filter - RF Cafe Forums
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Post subject: filter Posted: Tue Nov 14, 2006 6:57 am
Joined: Tue Nov 14, 2006 6:50 am
I'm facing a problem in High pass filter design realised using suspended substrate stripline. the filter cutoff frequency is 16 GHz. Return loss near cutoff is very poor (-6 dB) and rejection is not monotonically decreasing. it falls down very sharply upto -40 dB (typically) and then increases by 2 or 3 dB and remains constant at that level. I have tried various combinations of substrate parameters, number of elements etc.. Can u please help me out?
DEFENCE ELECTRONICS RESEARCH LAB
Post subject: Posted: Tue Nov 14, 2006 7:48 am
Joined: Fri Feb 17, 2006 12:07 pm
Location: London UK
We would need to know a little more about the filter design type: is it Butterworth, Chebychev or Elliptic Function? Also how many poles?
The last 2 types have strange behaviour near the -3dB cut off frequency.
If the cutoff roll-off rate in the stop band is very high it suggests an Elliptic Function or Chebychev filter, and the behaviour may be quite normal for that type.
They are used when you know the max frequency can be set somewhat below the max design frequency, but you have several products or external carriers not far removed from the wanted carrier, and you need max attenuation of these. The subsequent sidelobes of the filter then can remain the same level once they have been eliminated.
So, tell us more.
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