A mode conversion from the TE10 in WR3 rectangular waveguide to the TE53 in an oversized circular waveguide is required to perform the cold test of a Bragg resonator for a high power 250 GHz Cyclotron Auto-Resonance Maser (CARM) under development at the Frascati ENEA research centre. High operating frequency and oversized cylindrical cavity (exceeding any previous CARM experiment) make the design particularly challenging. An initial transition, aimed at transforming the TE10 in WR3 rectangular waveguide into the TE11, in a circular waveguide, is performed using a linear taper, a well-assessed device. Then a serpentine mode converter in circular waveguide with average radius of 1.48 mm and appropriate length has been adopted for the conversion into the TE01 mode. The subsequent transducers, for not-rotating (TE0m) and rotating (TE53) modes, are provided by means of rippled wall mode converters with small periodic perturbations of waveguide radius. The ripple period corresponds approximately to the beat wavelength (?b = 2p/(ßm-ßn)) with ßm and ßn the wavenumbers of the two interacting modes. For not-rotating modes an efficient conversion has been obtained by cascading single step TE01 to TE02, TE02 to TE03 and TE03 to TE04 mode converters. For each of them, profiles, radius and number of periods have been optimized in order to obtain low levels of unwanted spurious modes, low reflection and sufficient bandwidth. As the modes to convert (TE04 to TE53) present similar wavenumbers, the most appropriate method for the conversion consists in a three-periods rippled-wall mode converter made by a helically corrugated waveguide with an average radius of 4 mm and five azimuthal variations. The longitudinal profile of the ripples has been properly smoothed to avoid steps between the circular waveguide and the corrugated converter.In this paper the conversion chain will be described and the simulation results (S-parameters, efficiency and electric field) of each single converter will be reported. Finally some considerations about the high precision micromachining technique to be used, such as copper electroforming and Computer Numerically Controlled (CNC) milling, for the construction of these high frequency components, will be presented. © 2018 Electromagnetics Academy. All rights reserved.