Lorawan Bad uplink signal

[MViei.1]

I am trying to build a custom board with Nucleo L476 , RFM95 (sx1276) and the ICUBE lorawan library.

I have build multiple borads, connecting the RFM via jumper cables or on a breakout board. I have tried different antennas but the result is always the same:

I have a really bad RSSI/SNR for Uplinks: -115/-5 when the board is close to the gateway, and no signal at all above 3m distance.

On the other hand, when i get downlinks on the board, it is with -60/+7.

I checked the TX power and it is set to 0 ( max).

I also confirmed that it is not the gateway ( other devices work) nor the RFM's.

I took them from working boards and when i put them back there they worked perfectly. (Other boards are based on ESP32)

Any clue what it might be?

[Andrew Neil]

Post your schematics, and some photos of your setup(s).

Is your antenna good, properly tuned, and properly connected?

If any RF switch(es) and/or amplifier(s) is/are used, are you sure they are correctly connected, configured & controlled?

Someone tripped up on that one recently:

https://community.st.com/s/question/0D53W00001ZXmtGSAT/rssi-on-lorae5-vs-nucleowl55jc1

[MViei.1]

Hello,

yes, i tried the standard antenna we have on our older modules and i tried to solder a 8cm cable to the antenna port. Always the same result.

The RFM95 AFAIK does not have an RF-Switch, at least there is no port which would handle it. On the other hand it seems that it does not really switch to TX mode. Like i said. I can just get it to work when the module is <1m away from the gateway and using DR0. Everything else and i do not get any uplinks.

On the other hand the downlinks seem to be fine, did some tests around here and they always arrive.

Here the schematics. The RFM is on the bottom right.

Here some pictures. I tried different RFM on the boards, and also just attached by cables with the Nucleo. The result is always the same.

Any help would really be appreciated, i do not know what else to try

[MViei.1]

I also tried to bridge one of our older boards. When i drive the RFM via the ESP32 on that boars i get messages with around -40RSSI.

When i drive the same RFM via the Nucleo, i oly get -115RSSI and just around 1m range

[Tesla DeLorean]

The SX1276 typically use the PA_BOOST pin to transmit a stronger signal and there is typically an RF mux allowing for connectivity for 2 TX paths, and 1 RX path.

You should perhaps review the ESP32 software/design with more rigor and compare it to what you're implementing with the ST LRWAN code..

[Andrew Neil]

@Community member​ "review the ESP32 software/design with more rigor and compare it to what you're implementing with the ST LRWAN code"

+1 👍

Maybe put a logic analyser on those flying leads to capture what's going on - then capture corresponding traces from corresponding points in the ESP version?

[MViei.1]

Ok the problem seams to be related to PA_BOOST.

Probably because i'm using a RFM95 and not a pure SX1276

Anyway i modified sx1276.c

static void SX1276SetRfTxPower( int8_t power ){
 
...
 
power = 17;
 
 switch( Sx_Board_GetPaSelect(SX1276.Settings.Channel) )
 {
...
}
 
board_config = RF_PACONFIG_PASELECT_PABOOST;
 
..
 
}

It works!

I think that the RFM95 has the PA_BOOST pin hard wired to the RF cirquit. If it stays low during TX the signal gets pulled to low which explains the very poor RF performance.

Thanks for your help here!