RF Circuit for a Custom MKW41Z board, SMD antenna

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RF Circuit for a Custom MKW41Z board, SMD antenna

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muriukidavid
Contributor II

I am designing a compact custom BLE board with the MKW41Z controller for wearable application. I have some recommendations from the antenna datasheet on the RF matching circuit and also the 50 Ohm transmission line impedance matching from the NXP FRDM-KW41Z schematic. antenna_schematic.png

matching_network_for_antenna.png

Of course I have replaced the "J5 and ANT1" part of the circuit from the FRDM-KW41Z schematic with the Molex MID SMT antenna [with its recommended 1.3pF and 1.8nH RF matching circuit].

Custom_matching_circuit.png

And routed the PCB as close as possible to the recommended working layout in the application notes:

RF_Critical_custom_PCB_layout.png

RF_Critical_custom_PCB.png

My problem is I cannot figure out a specific trace width to use on the RF trace from the MCU ANT pad to the antenna

If I was to pick a PCB manufacturer who makes PCBs using FR4 boards of a specific dielectric constant, board thickness and top copper thickness, like OSHPark 2oz Copper with 0.8mm FR4, how do I figure out the width of the trace required and distance between the trace and the top ground plane. I need to fix this final part in order to do via stitching and make prototypes. Any assistance will be greatly appreciated.

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2 Replies

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estephania_mart
NXP TechSupport
NXP TechSupport

Hello,

By any chance have you checked the Hardware Design Considerations for MKW41Z/31Z/21Z BLE and IEEE 802.15.4 Device? I believe you will find a lot of useful information regarding our recommendations in the design for the KW products. 

Regards, 

Estephania 

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441 Views
muriukidavid
Contributor II

For some reason I had access to some older version of that document. That one is more updated and precise. I will do further tweaking. I also found a formula I can use to calculate the exact transmission line dimesions on top copper given the FR-4 material properties. I did that but it's amazing how the manufacturers cannot provide precise information for the properties of the FR-4 like dielectric constant like it's given in that document. I emailed OSHPark.com and they are yet to get back. I will check other manufacturers probably using better materials for high frequency RF, but FR-4 is more affordable and is worth prototyping for now as well. I will post some notes here when am done. Thanks.