Trace impedance for short traces , does it matter?

Thread Starter

neddie

Joined Sep 29, 2011
7
Hi all.
I'm working on a bluetooth design. The bluetooth chip rf out goes through a few passives (filtering and impedance matching) and then almost straight to the antenna. the track length to the antenna is only about 5mm. Is it necessary to design a 50ohm trace for such a short distance(relative to the wavelength).
I can understand if the track length is a significant portion of a wavelength , but at less than 5% does it make any real difference?
Cheers
Rob
 

Papabravo

Joined Feb 24, 2006
14,266
The answer is not simple or straight forward. What I can tell you is that with RF circuits every impedance discontinuity is significant. An impedance discontinuity occurs when the impedance of a transmission path changes abruptly. As you may or may not know what happens to any signal is that a portion is transmitted and a portion is reflected. If too much is reflected and not enough is transmitted the antenna may end up with next to nothing. Pay attention young Jedi and all will be well.
 

Janis59

Joined Aug 21, 2017
1,205
Most of cases (but not all!!) the electrical length of shorter than lambda/40 is sure non-reflective and shorter than lambda/10 is very small reflective. Be hardly beware if anything comes near like a lambda/4.
Bit demonstrate: 2,4 GHz lambda is 125 mm, however taking in account the nude round wire v(f) it will become rather about 115-120 mm. So, if the wire piece is shorter than 120/40=3 mm may ignore it. But if wire is longer than 120/10=12 mm You should worry much enough.
 
Top