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The problem under study here is the minimax design of linear-phase lowpass FIR filters having variable passband width and implemented through a Farrow structure. We have two main contributions. The first is the design of adjustable FIR filters without discretization, using 2D positive trigonometric polynomials, an approach leading to semidefinite programming (SDP) formulation of the design problem. The second is to modify the design problem by a special choice for the passband and stopband edges of the variable FIR filter. The advantage is a lower implementation complexity. The new problem is solved using positive hybrid real-trigonometric polynomials and their SDP parameterization. Design examples prove the viability of our methods.