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50 ohm filter, DC to light - RF Cafe Forums

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 Post subject: 50 ohm filter, DC to light
Posted: Mon Apr 09, 2012 6:09 pm 

Joined: Mon Apr 09, 2012 6:07 pm
Posts: 1
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?


 Post subject: Re: 50 ohm filter, DC to light
Posted: Fri Apr 13, 2012 5:40 pm 
User avatar

Joined: Wed Feb 22, 2006 3:51 pm
Posts: 104
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!

 Post subject: Re: 50 ohm filter, DC to light
Posted: Tue Sep 25, 2012 10:50 pm 

Joined: Wed Apr 08, 2009 11:07 am
Posts: 33
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.

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Posted  11/12/2012
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