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Design >> RF Design >> 50 ohm filter, DC to light
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Message started by rpaulsen on Apr 9^th, 2012, 4:17pm

Title: 50 ohm filter, DC to light
Post by rpaulsen on Apr 9^th, 2012, 4:17pm

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

Title: Re: 50 ohm filter, DC to light
Post by aaron_do on Apr 9^th, 2012, 5:33pm

Try reading up on "Absorptive Filters". I think there are a few papers on IEEE. Also, you can imagine that it can be done using a filter, and a pair of isolators.

regards,
Aaron

Title: Re: 50 ohm filter, DC to light
Post by rfcooltools.com on Apr 15^th, 2012, 4:51pm

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.

http://rfcooltools.com

Title: Re: 50 ohm filter, DC to light
Post by aaron_do on Apr 15^th, 2012, 5:27pm

Hi rfcooltools,

by a triplexor, you mean like a duplexor except with one more output right? Like this one...

http://www.tycc.cn/pics/new/1225336574.pdf

If so, how is that easy to design for 50-ohm broadband matching? To my knowledge, duplexors only pass two specific bands, so there is still a huge bandwidth that is unmatched.

Picking up on your idea though, I guess you could design a bandpass filter and a notch filter parallel, and terminate the notch filter in 50-ohm...

regards,
Aaron

Title: Re: 50 ohm filter, DC to light
Post by rfcooltools.com on Apr 15^th, 2012, 9:39pm

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

http://rfcooltools.com

Title: Re: 50 ohm filter, DC to light
Post by aaron_do on Apr 16^th, 2012, 2:07am

Hi rfcooltools,

thanks for the information. I see what you mean now.

regards,
Aaron