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LC duplexer - RF Cafe Forums

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 Facebook page, please do.

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


shai_s
Post subject: LC duplexer Posted: Sun Aug 07, 2005 9:22 am
Hi
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


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

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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.

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Guest
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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