The HJ-840 SiP module uses the nRF52840 SoC to design the antenna matching network and address layout issues.

[YiMing]

Hello,

I am currently using the HJ-840 SiP module that employs the nRF52840 SoC with an onboard chip antenna, 2450AT18B100E (Johanson Technology).

I am now facing some layout design issues. I'm still a beginner in this area; please excuse any shortcomings.

Since I have previously contacted their FAE, they advised me to use an external ceramic antenna, preferably connected through a PI.

Below is my SCH design diagram.

The capacitor and inductor are chosen and configured based on the specifications provided in the datasheet.

I now want to start drafting the layout design, but there are still details I'm uncertain about.

I used AppCAD and the impedance matching calculator from JiaLiChuang (JLC) Company to compute the trace width.

The following are the results I derived, as shown in the diagram.

(The green number 2.7097mm represent the trace width, and the orange number 1.465mm indicates thickness.)

I'm likely going to use a double-layer board and design using single-ended impedance, with a copper thickness of 1oz.

However, I'm still uncertain why the computed trace width is so wide. Is this normal?

(Compared to the lines drawn on the datasheet for connecting capacitors and inductors, it's much thicker.)

How should I proceed with such a design? Are there other considerations I should be aware of during the layout?

(For instance, the antenna should be placed at the edge of the board, the surrounding area should be kept clear, and copper should not be spread, etc.)

Thank you for reading.

[Bendik Heiskel]

Hi,

The trace is so wide because you are using a coplanar waveguide without a ground plane on the bottom layer. If you add GND on the bottom layer under the RF trace the width can be reduced to 1.6mm:

We recommend using this type of transmission line as there is less radiated losses from the trace, compared to a transmission line without a ground plane underneath.

The transmission line does only need to have an impedance of 50 Ohm between the matching networks, so in your case only the part of the trace between C6 and C7 needs to be this width.

 

As for the rest of the board its best to have as much of the board covered in a GND fill, the antenna performance is dependent on the size and shape of the ground plane, where a larger ground plane usually performs better.

The antenna you are using is designed to be mounted in the corner of the PCB and needs a ground clearance area around it. The antenna datasheet specifies the size and dimensions of this clearance area:

 

Best regards,

Bendik

[YiMing]

Thanks, it makes more sense now.