Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would
be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate
that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views.
It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if
you would like to post something on RF Cafe's
page, please do.
Below are all of the forum threads, including all
the responses to the original posts.
Post subject: filter design Posted: Mon Mar 05, 2007 2:52 pm
Mar 01, 2007 3:45 pm
When designing an interdigital or a combline bandpass
filter in stripline, I frequently have a S21 dip, about 0.5 to 0.8dB in magnitude, near either low side or high
side of the passband. For example, if the passband is 900MHz to 1.3GHz, this dip may occur at around 990MHz or
1.22GHz. There is a corresponding S11 spike at the same frequencies. What causes this? Can anyone enlighten me on
Post subject: Posted: Tue Mar 06, 2007 6:28 am
Joined: Fri Feb 17, 2006 12:07 pm
Location: London UK
This will always
happen when using an ellyptic function or a Chebychev design, and it is due to internal resonant paths within the
Butterworth Constant Phase types avoid this, but have a slower roll-off for the same number of
The attenuation variation (S21) across the band for a Chebychev should never exceed the design ripple
factor (eg. 0.25dB) by any more than the small margin for dissipation due to finite Q values of the elements. If
you are looking for maximum stop band roll-off rate, choose ripple factor at say 1.0dB, and that will be the
variation limit across the band.
Note also that the -3dB bandwidth and the equal ripple bandwidth are
different from each other, which is warning you that between the ripple factor frequency and the -3dB frequency
the S21 (and S11) will vary wildly. There is a formula relating them, but I will have to look for it.