Microstrip impedance vs. frequency - RF Cafe Forums
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Post subject: Microstrip impedance vs. frequency Posted: Sun Sep 25, 2005 10:14 pm
questions: 1) as the frequency increases what happens to microstrip characteristic impedance assuming dispersion
affects are included?
2) i think at high freq., impedance will decrease....to compensate for it do i make
width bigger or smaller? my understanding is that you have to increase the width to get higher impedance. answers
will be much appreciated. thanx
Post subject: microstripPosted: Wed
Sep 28, 2005 5:50 pm
I'm at work right now & don't have access to the papers - but Rogers Corp. has an on-line
calculator (downloadable too) and the formulas it's based on in this pdf:
Post subject: Posted: Wed Sep 28, 2005 7:05 pm
microstrip transmission line will tend to
"roll off" as frequency increases.
This is because the shunt
A 50 ohm line is a 50 ohm line regardless of frequency.
Post subject: microstripPosted: Thu Sep 29, 2005 2:26 pm
anormalhouse said: "A 50 ohm line
is a 50 ohm line regardless of frequency".
Not true. Most substrate materials have a dielectric constant
which is a function of frequency. Since the dielectric constant is part of the impedance formulas, the
characteristic impedance varies with frequency.
FR4 is a particularly blatantly bad example of this. "50
Ohm lines" on FR-4 vary from 50 Ohms as the frequency goes above 1 GHz. This is in the literature. Again, see the
Post subject: Posted: Thu Sep 29, 2005
Yeah, you kinda missed my point.
Post subject: Posted: Thu Sep 29,
2005 4:26 pm
you guys really didn't answer the question and i'd like someone to answer it again
for me also and for the person who orginally asked the question. taking into account frequency effects what will
happen to impedance when the width is increased?
Post subject: Posted: Thu
Sep 29, 2005 4:37 pm
whoa, whoa, calm down "guest". We'll aswer the questions in due time.
Let me have
a cup of coffee first, then we can all about frequency vs. line width, dielectric constants, and dogs.
Post subject: Posted: Fri Sep 30, 2005 6:46 am
When you increase the width the impedance
goes down, mor capacitive. Decreasing the width the impedance goes up, more inductive.