Meander antenna question when using nRF51822

RE: Meander antenna question when using nRF51822

Jørn — Thu, 11 May 2017 10:24:54 GMT

Keep in mind though the C_pcb is dependent on layout, board dimensions and substrate. If correctly designed it's impact shouldn't be very large, and the dominant capacitance would probably be the XC1 and XC2 pads. The traces of the DK are longer than strictly necessary due to the solderbridges, and the solderbridges themselves would also add some extra capacitance.

RE: Meander antenna question when using nRF51822

MANGO — Thu, 11 May 2017 09:01:40 GMT

Hi, Jørn. the pins connected to the crystal have a capacitance of 4pF alone. Meaning the statement in the blog is incorrect

Really?! That's quite surprising. I always used that equation when designing custom PCBs.

Then, Hmm... I ask questions about the load caps on another thread.

Thanks for your time handling this question.

RE: Meander antenna question when using nRF51822

Jørn — Thu, 11 May 2017 08:49:11 GMT

I have received word from our member of the design team, and while he didn't have access to the specifics of the DK at the moment, due to him traveling, he did point out that the pins connected to the crystal have a capacitance of 4pF alone (page 42 of the nRF51822 product specification). Meaning the statement in the blog is incorrect, and the C_pin+C_board will be larger than 4pF. It is therefore quite possible the actual total is 6pF as you suggested.

RE: Meander antenna question when using nRF51822

Jørn — Tue, 09 May 2017 12:33:10 GMT

My pleasure!

Correct. Both the matching networks and the radio hardware is identical for the nRF51822 and nRF51422.
I have passed the question on to a member of our design team, unfortunately he's travelling at the moment so I do not know when I'll get an answer.

RE: Meander antenna question when using nRF51822

MANGO — Tue, 09 May 2017 12:20:15 GMT

Thanks for your answer, Jørn!

In the product specifications, it is written as "nRF51x22". Just to double check,

the nRF51822-QFAA / nRF51422-QFAA or the nRF51822-QFAC / nRF51422-QFAC use the same RLC components for network matching, right?

I don't have the specifics as to why 12 pF was chosen in this particular instance. I will need to look further into it and get back to you on that.

I see. Take your time. I hope to hear the news, if possible.

-Best Regards, MANGO

RE: Meander antenna question when using nRF51822

Jørn — Tue, 09 May 2017 11:40:37 GMT

Hello MANGO

The chip on the nRF51 dongle is the nRF51422-QFAC. This package is a little larger than the QFAA and QFAB packages and the matching network is therefore a little bit different to compensate. For the correct output network, please see the reference schematics for nRF51822-QFAA QFN48 on pages 82-87 of the nRF51822 product specification. Specifically you will see that the inductor L2, and capacitor C5 have different values.

Correct, the meander antenna will not have the arm shorted to the ground plane as the IFA has. Please keep in mind that any changes to the PCA10031 antenna design will require changes to the antenna matching network encircled in green in your figure.

At this time I'm afraid I don't have the specifics as to why 12 pF was chosen in this particular instance. I will need to look further into it and get back to you on that.

Best regards

Jørn Frøysa