Mircostrip highpass filter - RF Cafe Forums
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Post subject: Mircostrip highpass filter
Unread postPosted: Wed Jan 12, 2005 7:31 am
Joined: Mon Nov 15, 2004 4:08 pm
I am working on a highpass microstrip filter for stop frequency 40GHz on substrate:
er=2.2, h=0.254mm (100mils).
The idea is that, one edge of the microstrip is shorted to the groundplane which ensures, that DC and TEM, qTEM modes are disabled in the structure.
Has anyone came across the problem?? Any sugestions??
Post subject: 40GHz high pass
Unread postPosted: Wed Jan 12, 2005 11:14 am
0.254mm=10mil, the circuit would nbever work with 100mil thick substrate. The dominate mode in microstrip transmission is quasi TEM (qTEM) so definetly do not want to squash that mode. There are surface modes which need to supressed, especially at the higher frequencise. If you have a board that you are testing then you will need to wrap the ground plane around the edge or stitch vias to ground. If this is part of a larger board then you will want ground plane at the surface ~3-4 times the transmission line width away from the transmission line. This will help suprress the surface modes. You do not want the ground plane to close the transmission line or it willbegin to look coplanar and the impedance will change.
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