Weird adjacent channel performance of the SX1268 LORA Receiver

[tony_lth]

Hi, Gurus,
I found one weird adjacent channel performance of the SX1268 LORA transceiver.
SX1268 datasheet as attachment "SX1268_V1.0.pdf".
The test setup as "Test_Setup1.lpg", the blue circle is the TX, and the red circle is RX board developed by us.
TX parameters: 484MHz, +10dBm, LORA SF5, PL=55, BW=500Khz,
At CH28, i.e. RX_Freq=484MHz, the RX performance is normal. As attached picture "CH28_RX.jpg", the RX level is about -30dBm.
One channel is 500KHz spacing.
But at CH24, the noise floor level is -70dBm, it should be -100dBm, so it's weird. As attachment "CH24_weird.jpg".
Only when another TX at CH24/482Mhz, and the RX signal level is more than -55dbm, then the RX CH24 performance is normal, as "CH24_normal", the noise floor level is -100dBm.
That is so weird, anyone has the solution?
Best,
Tony Liu

--- Updated Jan 27, 2021 ---

Attached "LORA_SF5_SA.jpg" is the spectrum analysis picture when TX at 490M/+17dBm. It can say that the noise floor @1MHz is -42.43dBm, if add propagation loss 30dB, the noise floor is about -70dBm. Is the aforesaid weird RX performance related to this spectrum floor?

 

[vfone]

I am not familiar with LoRa modulation, but did you try using the chipset in FSK mode (467kHz BW for example)?

In the datasheet is mentioned: "An optional pre-filtering of the bit stream can be enabled to reduce the power in the adjacent channels, also dependent on the selected modulation type."

All the channel filtering is done digitally inside of the chip, so may be a programming issue.

 

[tony_lth]

Hi, vfone,
Thanks a lot.
I tried GFSK mode, 484MHz, +10dBm, 100Kbps, BW=467KHz, PL=16, Fd=10KHz, BT=1,
and found the noise floor of CH26 (@-1MHz) is -95dBm, and CH24(@-2MHz) is -100dBm.
So did this verify that pre-filtering don't work?
Best,
Tony Liu

 

[vfone]

Could be a glitch in the programming of this pre-filtering. Maybe ask the vendor if has any idea how to fix this issue.
Looking to the datasheet, the difference that I see when using LORA_BW_500, this is the only setting that use IF=0. From this could be the starting point finding the problem.