Characterization of Causal Filters in The Frequency Domain
Let h(t) be a causal filter with corresponding Fourier transform H(ω). Define the function
which is non-causal. On the other hand, g(t) is Hermitian and, consequently, its Fourier transform G(ω) is real-valued. We now have the following relation
where Θ(t) is the Heaviside unit step function.
This means that the Fourier transforms of h(t) and g(t) are related as follows
where is a Hilbert transform done in the frequency domain (rather than the time domain). The sign of may depend on the definition of the Fourier Transform.
Taking the Hilbert transform of the above equation yields this relation between "H" and its Hilbert transform:
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