The paper presents a novel mm-wave filter design for extreme narrow passbands (0.1% bandwidth) which can be tuned to arbitrary center frequency allocations in a 900MHz band. The established design is based on a special side-wall configuration of TE113 dual mode cavities. Several details have been invented to overcome the structural constraints and to maintain the required high unloaded Q factor at all tuning allocations. Validation of the concept is provided by the measured results of a 5th order Chebyshev filter design at 26GHz with an equiripple passband of 32MHz (return loss 20dB) and a tuning range of 900MHz. These filter types are applied in the channel branching networks of short haul radio equipment for the combination of an arbitrary number of channels with a center frequency distance of 56MHz.