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Fourier Filters
Longitudinal periodic perturbation of a transmission line geometry leads to frequency dependent signal propagation. The response is related to the Fourier transform of the perturbation pattern. In the optical domain, periodic perturbation of the refractive index creates 'Rugate' filters that have an optical stop-band.
Similarly, in the RF domain, periodic perturbation of the characteristic impedance will create a band-stop response. Such devices are thus called 'Fourier' filters. Periodic perturbation of the characteristic impedance is achieved by patterning the centre track of a strip transmission line.
Owing to the Fourier relation between the response and pattern, a superposition of patterns creates a response that is the superposition of the individual responses. Unlike conventional distributed circuit filters, these 'Fourier' filters have a suppressed harmonic response; of importance in broad band systems.
