Post subject: LC duplexer Posted: Sun Aug 07, 2005 9:22
i would like to design LC duplxers consist of HPF and
in an eagleware software i got influence between the two
filters and i dont get the needed performance when combining the
i would like to know the right way to design it,
or to model it in eagleware or an application note regarding the
Post subject: Posted: Wed Aug 10, 2005 1:04 pm
Joined: Sun Aug 03, 2003 2:02 pm
Location: Erie, PA
It has been a while
since I have designed a diplexer, but if memory serves me correctly,
the way to go about a LP/HP combination is to design with an input
impedance of 1 ohm and an output impedance of whatever you system
impedance is (50 ohms?). Then, tie the two 1 ohm inputs together
as the common input.
The theory behind it is that between
the LP and HP filters, the input will "see" the system impedance
spanning in frequency across both filters. At least that's the way
I remember it.
- Kirt Blattenberger
RF Cafe Progenitor & Webmaster
Post subject: LC DiplexerPosted: Wed Aug 10, 2005 5:55 pm
In designing an LC Diplexer, it's important to have the right
kind of filters for the HP and LP filters. Neither should start
with a shunt element. (Use Tee filter sections instead of Pi sections
for the first stage).
Otherwise, each filter will "short
out" the signal in its stopband, with disastrous results.
If I remember correctly, you design both filters at the system
characteristic impedance (usually 50 Ohms). That way, the return
loss stays (more or less) constant over both bands.