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
two 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:
Posted: Wed Sep 28, 2005 7:05 pm
A microstrip transmission line
will tend to
"roll off" as frequency increases.
This is because
the shunt capacitance becomes
A 50 ohm line is
a 50 ohm line regardless of frequency.
Post subject: microstripPosted: Thu Sep 29, 2005 2:26 pm
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 Rogers website.
Post subject: Posted: Thu Sep 29,
2005 2:40 pm
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.
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.