nRF52840 2nd and 3rd harmonic out of spec

RE: nRF52840 2nd and 3rd harmonic out of spec

Johannes — Fri, 31 Aug 2018 13:41:39 GMT

Okay, then everything seems in order. Thank you very much for the insight! It'll really help us in our search.

Best regards,

Johannes

RE: nRF52840 2nd and 3rd harmonic out of spec

Andreas — Fri, 31 Aug 2018 13:39:51 GMT

Hi,

Sounds reasonable, output power is to be measured with peak detector. Measured with a probe cable on a DK you can probably account for at least 1 dB loss.

Best regards,

Andreas

RE: nRF52840 2nd and 3rd harmonic out of spec

Andreas — Fri, 31 Aug 2018 13:27:35 GMT

Hi Johannes,

The devkit should perform according to spec, yes. There will still be some variation as you say, do not count on every board hitting the typical dead on.

In 0dBm 2H is typically ~-70dBm, 3H ~-60dBm conducted.

Best regards,

Andreas

RE: nRF52840 2nd and 3rd harmonic out of spec

Johannes — Fri, 31 Aug 2018 12:33:22 GMT

With RMS detector the overtones do come under spec, but the carrier wave is well under the 8dBm I configured. With peak detector the carrier is about 6.5dBm. Is this expected behaviour?

RE: nRF52840 2nd and 3rd harmonic out of spec

Johannes — Fri, 31 Aug 2018 09:31:44 GMT

Hello Andreas,

Yes, the devkit is without a frontend. It has a sticker on it saying "nRF6828 rev 1.0.0 2018.21".

We're currently investigating all possible sources, and yes, the frontend is high up on our list of suspects, as is production variances in our matching network.

But regardless, the devkit should perform according to spec, right?

I'll redo our measurements with RMS detector and update with the results.

You mention that the harmonic generation is not as agressive on lower power settings. We're also operating on that assumption, but it would be nice to have some characterization or specification of how much it drops on lower output powers.

Best regards,

Johannes

RE: nRF52840 2nd and 3rd harmonic out of spec

Andreas — Fri, 31 Aug 2018 08:54:45 GMT

Hi,

Both plots are measured on a devkit without the external PA, right? The harmonics should be specified using RMS detector, as specified in the FCC DTS measurement guidance, this should drop the reading to somewhere closer to the spec.

The source of the harmonics is the RF PAs, in order to attenuate them you should take care when designing the external matching network and tune the components as needed. For the module with the external PA, the external PA is likely to be the dominant contributor as it is higher power and the nRF52840 should be operating in a lower power setting where the harmonic generation is not as aggressive.

Best regards,

Andreas