

Microstrip Impedance vs. Frequency  RF Cafe Forums

mike 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
mike
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guest 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 online calculator (downloadable too) and the formulas it's based
on in this pdf:
http://www.rogerscorporation.com/mwu/pdf/rt312.pdf
Good Luck!
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anormalhouse Post subject:
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 dominant.
A 50 ohm line
is a 50 ohm line regardless of frequency.
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Guest 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 FR4 vary from 50 Ohms as
the frequency goes above 1 GHz. This is in the literature. Again,
see the Rogers website.
Good Luck!
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anormalhouse Post subject: Posted: Thu Sep 29, 2005 2:40
pm Yeah, you kinda missed my point.
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guest
Post subject: Posted: Thu Sep 29, 2005 4:26 pm people,
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?
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anormalhouse 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.
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Guest 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.
Posted
11/12/2012



