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, Im back

the current state of me trying to copy the design is the following. I designed it as a design block in EAGLE in order to import it into my project later. Thats why GND-plane is not implemented yet.

Had to find an other crystal which fits the requirements mentioned by AmbystomaLabs.
So far the GND-plane is planned for the dotted area, whereas the antenna is planed to be free of GND or any other signals on top and bottom layer.  Anyway Im unhappy with this huge empty space on the left. I guess its okay to reduce the size, but Im unsure about the holes. Is it okay to remove them completely and basicly cut everything whats not needed on the left? By the way: Whats the intention behind placing all the holes? Whats their use? I cant find a clear pattern so for me it looks very random. Do I need some more holes to seperate the antenna from the rest of the pcb, which is not shown in the picture? Still learning about the whole RF design topic and I very appreciate all your advices.

Thank you :)

[Øyvind]

Hello, and welcome back!

This is starting to look good. 

JoE1205 said:

Anyway Im unhappy with this huge empty space on the left. I guess its okay to reduce the size, but Im unsure about the holes. Is it okay to remove them completely and basicly cut everything whats not needed on the left?

The open area on the left should be used to add pads and/or GPIO pins for physical connection to flash/program/debug. Ref. the reference design I referred to in this comment, the image shows that there is a connector placed in the left side of the board. The holes which you mention are via-holes and are used to connect and ensure that the GND layers are connected. 

I recommend that you remove the vias and finish your layout with components, GND-plane, and traces. Then send the layout to us, and we can help with adding vias. Have a look at this tutorial on Eagle from Sparkfun, and our tutorial on RF design.
Kind regards,
Øyvind

[AmbystomaLabs]

Good catch! I had forgotten you did that in the reference design.

[AmbystomaLabs]

And so JoE1205 is clear on this, you remove the ground pour on just the top layer (not the bottom layer) and only in that little rectangle.  You'll have to remove those two ground vias in the rectangle too.

[JoE1205]

[AmbystomaLabs]

Something funky happened in your app when you removed the copper pour.  It looks there are little dots of copper all over maybe even shorting some of the pads. Almost looks like a remnant of a hatch pattern.

[JoE1205]

Oh no, thats just the changed grid.

[JoE1205]

Hi guys,

so i soldered everything as good as Im able to, anyway the communication with this module is very poor.

For me there are 2 possible reasons:

-----------------------------------------------

1. Crystal:
While soldering I noticed that the wiring for the crystal is wrong, so i had to solder it upside down onto the pcb ( isolated from it so the is no unintended connection between pcb and crystal). Im not sure if this affects the RF connection.

2. My solder skills are to bad. I tried to check every connection direclty after soldering a component. Anyway the size is a big problem, so its hard to be sure that I get in touch with the pins of the components.

-----------------------------------------------

I assume that the design with the fixed crystal is correct, anyway the communication with this module is very poor. I rarely get a full functional connection at all, atleast sometimes its working. Most of the time I dont get a connection. Sometimes just for a millisecond.

[AmbystomaLabs]

Yeah, that is a drag that you got the footprint wrong on the crystal.  Always pay close attention to whether the spec's show bottom view or top view for the pins.

That's probably a good place to start with the communications issue.  The extra wiring from flipping it over will change the load on the crystal and alter the resonant frequency. You can easily tune it back to nominal but you need some sort of spectrum analyzer to figure out how much the RF carrier is off.  There are cheap WiFi module based spectrum analyzers out there.  Normally around $100. Alternatively you can output HFCLK/2 through gpio and try to measure its frequency with an o-scope.  The problem with this is most o-scopes have lousy oscillators and you need to get the nominal frequency to +/-40ppm for BLE.

Once you get that fixed you should verify the RF power output.  Easiest way is RSSI at one meter. Just put it in a link with a smartphone or something and look at the RSSI.  If you thumb around on the devzone you should find what people expect it to be for that chip.

[QuangCN]

Hi there, i'm currently working on the same project as yours. I just wonder if your board can work?