Output power nRF52840

[Francesc]

I am evaluating the output power of a testboard that uses the nRF52840-QFAA. 

The matching network closely resembles the recommended network in the datasheet (shown in figure below). In my network, C3 = 0.8 pF, and L2 = 2.7nH due to availability of the components in our lab. 

The nRF is configured for continuous transmission on 2.4 GHz at an output power of +8 dBm.

I am measuring using a spectrum analyzer with a 50ohm input directly on the RF connector. 

The measured power is only -5 dBm, which is 13 dB lower than expected.

I understand that the cables could add some losses and that the power spec is +/-4dB, But together it shouldn't add up to 13 dB.

 

I'm also including a screenshot of the PCB. For this test, the capacitor at the output is not populated and a pigtail is soldered directly to the RF output pin to interface with the spectrum analyzer.

I would appreciate any help to identify or verify this measurement result.

[Szabolcs]

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.

[Francesc]

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)

[Szabolcs]

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)

[Francesc]

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

[Szabolcs]

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?