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LC duplexer - RF Cafe Forums

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Below are all of the forum threads, including all the responses to the original posts.

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

thank you


Kirt Blattenberger
Post subject: Posted: Wed Aug 10, 2005 1:04 pm

Site Admin

Joined: Sun Aug 03, 2003 2:02 pm
Posts: 308
Location: Erie, PA
Greetings shai_s:

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.

Good Luck!

Posted  11/12/2012

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