Designing antenna for nRF52840

[Ray Breslin]

Hello,

we are currently developing a new prototype board with the nRF52840-chip.

In order to figure out the best performance for our bt-antenna we decided to plan our first prototype with a switch for a print-antenna and a ceramic antenna.

After going through some whitepapers, reference designs and board layouts from Nordic, we chose the ceramic antenna from the Thingy52-board and a self-designed print-antenna.

Now we have the following situation:

1. The proposed schematics from the Thingy52-board (see Fig. 5 here) uses C3 0,8pF, L1 3,9 nH, C13 2,2 pF and L4 3,9 nH - without L5 even though it was suggested by the antenna manufacturer (see here).

2. Since were using the nRF52840 instead of the nRF52832 from the Thingy52-board, we considered using C3 with 1pF and an additional C4 with 1pF as suggested in the nRF52840 PS v1.0 (see Fig. 212 here).

3. There is an additional C43 with 1,2 pF behind the SWT-coaxial-connector (MM8130-2600) that got introduced in the nRF52840 PDK (see PCA10056-schematics here).

Can we assume that using the ceramic-antenna from 1. with the minor value- and component-adjustments from 2. are the correct way to go?

If that should be the case, what should we do with that C43 capacitor from 3. and what is its purpose?

Any help would be greatly appreciated!

Kind regards,

Ray

[Andreas]

Hi Ray,

 

C3 and L1 ( and C4 in the case of nRF52840) should always be present and mostly as is for spurious rejection and impedance matching. They are a little dependent on the layout so you might need to tweak them a click up or down in order to optimize the harmonic levels. C13, L4 and L5 in the thingy correspond with C43 in the nRF52 DK, both are used for matching the antenna to 50 ohms. The antenna matching components will depend on a given PCB size, shape and layout, the same antenna in two different PCBs can have both small or large difference in matching components. A chip antenna typically need 2 or 3 components for matching (pi or T configuration is recommended to possibly accomodate any combination), and a monopole antenna as in the DK usually only needs a parallel capacitance. How to determine them is described in this document.

 

So yes, if you go with a ceramic chip antenna, you should have C3, L1 and C4 near the nRF52840, and C13, L4 and L5 near the antenna. If you go with a monopole antenna, you need C3, L1 and C4 near the nRF52840, and C43 near the antenna. Determining the antenna matching components should always be done for a new design as described in the document linked above, we can help with this if needed.

 

If you can accommodate it on your PCB we recommend going with a printed antenna, as it is lower cost and generally at least just as good as a chip antenna.

 

Best regards,

Andreas

[nameismahi]

Can u differentiate the RF filter and matching circuit in that design?