BLUENRG-LP Antenna Matching Guide

[MManj.1]

I am developing a new product using BLUENRG-355AC and need some help regarding the antenna matching circuit. I am planning to use an external 2.4GHz antenna connected to a SMA connector on the board.

The BLUENRG-LP series has two evaluation boards STEVAL-IDB011V1 and STEVAL-IDB011V2. Initially I had thought of replicating the antenna matching network from these evaluation board designs, but then both of these boards have quite a different antenna matching circuitry. So I am confused as to which one is more reliable.

On the other hand, I am also open to design an antenna matching network on my own but for that I'll require some guidance from the experts on this forum as I am not a very experienced in RF hardware design.

Could the experts in the forum kindly point me to some references which I can use to design the antenna matching network properly? I have had a look online but the information is so scattered that I am finding it difficult to make sense out of it. If someone could let me know the steps I need to follow, I will be happy to try my hand at it and gain valuable experience on this topic.

Thanks in advance!

[Laurent LOUAZON]

Hello,

Let me try to clarify.

First you should not confuse BlueNRG-LP matching/filtering and antenna matching.

BlueNRG-LP has a nearly 50 Ohms RF output but it requires few extra components to ensure best RF matching and also filtering to reject spurious emission (Harmonic2, Harmonic3 etc) and be compliant with regional certification (CE, FCC etc)

Then depending on antenna selected and also on layout or product mechanical parts, it is needed to add few components for antenna matching.

This will ensure antenna will radiate at proper frequency.

As you noted there are two different solutions depending STEVAL-IDB011V1 or V2.

With V1 version

• C30, C5, L1 and C6 are here to ensure proper BlueNRG-LP matching.
• U2 is a Low Pass Filter to ensure harmonic rejection.
• C42, L3 and C43 are dedciated to ensure proper antenna matching. Considering there is not a specific antenna here (SMA connector), no antenna matching is done (just 0 Ohm)

With V2 version, the idea was to replace the Low Pass Filter components was a full discrete solution.

• C30, C5, C42, L1, C6, L3 and C43 are acting for BlueNRG-LP matching and filtering
• Again, considering there is not a specific antenna here (SMA connector), no antenna matching is implemented or even shown here.

So hope that clarified the difference between both eval kits.

As said before, it is always recommended to have 2 or 3 components for matching an antenna.

Despite we will be perfectly 50 Ohms after BlueNRG-LP matching/filtering and despite antenna is also supposed to be perfectly 50 Ohms, in reality this never happens and having no matching will not allow to get an antenna radiating at proper frequency.

Antenna matching must be done with a VNA (Vector Network Analyser) by checking S11 parameter.

Regards

Laurent

[MManj.1]

Hi Laurent,

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Thanks for the clarification!

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So I guess now my best bet is to use the discrete matching + filtering solution as given in the v2 evaluation kit schematic and leave a pi network pads provision to fine tune the antenna if required. Is this correct?

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Thanks,

Mayuresh​

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[Laurent LOUAZON]

Hello Mayuresh,

Your undersatnding is fully correct !

Regards

Laurent