How to control the RF impedance?

[realealon]

Hi

I got some questions about the RF part of the ref schematic. 1, why the bias voltage for ANT1 anf ANT2 are not the same since they are differential lines? 2, what's the impedance from L1 to ANT1 and ANT2? Is it 15+85j? How am i gonna tell the PCB manufacturer to control the impedance?

BR ealon

[realealon]

Hi

It's NRF51822, and in the datasheet, it specifies the impedance seen from the ANT1 and ANT2 to the matching network is 15+85J.

The question is should I control the differential impedance to 15+85j? Normally we control single end to 50ohm and differential end to 100ohm.

Thanks.

[Reinier Gerritsen]

You should use very short lines from ANT1 and ANT2 to your balun. After the balun (discrete L/C or one single component) you'll have a 50 Ohm impedance and that's easy to handle long lines. For very short lines, the impedance doesn't really matter. Wavelength in FR4 is roughly 60 mm, so if you keep the distance from the nRF51822 to the balun much smaller (e.g. 5 mm), you don't have to worry about controlled impedance lines. If you are really worried about performance, take care of a good antenna!