# How to find cutoff frequency of the circuits

1. ## How to find cutoff frequency of the circuits

Hi, I want to find cutoff frequency of the two attached circuits. I just want to know is there any quicker way to find it. For the circuit2 I got it as 1/(2*pi*R*C) but I don't see the same in simulation, so I want to verify it with someone.

Circuit1: Circuit2: The cutoff frequency is defined as the frequency where the amplitude of H(jω) is 1/√2 times the DC amplitude (approximately -3dB, half power point).
https://electronics.stackexchange.co...ass-rc-circuit  Reply With Quote

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2. ## Re: How to find cutoff frequency of the circuits

Simply calculate voltage transfer function.
Then evaluate cutoff frequency by symbolic math tool such as Mathematica, Maple, Maxima, Symbolic Math Toolbox in MATLAB, etc.

Or use SapWin.
https://en.wikipedia.org/wiki/SapWin  Reply With Quote

3. ## Re: How to find cutoff frequency of the circuits

Hi,

For the circuit2 I got it as 1/(2*pi*R*C)
Circuit2 includes two "L", but your formula does not.

According to your formula the cutoff frequency is independent of L .... I don't think this is correct.

Klaus  Reply With Quote

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4. ## Re: How to find cutoff frequency of the circuits

I don't think you can get a simple equation for finding the 3dB bandwidth for such multi element circuits.
But there are many freeware circuit simulators that can do this job:

http://qucs.sourceforge.net/screenshots.html
https://www.analog.com/en/design-cen...imulator.html#

1 members found this post helpful.  Reply With Quote

5. ## Re: How to find cutoff frequency of the circuits

circuit 2, because it has a resistor in series with the capacitor, does not have a traditional "cuttoff frequency". It has a frequency where insertion loss increases, but then quickly changes attenuation slope to only look like one series inductor.  Reply With Quote

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6. ## Re: How to find cutoff frequency of the circuits

Of course there is an equation. Just the complexity baloons quickly with multi-order filters.

I second sapwin.

And suggest LTSpice which has a great AC analysis.  Reply With Quote

7. ## Re: How to find cutoff frequency of the circuits Originally Posted by asdf44 And suggest LTSpice which has a great AC analysis.
circuitslave does his EDA Tool Play using both Keysight ADS and Cadence Spectre.
So LTspice is not required.

The followings are also not required. Originally Posted by vfone   Reply With Quote

8. ## Re: How to find cutoff frequency of the circuits

circuitslave does his EDA Tool Play using both Keysight ADS and Cadence Spectre.
So LTspice is not required.
Other members might take pleasure in giving "funny" nicknames to you as well. Better stop it.

The original poster has apparently left this thread. I see the Ltspice suggestion addressed to the Edaboard community which sounds reasonable.  Reply With Quote

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