Output power nRF52840

RE: Output power nRF52840

Francesc — Wed, 06 May 2026 12:23:59 GMT

Wauw.. Thank you. Sometimes something so obvious is hard to spot when you are looking for the error in other places.

RE: Output power nRF52840

Szabolcs — Wed, 06 May 2026 11:32:49 GMT

Hi Francesc,

It seems to me that the cable is connected to the wrong port on the instrument, shouldn't it be on the RF Input?

RE: Output power nRF52840

Francesc — Wed, 06 May 2026 10:59:34 GMT

Hi Szabolcs,

We are testing the FW on a nRF52840-DK, however I'm already running into some issues for the Radio Test example.

Here is what I did:

- I programmed the nRF using the cli "nrfjprog --program radio_test_pca10056.hex --chiperase --verify -–reset"

- The programming was successful, and my PuTTY terminal shows expected behavior.

However, when I configure the tx carrier to start, nothing happens.

I include a screenshot of my putty terminal and a picture of the setup. I'm using a 'MXHS83QE3000' cable to connect the DK to my VNA. As you can see, the VNA (showing 1 GHz - 3.2 GHz) measures nothing above the noise floor, while I'm expecting to see a carrier of +8 dBm.

Any suggestions what could be wrong here?

I'm suspecting something might be wrong with the DK at this point..

Thanks,

Francesc

RE: Output power nRF52840

Szabolcs — Tue, 14 Apr 2026 11:57:17 GMT

The radio is a bit more complicated than that. It is possible, for example, that the VCO starts but the PA is not configured properly, and that's why you see a peak, but at lower level. That is why it is important to use a known-good firmware for radio testing (e.g. Radio Test or DTM sample project)

RE: Output power nRF52840

Francesc — Tue, 14 Apr 2026 10:51:38 GMT

Hi Szabolcs,

Thank you for the reply. The radio is starting, because we see a clear peak at 2.4 GHz as we send the command, though it is lower than we expect. So I'm not sure if the problem identified by your colleague can be the cause?
From what you describe, it seems that the radio wouldn't start at all but that is not the case for us.

(I am also not a firmware engineer, so I need to relay this to someone else as well)

RE: Output power nRF52840

Szabolcs — Tue, 14 Apr 2026 10:34:53 GMT

I just got the feedback from my coworker, you are missing the READY_START short enabled, TXEN by itself is not enough to start the radio. (https://docs.nordicsemi.com/bundle/ps_nrf52840/page/radio.html#register.SHORTS) The IsRunning flag variable is also not handled correctly, so you should review that.

RE: Output power nRF52840

Szabolcs — Mon, 13 Apr 2026 12:21:59 GMT

Hi Francesc,

Your code looks good to me, but I'm not an expert on firmware, so one of my coworkers will also check it. In the meantime, could you please also test your device with the Radio Test sample project?

RE: Output power nRF52840

Francesc — Fri, 10 Apr 2026 15:07:59 GMT

Hi Szabolcs,

Thank you for the suggestions, we will test the firmware on a devkit and improve the pigtail connection.

In the meantime, here is the fw file that we use. It uses the nrf_radio HAL driver from nRF5 SDK v.17.1.0.
Perhaps you can already flag some issues there?

devzone.nordicsemi.com/.../rf.c

RE: Output power nRF52840

Szabolcs — Thu, 09 Apr 2026 10:51:31 GMT

Ok, thank you for clearing things up.

If you haven't already, you should test your firmware on a Nordic DevBoard, and see if you get the expected power output, just to make sure.

The pigtail connection is not very good, the ground connection is not sufficient on these frequencies. You should solder the outer shield at the end of the coaxial cable to the ground very close to the connection point, trying to minimize the length of the discontinuity. Remove the large inductor (you could even remove the pads under it), and short the pads between the capacitors. (Ideally, you could also solder a 2.2nH 0201 inductor between the caps, if you have it, but it's not critical for this test, a short will do.) You can see what I mean in this sketch below:

Do you know the part number of the 4.7nH inductor?

RE: Output power nRF52840

Francesc — Wed, 08 Apr 2026 13:24:41 GMT

Hi Szabolcs,

Thank you for your reply!

The shortest path from the first shunt capacitor to the PAVSS pin is through the ground plane on the bottom layer and coming up to the exposed pad. See the included figure.

I am measuring at point two in your annotated screenshot. (The picture is from the backside of the pcb)

The FW during testing is custom (developed in-house). It configures the nRF in continuous transmission mode at 2.4 GHz with the power level configured at +8 dBm.

RE: Output power nRF52840

Szabolcs — Wed, 08 Apr 2026 12:58:37 GMT

Hi Francesc,

The layout in the picture has several issues. The first shunt capacitor must only connect through PAVSS to the exposed pad, and must not be connected to the top GND pour. I cannot see how PAVSS is connected on your picture, could you post a screenshot in the 2D design view?

There is also an inductor which is way too big, it should be 0201 size and placed inline with the rest of the components. Larger components have more parasitics, and could behave totally differently on high frequencies, even with the same value.

Small deviation from the reference value could cause problems with harmonic emissions, but for the output power, there shouldn't be much difference.

Please see the reference layout. This is how you should implement it:

Just to be clear, at which point are you measuring the output power, in the picture below? Can you include a photo of the soldered pigtail?

Also, what firmware are you using for the test?