How to design the matching circuit from ANT1/2 to 50 ohm?

[lyf9908]

Hello, I am useing NRF51822-QFAC in my BT bridge project; Now I need to design the matching circuit; there was some recommended matching circuit in the datasheet(11.3--11.10)(Picture2 in attachment); But the reference PCB (picture1 in attachment) is just two layer with matching circuit in top layer while GND plane in bottom layer, the thickness is about 1.6mm; Now I just copy the matching circuit in my schematic(picture3 in attachment),it is not right, this matching circuit is not fit for my application,becasue My pcb(picture4 in attachment) is four layers, the layer definition is like picture 5 and picture 6 in attachment; matching circuit is in top layer, GND plane is in the second layer; the thickness between them is just about 4mils; So I need to redesign the matching circuit, converting 15+j85 into 50 ohm; I need to convert the differential 15+j85 into a single 50 ohm; in the recommended circuit, you connected VDD_PA to ANT1 by L1 with decoupling by C3, how to concern the affect of L1 and C3 on the matching circuit? I've been checking on it for a couple of days, but can't get start... how to design the matching circuit to match the differential 15+j*85 to 50 ohms? picture.png thanks;

[MartinBL]

Hi,

The nRF51 series should have the GND layer at the bottom to get the optimal substrate thickness between the matching network and GND. Strictly speaking, it is possible to have GND in the the inner layers, but the thickness between the matching network and the GND layer will then be so small that the matching network shown in the Product Specification is useless. The reference design in the PS is designed for a substrate thickness of 1.6mm and designs with anything less than 0.8mm usually have to be retuned and you should expect a hit on maximum output power. When you are satisfied with your design there is not much left to do other than to measure the output power and spurious emissions and then tune your design according to your results.

Please have a look at this PCB guideline for more general tips and note that you can always send us your schematics and layout files for review (You can send your files in confidence in MyPage). We are also happy to help you out with tuning of your final design free of charge.