How to control the RF impedance?

RE: How to control the RF impedance?

Reinier Gerritsen — Thu, 15 May 2014 14:51:49 GMT

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!

RE: How to control the RF impedance?

realealon — Thu, 15 May 2014 00:18:45 GMT

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.

RE: How to control the RF impedance?

FormerMember — Wed, 14 May 2014 14:34:21 GMT

Which chip do you use?

There are two matching networks that are needed, one for matching the chip to 50 Ohm, and another for matching the antenna to 50 Ohm.

The reference design for a specific chip contains the matching network for matching the chip to 50 Ohm. In addition, a matching network (a shunt component or a pi-network) is needed for tuning of the antenna, see the Antenna tuning white paper chapter 4.

Yes, we help out with antenna tuning. For request regarding antenna tuning, please open a case in Nordic support system.

RE: How to control the RF impedance?

Marc Nicholas — Wed, 14 May 2014 13:10:07 GMT

The reference schematic does not have a matching network. You will need a Pi matching network that's appropriately tuned for your design.

I believe that Nordic still offer a free antenna tuning service, but you should speak to your sales rep about it.

-m

RE: How to control the RF impedance?

realealon — Wed, 14 May 2014 00:02:40 GMT

And the ref schematic/layout doest not specify the matching impedance.

RE: How to control the RF impedance?

realealon — Tue, 13 May 2014 23:57:51 GMT

Kristin

Yet the impedance seen from the ANT1 and ANT2 to the matching network is 15+85J in the NRF51822 datasheet.

RE: How to control the RF impedance?

FormerMember — Tue, 13 May 2014 08:57:47 GMT

In order to get the right impedance the reference schematic/layout should be followed. The matching network in the reference layout matches the chip to 50 Ohm.