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# Calculation of component values from Transfer function for desired cut-off frequency

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#### chandresha1

##### Member level 2
I am designing a LC low-pass filter for reactive load(Combination of capacitor and Resistor). I want to design 3rd order butterworth filter in order to filter out all the high frequency component present in amplified PWM signal. My switching frequency is 2MHz and desired output signal frequency is 300KHz. Therefore the cut-off frequency of designed filter will be 300KHz. I can't use computer application for filter designing because it always deals with resistive load but in my case the load is reactive. Therefore i have derived transfer function for T-section LC filter and i want to calculate components value for required cut-off frequency of 300KHz and damping factor of 0.7. How to calculate the values of L1,L2 and C1? Due to the presence of capacitor at load side, transfer function has four poles and one zero.

As a general guideline, L & C values are determined by the Amperes going through your circuit. Small current is associated with small C, large L.

RS appears to be your input resistance. As a starting point, try L & C values which yield the same reactive impedance (RS) at your rolloff frequency.

Time constants (RL and RC) are important in your calculations. Example, RS creates a time constant with L1. Get an idea how these behave at the frequencies you specified. The other sections of your filter are intended to enhance this action.

I think it will be easier to use a simulator for this exercise. It is cumbersome to try to do it by mathematical formula.

I think it will be easier to use a simulator for this exercise. It is cumbersome to try to do it by mathematical formula.

In addition, some Spice-based simulators contains manufacturer libraries with models for real-world components, which considers its parasitic elements, so doing this way the simulation could bring a more accurate result, particularly at this bandwidth.

Ok, I think i have to tune the component of filter and need to check frequency response in LTspice.

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