NRF51822 PCB layout review

[dingari]

Hello, a few days ago I was having trouble with RF performance on a custom PCB using the NRF51822. I posted here to get feedback and figure out where the design went wrong.

I've since then re-designed the board, taking into account the faults of the previous design. Could someone review this design for any obvious mistakes / design errors before I send it off for manufacture?

Relevant part of schematic:

PCB top:

PCB bottom:

Main changes since last time:

• Use integrated balun instead of discrete matching network
• Antenna on same side of PCB as SoC and matching network
• More free antenna placement
• RF and SoC grounding prioritized
• Four layers (Bottom components and signals - ground plane - power traces - top components and signals)

[Einar Thorsrud]

This seems much better than the initial board, but it is difficult to see all the details of the layout in the pictures. It would also be interesting to see the other planes, particularly the ground plane. Can you upload Gerber files, which would make it easier to review it?

Also:

• Which balun do you use? It must be one of the supported baluns. Remember to take care to copy the placement of the capacitor (your C20) exactly as the reference design.
• Even though the antenna datasheet states that "No external matching required", this is generally only true if your board is close to identical to the antenna reference layout. In practice, most boards require a pi network in order to tune the antenna, so you should include that.

[dingari]

Hi, I've uploaded gerber files here: www.dropbox.com/.../gerber.zip - I started out with two layers, but since the board is so crowded I had to add a seperate ground plane. Since you don't really make three-layer boards I utilized the other inner layer for the supply voltage traces. There aren't a lot of traces on that layer, would you recommend filling the extra space with ground copper or leave it blank?

I'm planning to use the Johansson 2450BM14E0003T balun, listed here: www.johansontechnology.com/nordic

[Einar Thorsrud]

This seems much better than the initial board. I have few comments, though:

• You should have a pi-network to tune the antenna, as it is always required in practice in order to get good performance (regardless of what the antenna manufacturer claims). In order to make room for it you probably need to move some components.
• In general it is a good idea to use all empty space for ground, so I would put ground copper on the power layer as well.
• I have not compared the placement of your balun to the reference design in detail, but from looking at your layout it seems OK. If you have not done so you should measure trace lengths etc., as it is important to exactly copy the placement of the balun relative to the nRF.