HopeRF in Narrow Band

Question

I am new in this group, and I would first like to say thanks to this community, I have used the Stack groups to solve many many questions I have, so it is a powerful knowledge place. So thanks for this.

I am using a RFM98PW module, and I would use it in narrow band, 12.5 kHz. I am able to work with it in 25 kHz bandwidth, but when I try to use it in 12.5 kHz, something doesn't work. I tried with 5 kHz Fdev, and 20.8 kHz BWRx,and 19.2 kbps and I need to down to 12.5 kHz, but when I set the Fdev to lower values, I can see it perfectly in the Spectrum analyzer, but the link with other module doesn't seem to get any packets. (Fdev 3.05 kHz, BWRx 12.5, Bitrate 9.6kbps). Does anybody tried it? If so, I will have a deeper look into my settings.

Many thanks in advance Jordi

Welcome to ham.stackexchange.com, and thank you for the kind words! — rclocher3, Apr 19 '20 at 17:14
What's an RFH98 module? I searched the web but I couldn't find anything. — rclocher3, Apr 19 '20 at 17:20
Hi! Thanks for your reply! I did a mistake... The module is RFM98PW, based on RFM98. (https://www.hoperf.com/modules/lora/RFM98P.html) — Jordi NC, Apr 20 '20 at 10:39
Hi Jordi, I edited your question. I hope I didn't change your meaning, but if I did, please feel free to edit again. — rclocher3, Apr 20 '20 at 13:53
Since you mention FDEV, I take it you're using it in FSK mode, not LoRa mode? — hobbs - KC2G, Apr 20 '20 at 18:22

score 1 · Accepted Answer · answered Apr 20 '20 at 22:16

Most likely, the device's crystal isn't stable/accurate enough for such a small bandwidth. Those little modules often have cheap crystals that are specced to be within +/- 50 ppm of correct. At 450MHz that's +/- 22.5 kHz. With two such modules, one's transmissions could be as far as 45kHz away from where the other is listening. Even with AFC magic, that's not going to work to bring in a 12.5kHz-wide signal.

It sounds like yours aren't worst-case, since they agree well enough for 25kHz to work... but they still aren't good enough for 12.5. Most applications of these things use wider modes for this reason.

Thanks for your reply. You are right, this is not stable at all at this small bandwidth. Finally I was able to use it at 12.5KHz BW, modifying the allowed errors in the preamble, but, after several time working I lost most of the packets, . I check with Spectrum Analyzer for some hours and It was a deviation of 2.5KHz in the carrier, so it is not stable at all... — Jordi NC, Apr 24 '20 at 14:56

HopeRF in Narrow Band

1 Answers1