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John Jardine wrote:
Diego Stutzer wrote in message m... Hi, Every one knows, that e.g. a simple RC-parallel circuit has a frequency-dependent impedance-characteristic (Absolute Value) - the impedance (Abs) raises as the Frequency approaches zero. As a formula: Zin = 1/(1/R + i w C) , where i ist the imaginary number and w the frequency. Now the hard part. How does one create an Impedance, which decreases "slower", for frequencies close to zero but then decreases "faster" for higher frequencies, than the simple parallel RC-Circuit? Is there some kind of procedure like the one for syntesizeing LC-Filters (Butterworth, Chebychev,..)? You need to graph out the required frequency-impedance slope then approximate the required roll off rates using a segmented breakpoint scheme consisting of a number of CR series sections in parallel. Essentially it's a straight line approximation to the required Z-F curve. The CR's adding zeroes as the frequency goes up. Estimating the individual time constants can be irksome as each has effect outside it's area of interest. Use a 'least-squares approximation' to obtain a best curve fit for the number of sections involved. It's an interesting subject but I've come across nothing out there that's of use. ___ o-|___|--+--------+--------+---o 10k | | | | | | --- --- --- --- --- --- |100n |10n | 3n3 .-. .-. | | | | | | | |15k | |10k | '-' '-' | | | | o--------+--------+--------+---o created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de use fixed font to view This does exactly what you want, in the beginning the slope is less than 3dB/oct. and at 10kHz it goes to 6dB/oct. This is how to produce a pink noise that rolls off faster at the end of range, or to make some weighted filters (dBA) etc. ciao Ban |
#2
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Ban wrote in message ... John Jardine wrote: Diego Stutzer wrote in message m... Hi, -clip- ___ o-|___|--+--------+--------+---o 10k | | | | | | --- --- --- --- --- --- |100n |10n | 3n3 .-. .-. | | | | | | | |15k | |10k | '-' '-' | | | | o--------+--------+--------+---o created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de use fixed font to view This does exactly what you want, in the beginning the slope is less than 3dB/oct. and at 10kHz it goes to 6dB/oct. This is how to produce a pink noise that rolls off faster at the end of range, or to make some weighted filters (dBA) etc. ciao Ban [Slightly OT].These 'spread CR' things are *weird*. How else can 1Hz to 1MHz be set with just one pot!. ,-------------------+--------------. | | | .-. .-. | 2k7| | | | | | | | | | '-' '-'220 | | | | | ¦ V+ | | | |\| | ,---+---+--++---+---+--------- | ------|-\ | | | | | | | Min.-. | ---'-o .-. .-. .-. .-. .-. .-. | |-----|+/ Square wave out | | | | | | | | | | | | | | |/| | | | | | | | | | | | | Max'-'Pot V- '-' '-' '-' '-' '-' '-' | 10k | | | | | | | --- --- --- --- --- --- Comparitor --- --- --- --- --- --- | A| B| C| D| E| F| .-. '---+---+---+---+---' | |680 | | | 0V '-' A=10K:10u | B=4k7:1u 0V C=2k2:100n D=1k2:100n E=680:1n F=330:100p created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de regards john |
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