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In this study, a theoretical analysis leading to the derivation of the overall incoherent transfer function of a microwave photonic filter composed of the cascade of two optical structures is developed. Two cases that are more common in practice are considered, i.e., 1) a filter illuminated by a single optical source and 2) a compound filter illuminated by an optical source array. In both cases, the conditions on which the overall incoherent transfer function can be expressed as the product of the individual incoherent transfer functions of each of the optical structures that compose the filter are investigated. In other words, the conditions where the end-to-end electrical linearity is preserved when cascading incoherent optical structures are studied. These results are of importance for the design of complex incoherent filter structures aiming to provide high Q or single resonance performance.