I need to design a bandpass filter with 50 ohm input and output impedance.  This would be a simple task except that I need the input and output impedance to be 50 ohms in the passband AND in the stopband.

Most of the bandpass filters I've designed in the past have a very low impedance at frequencies below the passband and a very high impedance at frequencies above the passband.  Has anyone seen any design material on matching the stopband of a filter to 50 ohms?  I'm sure this is going to be very challenging because the impedance swings all over the Smith Chart.  Can anyone help?

Thanks

rpaulson,

The simplest way but higher component count would be to design a triplexor where the stop bands are pass bands to unused paths simply terminated with 50 Ohm resistors.

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Aaron,

Duplexor is a nomenclature intended to mean talking and listening simultaneously where as a diplexer is to frequency separated combination of two bands.  A duplexor in cell phones for example is a type of diplexor. Combining a type of low pass and a highpass to create a three port is a diplexor where the common port see the response of both filters and one port observes the response of the low pass and the other the highpass.  A triplexor is to combine three filters separated in frequency.

Yes, its a four port with one common, one low pass only for match, one high-pass only for match and one band-pass as the desired.  

The link you put in your post is a type of triplexor but it is made up of three bandpass filters where as what I am talking about is wider in bandwidth.  Consider this link to demonstrate.

http://productfinder.pulseeng.com/product/C6178NL

click on data sheet and look at the return loss characteristics.

I hope that clarifies

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