when does controlled impedance matter in RF

Thread Starter

lupal

Joined Apr 1, 2020
10
Hello,
I am wandering when impedance matter on PCB. Assuming 868MHz circuit and 10mm trace length - does it matter in practice?
Thank you.
 

crutschow

Joined Mar 14, 2008
27,761
It generally matters when the propagation delay is no more than a quarter wavelength, sometimes even less.
So for 10mm @ 866MHz there should not be a concern about the trace characteristic impedance.
 

Thread Starter

lupal

Joined Apr 1, 2020
10
It generally matters when the propagation delay is no more than a quarter wavelength, sometimes even less.
So for 10mm @ 866MHz there should not be a concern about the trace characteristic impedance.
Thank you, crutshow.

One more thing. If there are more components in the way (passive, like a DC blocking capacitor for example), should I sum the length together as one transmission "line"?
 

crutschow

Joined Mar 14, 2008
27,761
If there are more components in the way (passive, like a DC blocking capacitor for example), should I sum the length together as one transmission "line"?
Yes.
Since a series capacitor has little effect on the AC signal, you need to include the total electrical length of the signal through and past the capacitor.
 

Thread Starter

lupal

Joined Apr 1, 2020
10
And one more thought. An active component such as low noise amplifier splits the paths in two. Theoretically it could change the impedance - one (say 50Ohm) on the input circuit and other one (e.g. 75Ohm) of the output circuit if I understand it correctly. Right?
 
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