nrf24l01+, pcb mifa antenna design

[JoE1205]

Hello guys,

I tried to design a pcb antenna for a nrf24l01+, since my company doesnt allow to buy cheap smd breakout board version from china anymore

The pcb stats of the producer:

• 2-layer FR4 1.55mm (61.023mil)  thickness, εr = 4.6
• Copper thickness, 35μm (1.378mil)

What I used:

• Guideline for pcb mifa antenna design:https://www.cypress.com/file/136236/download :
  Antenna basic dimensions: Page 10
  Cutting antenna length to ~115mil  as told on page 14
• nrf24l01+ schematic pictured in the datasheet page 66 (rev1.0)
• https://www.eeweb.com/tools/microstrip-impedance for calculation resulting into a feedingline width of 2.82mm (111.023mil), this is double the value of the recommended value in the cypress file. Not sure whats right.

This results into following desgin block:

The antenna area is clear of any signals aswell as ground on top or bottom layer.

Since I have absolutly zero experience with RF antenna design I would like to have some opinions regarding my design.

Thanks so far,

Joe

[JoE1205]

Hey guys,

first of all, thank you very much for your advice.
After reading your posts and checking the files Oyvind has attached to his post I came up with following plan:

1. Place all components and use the same size shown in the files

2. Copy the pcb antenna

3. Check routing

Since I dont have very much experience with RF, as you already found out, I obviously have some (stupid) questions:

• My pcb is a little bit thinner than the pcb used in the files. Does this have a big effect on the performance if I just use the same dimensions?
• What about the line thickness, Antenna length etc.? I there a way to obtain the exact dimensions like width an length? Does it have to be very precise or is it not that important so I can try to measure the lines via grafic tools or something like this? This will result in little differences I guess.

[Øyvind]

JoE1205 said:

I came up with following plan:

 We recommend that you copy and place the antenna first, as this is the most important part of the RF design. Keep in mind that the reference design is designed for optimal performance.

For best result, copy the exact antenna layout with the nrf42le1, and antenna components (connected to ANT1 and ANT2). Keep space around the components and GND layer. For the antenna itself, remove any fill around and on the bottom layer.

Then place other components and route them as wanted. 

JoE1205 said:

My pcb is a little bit thinner than the pcb used in the files. Does this have a big effect on the performance if I just use the same dimensions?

 No, this should be fine. Down to 0.8mm will not affect the performance.

JoE1205 said:

What about the line thickness, Antenna length etc.? I there a way to obtain the exact dimensions like width an length? Does it have to be very precise or is it not that important so I can try to measure the lines via grafic tools or something like this? This will result in little differences I guess.

 For the antenna and circuit, keep everything exactly the same to be on the safe side. The length of the antenna, seen in the image above, is that length in case you need to tune the antenna. See White Paper - Antenna Tuning v1.0.

Kind regards,
Øyvind

[JoE1205]

Some questions regarding the crystal.
I cant find a crystal with +-60 ppm. Is +-50 also possible?
The BOM is listing a HC49. Does this include any HC49/xxxx versions, for example HC49/4HSMX?

[AmbystomaLabs]

You can use anything with a tighter tolerance (ie, 50 or 30ppm) as long as the load capacitance and ESR are the same as what Nordic specifies.  Since I notice that this design is rather old and Nordic specified 12pF which is no longer shipped you may want to switch to a different packaged part. There are plenty of 4-SMD crystals in 12pF with an ESR of 30 to 50ohms which is similar to the original crystal.  The rerouting of the crystal is pretty low risk and any of us can guide you on getting the grounding right. Since it would be 4-SMD you can also just mimic the routing for the crystal on a nRF52 reference design.

One question, you know the nRF24L01 is slated to be discontinued, right? You sure you want to put so much effort into this.  You might be able to find an nRF51/52 module that is footprint compatible with your old module and then you can just port your code over.  There is softdevice support on 51/52 for ESB, ANT, BLE, etc. 

[JoE1205]

Hey its me again.
I was able to recreate the layout by using the pick and place file. But im stuggling with the antenna positioning and dimensions. Is there a way to get the exact dimensions?

[AmbystomaLabs]

Not sure whose antenna you are referring to since a few were mentioned.  On the Nordic ones you can just measure in the design files. If they are Altium files you can open the library model and it will give the dimensions.  If you are using the gerbers then any gerber viewer lets you measure distances.

With the TI mifa they give the dimensions in the pdf.  See enclosed.swra117d.pdf