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differential low pass filter - 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.


wolf80
Post subject: differential low pass filter
Unread postPosted: Mon Nov 15, 2004 1:57 pm
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Joined: Tue Sep 14, 2004 2:46 pm
Posts: 5
Location: San Diego, California
Hey guys, can anyone explain what a differential low pass filter is used for and how I can design and simulate one? Thank you.

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Post subject: Differential LPF
Unread postPosted: Tue Nov 16, 2004 8:17 pm

Hi!
A differential low pass filter is used on a (usually!) balanced line, to suppress high frequencies and pass low frequencies. For example: your telephone line is a differential (balanced) line; if you wanted for some reason to get rid of all frequencies above 2.8 kHz, you'd use a differential low-pass filter. This would keep the balanced quality of the line so you wouldn't pick up hum.

The standard approach is to design a standard LPF, put half of the inductance in each lead:

----mmm---------
|L
---
---C
|
-------------------

--------mmm------------
|L/2----/
---
---C
|L/2----\
--------mmm------------


Simulation (if you're using SPICE) requires two transformers - built out of coupled inductors if your version of SPICE doesn't provide transformers as a basic block.

Good Luck!



Posted  11/12/2012
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