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I am using Matlab fdatool to implement highpass IIR butterworth filter But 'Fs' value cell is gray and i am not able to change sampling frequency, what is the actual value in this case of attached photo?

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    $\begingroup$ Fc doesn't matter because your frequency units are set to relative, so nothing is measured in absolute frequencies. It's literally the box above the one you've marked yellow... $\endgroup$ Aug 8 '20 at 15:38
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Cutoff frequency for a discrete time filter can be expressed in different ways. It can be expressed in radians, which is mathematically useful when analyzing the transfer function in the z-domain. It can be expressed normalized to the sample rate, as it is here, which is handy because it is a simple number to work with and is not dependent on the sample rate. Or you can express it as a true frequency, which requires a sample rate and is handy because it’s the actual frequency in the implementation.

It looks like you just need to change the units in the drop down box to set the sample rate. Probably something like Hz, but I’ve never used that tool.

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