Post subject: LC duplexer Posted: Sun Aug 07, 2005 9:22 am
i would like to design LC duplxers consist of HPF and LPF
in an eagleware software i got influence between the two filters and
i dont get the needed performance when combining the filters
i would like to know the right way to design it, or to model it in eagleware
or an application note regarding the subject
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
- Kirt Blattenberger
RF Cafe Progenitor &
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.