FCC Testing: "Hump" in continuous signal

[josh-m]

Hello,

We're beginning FCC certification next week and I've been creating radio test and DTM versions of firmware that can run on our hardware for the lab. Everything looks good with the DTM example so I think we'll be good for the ETSI Rx blocking test.

On the radio test example, when I put the radio into continuous Tx mode I see the signal on our spectrum analyzer, but there is also a "hump" about 2 MHz above the center frequency. It's down at -40 dBm, so hopefully it will not be an issue. Is this normal or is there a potential issue with our firmware or hardware?

Here is a screenshot, the "hump" is present with both a modulated an unmodulated carrier, the screenshot is from a test with the unmodulated carrier:

Thanks,

Josh

[Andreas]

Hi,

 

I did a quick test with the radio test example, no hump on my side.

You tested this with your custom HW right, is it there with a Nordic DevKit also?

Is there any switching at ~2 MHz on your board, e.g. a regulator? Noise on the supply rail can get modulated into the carrier.

 

Best regards,

Andreas

[josh-m]

Hello Andreas,

Thank you for the response. We think the hump may be some noise around 2 MHz generated by the backlight on our display. Most of us our out of the office this week for holiday but we'll try running some tests with the display disabled when we return.

I did try running the Radio Test example on an nRF dev board, and using the same spectrum analyzer I got a waveform similar to the one you posted (no major spikes or humps), so I'm thinking it is probably something specific to our hardware.

Thanks,

Josh

[Andreas]

Hi Josh,

 

That sounds quite plausible and we see similar things from time to time. I suppose you just have to experiment with various countermeasures if you want to suppress it.

Off the top of my head it should not cause any issues, the main points of concern would be FCC restricted band limits. Your board could potentially violate the 54 dBµV limit (~-41.2 dBm conducted with 0dBi antenna gain), but at channel 39, which is the highest BLE channel (2480MHz) it will fall outside the 2483.5-2500 MHz band and not subject to this limit. Requirement for in-band emissions is <-20 dBc which you have quite a bit of margin to.

 

Best regards,

Andreas