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# differential low pass filter - RF Cafe Forums

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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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Guest
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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