nRF52840 matching network for pin 31(ANT), matching circuit needed for regulatory requirements, EMC

[bjornshnrf]

Hi,

We are close to going into a certification process but before we start we need to be sure we are not missing out on your recommendations with regards to EMC performance.

Is the radio matcing network needed for other purposes than matching towards 50ohm? Is it needed for regulatory approval? 

Can you share the impedance at pin 31 (ant port) of the nRF52840?

Thanks,

Regards

Bjørn

[bjornshnrf]

Hi,

Thank you for your answer.

Can you tell what specific spurious you was challenged on that triggered the design of current recommended nRF matching network?

The suggestion you gave hint that you needed a notch at 6GHz. 

I can read up om the standard to check if our design is above limits at various frequencies - I will be able to find any spurious with the spectrum analyzer and the GTEM cell i use for desktop RF measurement (for those below roughly 6GHz).

Regards

Bjørn

[Kaja]

bjornshnrf said:

Can you tell what specific spurious you was challenged on that triggered the design of current recommended nRF matching network?

2nd and 3rd harmonic emissions, as listed in the description below: resonant freq. between 2nd and 3rd harmonic. 

Best regards,
Kaja 

[bjornshnrf]

Hi,

We measured the unit in a GTEM cell that should be suitable for up to 8GHz. With 0dBm output power, we could not find any 2nd harmonic and a weak 3rd harmonic with level below -70dBm. (added 13/10: this is per GTEM port and needs to be adjusted . reading up on the EN 610000-4-20 standard for the 250 design, one should ideally measure EMC with 3 vectors - I only used one and this gives not accurate results! )

We made the layout prior to the final recommendation on the matching network and did assume 50ohm into ANT (back in the days). The potted antenna has 50ohm at 2.45GHz. I believe we could gain some dB on power out with the recommended 0dBm output power matching network. 

Regards

Bjørn 

[Kaja]

Did you measure radiated or conducted? 

Best regards,
Kaja  

[bjornshnrf]

Hi Kaja,

I measured radiated but in a GTEM cell - see added comment in my previous post. I selected to keep my "mistake" transparent rather than just delete the post.

This is a bit of a learning curve for me. I am new to the GTEM IEC 610000-4-20 standard. See https://www.npl.co.uk/special-pages/guides/gpg65_gtem

By summarizing the E-field /V/M) vector sum for X, Y and Z, adding variability of the cell, etc. etc. I might end up above the allowed level at 3rd harmonic.  

I will post my findings.

Regards

Bjørn