What is the lowest TX power you have measured using the RFM95W?


(Pomplesiegel) #1

Hello Everyone,

Just like many of you I am using the HopeRF RFM95W module with mcci-catena/arduino_LMIC.

My question to you: What is the lowest current consumption you have been able to achieve while transmitting with this module? I am trying for a low-range, low-power application, so I would like to lower the power basically as much as possible.

I have found that LMIC_setDrTxpow() does not work, as reported by other open issues. Therefore, for testing purposes I hard-coded a value in lmic_us915.c : void LMICus915_updateTx(ostime_t txbeg)

// LMIC.txpow = 30; //old value
LMIC.txpow = 0; //edited value

I believe this is one of the lowest sanctioned transmit power values, but to be honest I’m not sure.

Even after this change (which did lower the power somewhat), I have found that the actual measured full-system current consumption values during a transmit are much more than I would expect: Like 35mA = 115mW @3.3V. This is with the constant overhead of the microcontroller (15mA when on) already subtracted. I know there are system overhead costs and such, but this is significantly more than the stated 25/50mW and numbers like that I have seen thrown around.

Any thoughts on how to further lower these consumption numbers, as well as your personal experiences with the power consumption of the RFM95W + LMIC?

Thank you!


(Grazy) #2

Hi,

In LMIC the minimum TX power programmed value to the chip is +2dBm and maximum TX power is +17dBm and this on PA output. (see https://github.com/mcci-catena/arduino-lmic/blob/master/src/lmic/radio.c#L445)

There is two ouput on SX1276 (the chip on RFM95 module) RFO and PA but I think the RFO output on RFM95 is not usable.

For me when I reduce programmed TX power to +2dBm, total consumption is 48mA (for 60ms) so it must be around 33mA for the RFM only. (I don’t have precise tool to measure)
If you look look at the documentation it says : 87mA when power is set at +17dBm.

I think your current consumption is normal.


(LoRaTracker) #3

Those figures seem about right, the DC power consumed is always going to be significantly more than the RF power output, 3 to 4 times greater probably.

What you really need to know is what the actual power output is by measuring it …


(Pomplesiegel) #4

Got it. OK, so it’s normal for the unit to have only about 33% to 25% efficiency when transmitting?


(LoRaTracker) #5

Sure, and if you take a look at the datasheet for the SX127x you will find it agrees as well.


(Pomplesiegel) #6

Great, thanks so much for the clarification! I’m examining it right now