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18th October 2019, 21:58 #1
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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...assrccircuitLast edited by circuitking; 18th October 2019 at 22:18.

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19th October 2019, 04:17 #2
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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/SapWinLast edited by pancho_hideboo; 19th October 2019 at 04:15.

19th October 2019, 07:22 #3
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Re: How to find cutoff frequency of the circuits
Hi,
For the circuit2 I got it as 1/(2*pi*R*C)
According to your formula the cutoff frequency is independent of L .... I don't think this is correct.
KlausPlease don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.

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19th October 2019, 16:12 #4
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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/designcen...imulator.html#
https://www.ad5gg.com/2017/04/06/fre...ionsoftware/#
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25th October 2019, 12:03 #5
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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.

26th October 2019, 14:02 #6
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Re: How to find cutoff frequency of the circuits
Of course there is an equation. Just the complexity baloons quickly with multiorder filters.
I second sapwin.
And suggest LTSpice which has a great AC analysis.

26th October 2019, 14:38 #7
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Re: How to find cutoff frequency of the circuits
Last edited by pancho_hideboo; 26th October 2019 at 14:59.

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26th October 2019, 15:32 #8
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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.
The original poster has apparently left this thread. I see the Ltspice suggestion addressed to the Edaboard community which sounds reasonable.
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