Pi filter design pdf4/21/2024 ![]() ![]() There is no way DC42 actually calculated anything he is just using standard values LCL, Pi, and T filters are quite another step up from this, but they succumb to the same analysis The OP needs to decide what frequencies he needs to cut out, he also needs to know what frequencies are present in the system so he can keep the resonant frequency well away, a decade away is common with the cut off point a decade above thisīut thats still not the full picture!, the equations assume no load which for DSP applications is just fine, however if this is a power filter, or an appreciable load is flowing then the plot thickens, if its complex terminated then the orders of the equations increase, hell I can think of many a time when the transfer functions need to go out the window and all the state space stuff comes in, its basically as complicated as you want it to beįor a Pi filter you make the values of the capacities the same. not very many I am afraid, whoever posts the correct solution first wins a mars bar!, the work is done Its surprising how many engineers don't know how to do stuff like this, how many engineers can solve the quartic equation the above yields?. Set to 0.707 and solve for W, This will give you the equation for the -3db point, I won't post the equation here, its not common to find it online, its much more complicated than you are giving it credit for, Its not as easy as working with first order filters, which as far as I can tell is all the OP needs anyway Insert S=jw, square, equate real and imaginary then take the square root Plot the circuit, add in a parasitic R, then calculate the transfer function Sorry but this isnt right, you set them equal to each other, put in your W (desired resonant frequency) in order to work out the values for resonance!, this way you are designing a system to resonate Suppose you want the cut off frequency, that is the frequency where the input value is cut by a half at the output, called F, you make This is where the 1/(LC)^0.5 equation comes from Thats the resonant frequency, exciting an LC filter with its resonant frequency is normally what you design to avoid! You want the inductive reactance to be the same as the capacitave reactance ![]()
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