NRF51822-CFAC-WLCSP + BAL-NRF02D3 + onboard PCB (passive antenna)+ 2layer flex PCB

[Bhaskar]

Hi,

I'm designing a 2 layer flex PCB with the nRF51822-CFAC WLCSP package and recommended ST BAL-NRF02D3 balun and on-board PCB passive antenna (not ceramic chip antenna) with pi network for tuning.I have a couple of questions.

1. How to achieve 50 Ohm transmission line impedance to match on both nRF side, and the antenna side to meet 20m distance. its a completely flex PCB of 2 layer (not a rigid PCB) and all components should be mounted on flex PCB only.

2. What reference files should i consider to satisfy my requirement. Let me know is there any design reference available.. My project running with critical deadlines..Please help me how to proceed..

Thank you....

[Dave_couling]

You need to tune your impedance trace for your PCB. There are many free impedance calculators available online for this.

You will also need a tuning network (T or Pi), but to tune these values you need to test your board on a Network Analyzer.

[Einar Thorsrud]

I do not think there are any particular things you need to think of when using a Flex PCB compared to a normal rigid PCB. The only thing is that you need to tune the impedance trace to match the material and thickness of the PCB, as Dave_couling wrote.

You can download the nRF51x22 CxAx Reference Layouts here.

[Bhaskar]

Thank you Dave and Einar... I have one more query related type of PCB antenna suitable to meet 20mts- 30mts distance.. Micro-strip “L” antenna (or) Micro-strip bow-shaped antenna (or) IFA. I have enough space on flex PCB for antenna purpose..

[Einar Thorsrud]

A properly tuned λ/4 printed monopole antenna, such as the one that is used in the nRF51 DK has a good efficiency. I am not an antenna expert, but I would be surprised if you could gain much using a different PCB antenna shape. The most important is to let have a decent distance to ground and that the antenna is as straight as possible. (The performance is better if the antenna has a 90-degree angle to the ground plane, but a normal PCB antenna is naturally parallel to ground.)

[Bhaskar]

Hi Dave,

How are you. I hope your doing great. In my design i am planning to use monopole PCB trace antenna(not chip antenna) because i have enough free space on PCB. I have seen reference design nRF51-DK and i have following queries related to antenna tuning network.

I understood from the reference design :

1. only one shunt component recommended (C15 -1.2pF) for tuning a PCB trace antenna? Is it true?

2. Are you assuming that we are also changing the length of the antenna in this case? but in my case PCB dimensions are fixed and i can't change the length of the antenna

3)Why wouldn't you also use a pi-network for a pcb trace (assuming the dimensions of the antenna are fixed).

4. Also could you replace the tuning network up to component C6 with a balun? In my design i am using BAL-NR02D3. so,will it replace C4,L3 and C6 components also?

I have given provision for BALUN, pi-network(C4,L3 and C6) in my design but i have not included C15 ( 1.2pF) in my design. Let me know if am doing anything wrong. Thank you..

Best Regards,

Bhaskar M