4/6/2024 0 Comments Rc pi filter design![]() That's way off from what DC42 suggested here: I have no idea what I should set the impedance to, but setting it to 1466 ohms gives me 330mH for the inductor, and 153.5nF for the two capacitors. That only calculates PI filters, not LC, and I find the fact that it accepts frequencies only in the MHz or GHz range suspicious, but it does seem to allow me to enter. I have tried adding all sorts of capacitors to remove the noise, and had only moderate success by adding several caps of different values (up to 4700uF), and I've read using inductors alone is no more effective at reducing noise. Is a first order filter one with just a capacitor or just an inductor? Wikipedia wasn't clear on that. (I had to check to make sure you didn't mean to write quadratic.) I don't know how to deal with imaginary numbers, and until today I'd never even heard of a quartic equation. I appreciate the help, but I don't understand a thing you just said. I haven't seen the post you are talking about but going on dc42's posts then I would be surprised if he hadnt calculated something,įor me rules of thumb, standard values etc are nice to use when you know what you are doing, its such a richer experience to do things the hard way sometimes 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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