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]

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?

[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?

[Francesc]

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

[Szabolcs]

Could you measure the expected output power with the setup?

[Francesc]

Yes, I verified the output power with both the Test Radio example and our own FW are similar. When configured at +8dBm, I measure a signal of +5.5 dBm on my VNA with both designs.

With this we confirm that the rest of the losses are attributed to the layout of our design.