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# microstrip BP filter from an elliptic prototype - 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.

guest
Post subject: Matching networks for differential design Posted: Tue Dec 27, 2005 9:12 pm
i need to design an amplifier that has differential input and output and match it for 50 ohms. however, since i've differential input/output how do i match for 50 ohms...some one told me you match it for 100 ohms but that didn't make sense

also, what's the best way to test for matching in the case of differential, meaning how should the match be tested??

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Guest
Post subject: Differential ImpedancePosted: Thu Dec 29, 2005 8:29 am
In the amplifier you're talking about, there will be 3 input impedances: Input 1 to ground, input 2 to ground, and input 1 to input 2. You can't assume Zdiff = Zin1 + Zin2 at all. Consider a 1:1 transformer terminated in 50 Ohms - you could ground either input and not affect the impedance between the two inputs at all. Or you could place, say, a 1k resistor from one input to ground, and a second 1k resistor from the other input to ground. Zin1 = 500 Ohms, Zin2 = 500 Ohms, Zdiff = 48.78 Ohms (50 || 2k).

The easiest way to measure differential input impedance is with a transformer of known behavior. The best way is with a differential vector network analyzer - and that's \$\$\$\$.

Good Luck!

Posted  11/12/2012
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