Resistor tolerance and requirement for RF matching network

[floatcloud]

For NRF51 and NRF52 antenna matching network, is there any requirements for resitors? Tolerance? 

I know there are some requirement for capcitor and inductor types, how about resitors?

Can I use regular 0402 resistor with 1% tolerance for tunning?

[haakonsh]

Hey floatcloud,

Yes, NP0!

You can use any 0402 1% NP0 components. You should never use any other resistor than 0ohm resistor, and only as a bridging component or as a cheaper alternative to a 1nH inductor. 

Cheers,

Håkon.

[floatcloud]

Hi haakonsh, thank for the reply,

Could you explain a little bit why a resistor other than 0ohm should never be used?

The reason why I ask this is that, in one of our boards, I tried using a single 24ohm resitor as the bridging component without any shunt component for antenna tunning, resulting the desired frequency right at the bullseye of the smith chart. 

The same goal wil be more complicated to achieve if using only capacitors and inductors.

What do you think of that?

thanks

[haakonsh]

An impedance with a real component larger than zero will burn energy that would otherwise get transferred to the antenna.

The ideal impedance match is 50ohm, but it is a complex value where the resistive component is zero and the reactive component is 50ohm; Z = 0R + 50i ohm. 

 

[floatcloud]

thanks for the clarification 

[AmbystomaLabs]

Just to clarify, part of what you stated is correct, part is wrong.

Yes, you should not use resistances in RF matching.  Just like any dc power divider, the power is split between the two connected loads. This is a simplifications since VSWR plays a heavy role in anything RF.  But the idea is similar.

Occasionally, real resistances show up in networks either for loss to reduce gain, or similarly as a way to ensure stability.

Now on your last bit, about match being a reactive device.  No that is not true.  Only real resistances can consumer power.  So 50 ohm loads are real 50ohm loads (ideally) at their intended frequency.  So an ideal antenna in a 50ohm system looks like (50 + j0) ohms. Similarly the ideal source looks like (50 +j0).

The reason then for not using resistances in matching (as you pointed out) is that they consume power. Another reason is that unless you use a pi or t network, the resulting match won't be symmetric.  So although your source might see a 50 ohm load, the load will not and problems will happen.

A well designed match uses only complex impedances and can do anything from turning 20 ohms real to 50 ohms real, or (5 - j75) into (50 + j0), etc., etc.