my links fail on cold mornings

Aoife’s question is the right one. Cold moves the crystals.

The four in RX:

RSSI is how strong the received signal is, read from RegRssiValue (0x11), value / 2 as negative dBm. AGC picks the LNA gain from that so strong signals don’t saturate the receiver; your 0x0E already has AgcAutoOn (bit 3) set, leave it. FEI (frequency error indicator, RegFeiMsb/Lsb 0x1D/0x1E) measures how far the incoming carrier is from where the receiver expects it. AFC uses that measurement to retune the receiver onto the signal, at the start of a packet if AfcAutoOn (bit 4) is set. Yours is off.

A plain crystal is good for maybe ±10 to 20 ppm, and drifts with temperature. At 868 MHz, 20 ppm is about 17 kHz. Probe and receiver drift differently on a cold morning, and once the offset plus your deviation falls outside a 10.4 kHz receive bandwidth, the packet is gone. That’s your dropouts.

RegRxConfig (0x0D) = 0x1E   // AfcAutoOn + AgcAutoOn + preamble detect trigger
RegAfcBw   (0x13)  = wide enough to catch the offset before AFC (e.g. 41.7 kHz)
RegRxBw    (0x12)  = 20.8 kHz after AFC
// or simply widen RxBw if you don't want AFC

Then send the FEI and RSSI of each probe as values next to the moisture reading. In Hermetry a chart of FEI against air temperature shows you which probes drift; ours showed 11 kHz swings on two units before we changed anything.