We propose a novel synthesis method for designing flexible, tunable non-periodic filters. It is based on a building block which is presented by first time for these purposes, being the poles position tuned by means of a coupling coefficient and an amplifier gain. We present the device, the design equations and a design of a filter with flat response, 54 dB of crosstalk and less than 1.3 dB of ripple to explain and validate the method. These filters can be used as part of optical cross-connects for selecting channels avoiding very restrictive free spectral ranges. Also we check the correct operation of the device, against fabrication tolerances of coupling coefficients. Frequency dependence of the transfer function poles as a unique feature improving the crosstalk of the device spectral response is also analyzed.