raw lora packets multiple of 5 bytes?

[robert-hh]

@kjm So I made a few spectrum capures using the same set source but three different methods for capturing.
test source: LoPy4 in Lora raw mode, sending 4 bytes at 863 MHz, SF7BW125.
Capture 1: SDR receiver (SDRPlay1a) with spectrum analyzer software
Capture 2: SDR Receiver with GQRX software
Capture 3: Oscilloscope with FFT software.
Capture 1 show sides peaks at +/- 5 MHz. And other peaks from other sources, e.g. at 868 MHz. If the IF setting is changed to Zero-IF, the side peaks are not present.
Capture 2 does not show that, if the center frequency is at 863 MHz. When moving the center frequency to e.g. 860.5 MHz lower level echo appears at 858 MHz. That is typical for these receivers, that they mirror events at the center frequency.
Capture 3: Very low level side bands at +/- 7.4 MHz.

Given that what we see varies with the test tools, I assume these side bands are artifacts created by the test instrument/method.

[kjm]

@robert-hh Unreal Rob! I'm used to artefacts (mainly missing events) on a DSO due aliasing but I haven't had a lot of experience with spectrum analysers. This experience of seeing frequencies that aren't really there is bothersome. But then any instrument beyond a ruler is probably measuring not what you are interested in but some other metric supposedly indicative of what you are interested in, so anomalies are only to be expected I guess. Really interesting that the artefacts vanish at zero IF.

I might get myself a HackRF1 as a Christmas present & see what it has to say about side lobes as a function of bytes sent from the SX1272/1276.

Regardless of lobes/no lobes lora is pretty impressive. I ran a swept frequency source over the top of a raw lora link here on the bench & the link ploughed on regardless. I'm sure if I fiddled the timing I could break the link but I don't much see the point.

Thnx to you & jcaron for your contributions these last few years, always constructive & enlightening.