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

There's only one way to get what you need: an "absorptive" filter.

That looks like 2 filters, connected in parallel at the input, with one going to the
output and the other going to a resistive termination capable of absorbing all the
applied power in the stopband. If you need bidirectional termination, looking like 50 Ohms
at each port, it starts to look like 3 filters - doable, but rather a mess.

Usually, the textbook examples of these absorptive filters are for high-pass or low-pass
filters, but the same principles apply.

Good luck!

a couple ways to do it:
1) put 3 dB pad in front of BPF, and you will have at least 6 dB return loss out of band
2) put broadband circulator in front of BPF, and you will have at least 15 dB of return loss over a much bigger bandwidth
3) IF you know what you are doing, you can design LPF/BPF/HPF (or some similar variation) triplexer using singly terminated design. Done properly, you could have DC to 20 Ghz good return loss. Done poorly and you will have a tuning nightmare.

_________________
Rich
Maguffin Microwave Consulting
www.MaguffinMicrowave.com