Low-pass LC butterworth filter cutoff frequency question

In summary, a 5-pole low-pass Butterworth filter with a cutoff frequency of ~50 Hz is being designed for a source impedance of ~1 Ohm and a load impedance of ~50 Ohm. The "T" configuration is being used and values for L1, C2, L3, C4, and L5 have been determined using a table from The Art of Electronics by Horowitz and Hill. However, due to the inability to order custom parts with exact values, parts with slightly different values have been chosen. The question now is what will be the actual cutoff frequency for these values, and the suggestion is to use LTSPICE from Linear Technology to simulate the filter before building it.
  • #1
mehrlin
3
0
I am designing a 5-pole low-pass butterworth filter to have a cutoff frequency of ~50 Hz. It has a source impedance of ~1 Ohm, and a load impedance of ~50 Ohm (The load impedance can be changed around if that will help later). Because of these, I am using the "T" configuration. I used the table in the back of The Art of Electronics by Horowitz and Hill, to determine the following values:

L1=245.9 mH
C2= 107.9 uF
L3 = 220 mH
C4 = 56.9 uF
L5 = 49.18 mH

The problem, however, is that I am unable/unwilling to order custom parts with these exact values. Thus, I have chosen parts with the following values:

L1=250 mH
C2 = 100 uF
L3 = 220 mH
C4 = 56 uF
L5 = 50 mH

My question is now what will be the actual cutoff frequency for these values? I have thus far been unable to track down an equation that will help me find this.
 
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  • #2
Download LTSPICE from Linear Technolgy (linear.com) and simulate it. You should always simulate something before you build it, anyway.
 

1. What is a low-pass LC Butterworth filter cutoff frequency?

A low-pass LC Butterworth filter cutoff frequency is a frequency at which the filter begins to attenuate or reduce the amplitude of higher frequency signals passing through it. It is a parameter that can be adjusted to control the filtering characteristics of the filter.

2. How is the cutoff frequency of a low-pass LC Butterworth filter determined?

The cutoff frequency of a low-pass LC Butterworth filter is determined by the values of the inductance and capacitance components in the circuit. These components are chosen to create a filter response that meets the desired cutoff frequency.

3. What is the advantage of using a low-pass LC Butterworth filter?

The advantage of using a low-pass LC Butterworth filter is its ability to provide a flat frequency response in the passband, meaning that it does not introduce any distortion or phase shift to the signals passing through it. This makes it useful for applications where a clean, undistorted signal is needed.

4. How does the cutoff frequency affect the filtering characteristics of a low-pass LC Butterworth filter?

The cutoff frequency determines the point at which the filter begins to attenuate higher frequency signals. A lower cutoff frequency will result in a more gradual attenuation of higher frequencies, while a higher cutoff frequency will result in a steeper attenuation. This can impact the sharpness of the filter's transition between the passband and stopband.

5. Can the cutoff frequency of a low-pass LC Butterworth filter be adjusted after it has been built?

Yes, the cutoff frequency of a low-pass LC Butterworth filter can be adjusted after it has been built by changing the values of the inductance and capacitance components. However, this may require recalculating the values and replacing the components, so it is best to determine and set the desired cutoff frequency before building the filter.

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