Setting power on RFM95PW/RFM98PW (High-power LoRa modules)

This message was edited 1 time. Last update was at 10/11/2016 08:56:28

Regarding the 30dBm/27dBm high-power modules now available:

868/915MHz Enhanced Power RF LoRa Transceiver Module

After receiving some of these and testing to be sure they are compatible with RFM95W code (they are), I realized there's no reference anywhere that I can find on setting the power registers for full power output. Libraries such as RadioHead limit the values per the requirements for the RFM95 series, and while I can change that in the library code, I'm not sure what values I can/should use. It may seem obvious, and it probably is what I assume it to be, but that's the point of a datasheet, right? To make clear such important parameters?

The datasheet on the above page is very limited. It seems to cover only the basics and offers no advice on handling the difference between the models.

Help?

If these are like the RFM23BP, the module adds a power amplifier with a fixed amount of gain to the transmit path (and presumably via the PA_BOOST pin; you can probably tell by looking at the blown-up photograph to see which pins are connected). The question is do you configure the SX127x for +17dBm or +20dBm; experimentally you could try both, I suppose - it is not likely to damage the power amplifier. I'm highly interested in obtaining a handful of these modules for evaluation, as well.

Cheers,
Dana

That's probably correct. Set it for 20dBm and PA_BOOST and get the full module output of 27/30dBm at the antenna. We can't measure the RF output here with any precision though, so I don't know exactly what it's doing.

They definitely draw a lot more current during transmit vs. the RFM95W. It wasn't on the scope at the time, but when sending long packets at the 20dBm setting the bench supply was hitting upwards of 450mA. Not to forget - this is not a 3.3V module like the RFM95W. It's 5-6.5V and has an on-board regulator for providing 3.3V to a connected microcontroller. That makes for about 3W of dissipation. Logic is still 3.3V.

One of my team ordered these units. I'm not sure where they came from but they look similar to if not the same as the photos on the product page except it's shielded by a metal enclosure on the component side of the board.

On further thought, it is possible that +20dBm setting isn't required for full output power of this module. Specified output of +27dBm (900MHz) is probably from a device with gain of greater than 10dB and pretty surely more than 7dB. So I'd try setting the SX127x to +17dBm for full output. There's a note in the Semtech datasheet that +20dBm operation is limited to a maximum of 1% duty cycle, too. The 430MHz power amplifier probably has a bit more gain, too. So +17dBm is likely the max power level to run the part at.

Cheers,
Dana