Antenna tuning for BGM12x (coin-cell usage example)

by <a href="http://community.silabs.com/t5/Welcome-and-Announcements/Community-Ranking-System-and-Recognition-Program/m-p/140490#U140490"><font color="#000000"><font size="2">Master Employee</font></font> </a> tmonte ‎01-10-2017 08:13 AM - edited ‎01-10-2017 08:23 AM

Introduction

 

This article shows how tune the antenna integrated into the BGM12x with the specific use case of placing a coin-cell battery behind the SiP, therefore enabling extremely compact designs.

 

To better follow this article it is recommended to watch our BGM12x related videos, BGM121/123 Hardware Design Guidelines and BGM121/123 Antenna Robustness.

 

 

Effects of placing coin-cell battery in tuned design

 

To understand the effects of the coin-cell battery on the antenna matching we took a BGM121 test board and placed a coin-cell in the back with a piece of blu-tak to keep it in place.

 

Figure 1 - BGM121 test board with coin-cell battery attached

Original board (lower resolution).png

 

The antenna matching can be seen in the following figures, first for the test board without battery and then after attaching the coin-cell battery.

 

Figure 2 - Original antenna matching for the BGM121 test board

BGM12_50_PLA3.png

 

Figure 3Antenna matching for BGM121 test board after attaching the coin-cell battery

BGM12_50_PLA4.png

 

Markers 1, 2 and 3 correspond to the beginning, middle, and end of the Bluetooth band. As it is visible in Figure 3 the presence of a metallic body (coin-cell) shifts the antenna tuning outside of the Bluetooth band. This is mostly due to the fact that the coin-cell is covering part of the copper clearance area and therefore reducing the antenna current loop.

 

 

Re-tuning the antenna 

 

To counteract this frequency shift we can extend the clearance area simply by scrapping away the ground layer as shown in Figure 4. Due to the the underlying uncertainty on how much it needs to be extended we’ll just extend it about 50% more as it is then easier to re-add a metal plane in order to fine-tune the clearance area.

 

Figure 4 - Extending the copper clearance area

IMG_1517.JPG

 

The antenna matching of the modified board is show in the next figure.

 

Figure - Antenna matching after extending the copper clearance area

BGM12_50_PLA2.png

 

The matching is still off-band but this time it is towards the lower frequencies which can be compensated by reducing the copper clearance. To reduce the copper clearance we simply cover it with a piece of metal (a shield from one of our modules in this specific case) until we get the correct matching as shown in the next figure.

 

Figure 6 - Reducing the copper clearance area to fine tune antenna matching

IMG_1514 - Copy.JPG

 

The antenna matching is now back into the Bluetooth band and very close to the original design.

 

Figure 7 - Antenna matching after reducing size of the copper clearance area

BGM12_50_PLA.png

 

 

Conclusion

 

The presence of metal very near to the BGM12x can have a negative impact on the antenna matching. However, it is easy to compensate the effect by adjusting the size of the copper clearance area to bring the antenna matching back into the Bluetooth band and ensure the best RF performance for your design.

Comments
by mobilenok
on ‎02-06-2017 06:31 AM

Hi 

 

What is the recomended thickness of the FR4? 

 

Also is there any thickness reccomendation for the groundplane? 

 

best regards Johan 

by <a href="http://community.silabs.com/t5/Welcome-and-Announcements/Community-Ranking-System-and-Recognition-Program/m-p/140490#U140490"><font color="#000000"><font size="2">Master Employee</font></font> </a> tmonte
on ‎02-07-2017 02:01 AM

Hi,

 

There is no recommended FR4 nor copper thickness, those do not impact the antenna performance.

 

Regards,

Tiago

by <a href="http://community.silabs.com/t5/Welcome-and-Announcements/Community-Ranking-System-and-Recognition-Program/m-p/140490#U140490"><font color="#000000"><font size="2">Super Star</font></font> </a> Tom_Kawala
on ‎03-10-2017 08:16 AM

Good points raised here, and I have some ideas to share:
- Wouldn't better to test the battery with leads.. with at least 1mm spacing from the carrrier FR4 PCB?

- As in that case of battery proximity, we need to compensate for several per-cent of loop length, placing the unplated holeson the inside of loop, and tuning with wider drill/mill bit should bring us to desired lower frequency whilst tuning the samples. It would require changing just hole diameter in final product design for manufacturing.