Question about CPWG of nRF52840 Antenna

[dong-un lee]

Hi
I am reviewing the nRF52840-DK hardware documentation to make boards with good antenna performance.

Q1. I have divided it into three areas as shown below. Is my understanding correct?

Q2. I found the following information in the document that I needed to calculate the CPWG.

Dielectric thickness = 1.4mm

Dk = 3.72

RF line width = 0.762mm

Clearance between RF line and GND = 0.254mm

I calculated the CPWG value from the link below.

http://www.rfdh.com/rfdb/cpwg.htm

The results are as follows.

To obtain a 50 ohm line, W should be 0.078 mm. Calculated as 0.254mm according to the DK document, it becomes 66ohm.

Why is it made with 66ohm like the above?

Q3. I think the difference between 66ohm and 50ohm is compensated by C4 and C22 connected to the RF line. Am I right?

Q4. I'm going to make a 4-layer PCB (with a dielectric constant of 4.3) with 1.6T thickness. Can I make a CPWG with the same specifications as DK?

I'm looking forward to your reply.

Thanks

[Andreas]

Hi,

 

From what I can see thickness = 1.6mm, width~=0.75mm, gap~=0.25mm will give 50 ohm when the metal thickness is 0.1mm. With 0.036mm, which is a fairly common thickness, it is closer to 50 ohm. You should check what is specified for your PCB and adjust accordingly, if it is 0.1mm it will be fine.

 

I am not sure what the PCB manufacturer is on about, for the CPW itself it is not important as long as you know that the substrate thickness will be different when calculating width and gap. With 0.2mm thickness between 3rd and 4th layer it is only minor though. One positive change might be that if using the 3rd layer, you will be able to use the 4th layer for other things, e.g. component placement.

 

Best regards,

Andreas