All About Circuits Forum Low Pass Response in LTSpice
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#1
12-09-2012, 08:52 PM
 Sparky49 Senior Member Join Date: Jul 2011 Location: Cambridgeshire Posts: 764 Blog Entries: 12
Low Pass Response in LTSpice

Hi guys.

I've found this awesome program called LTSpice which lets you model circuits, and I've been playing around with it today.

I put in a simple low pass filter, and when I tested it, I got the response.

May I ask what causes the sudden change in attenuation?

Also, I am not aware of an equation to find the gradient of the response - does anyone know one?

Sparky
Attached Images
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#2
12-09-2012, 11:55 PM
 Papabravo Senior Member Join Date: Feb 2006 Location: Michigan, USA (GMT-5) Posts: 5,786

It is a nice discovery. Some of us have known about it for almost a decade or more.

It is a "feature" of the capacitor that you cannot chnge the voltage across it instantaneously. As the frequency of the generator goes up tha capactitor cannot keep up with the input because it takes time to move charge onto and off of the plates.
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#3
12-10-2012, 07:26 AM
 crutschow Senior Member Join Date: Mar 2008 Location: L.A. USA Posts: 6,323 Blog Entries: 1

The equation for the filter is here. The reason your response looks rather odd is the the rolloff frequency of your filter is .006Hz and you are plotting the circuit with a linear frequency scale out to 20kHz. Typically you use a log frequency scale (octave or decade in LTspice) to plot frequency response. So on your simulation command, change the Type of Sweep to Octave or Decade, the Number of Points to 10, the Start Frequency to .0001 and the Stop Frequency to 10, to get a more normal looking plot.
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 Tags low, ltspice, pass, response

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