Copyright: 1996 - 2024
BSEE - KB3UON
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Designing Microstrip on a PCB with unknown substrate - RF Cafe Forums
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lots of great posts in the archive that ware worth looking at.
Below are the old forum threads, including responses to the original posts.
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Post subject: Designing Microstrip on a PCB with unknown
Unread postPosted: Fri Mar 25, 2005 12:24 pm
I have bought some double sided PCBoard for Microwave work from
Ebay, it was cheap, looks of good quality and is in bulk but unfortunately
I don't know what the substrate material used in it is, or how thick
the copper is. How can I find out, is there some test that I can perform?
I will be etching 50ohm microstrip lines on it, how can I determine
the right thickness for the lines without any knowledge of the board
material or Er (relative substrate electric permitivity)? Any ideas?
Unread postPosted: Sat Mar 26, 2005 9:07 am
First check if there
are any labels or markings attatched to any pieces. Post this info up,
as it may contain the information you are looking for.
Rogers, Arlon and Taconic websites ( I know there are more manufacturers,
but these are the most common)
The physical dimensions are easily
measured with a micrometer or vernier caliper.
As a first guess,
use a line width that is the same as the material thickness, although
absolute value of impedance will be unknown, it will be reasonable to
To measure Er, I would suggest that you etch some
test resonators of known dimensions using this line width onto a spare
piece, and measure the response on a VNA. If you have no VNA available,
any means of measuring VSWR with frequency will do. I would recommend
a ring resonator, coupled lightly as possible. Using ring resonator
eliminates the capacitive end effects of a open line. The S11 (VSWR)
response will dip at the point where the ring is electricaly one wavelength
around (use centre line of strip). Be aware that this will repeat at
integer wavelength multiples.
You can now apply the microstrip
design equations (or goal seek in excel) to give a reasonably accurate
value for Er.
Unread postPosted: Mon Mar 28, 2005 3:40 pm
There are no markings
or stickers on the board itself but I'll try your procedure, thank you
for the tips!
Unread postPosted: Tue May 17, 2005 9:29
The guest who suggested the resonator is correct, the method
suggested is the standard procedure used to determine er ( dielectric
constant).. The equation he was refering to is: Lambda=C/(sqrt(er)*F);
where Lambda is the wavelength , c is the speed of light, f=the frequency.