Design of millimetre wave diplexers with relaxed fabrication tolerances

A novel millimetre wave waveguide diplexer topology that can be fabricated using relaxed fabrication tolerances is presented. This is achieved by using a highpass filter/hybrid coupler topology in addition to a low order capacitive iris bandpass filter. By targeting the most sensitive centre resonators in the bandpass filter and increasing their length to full wavelength, the sensitivity to fabrication errors associated with changes to the resonator length can be reduced without the introduction of spurious responses close to the passband. The main benefit of this topology is that only one bandpass filter is required to realise the final diplexing response. This is advantageous when tolerance is considered as it is found that resonating structures associated with bandpass filters contribute to tolerance effects. In addition to this, the authors find that the choice of iris discontinuity used to realise the impedance inverters in the bandpass filter can help to relax fabrication tolerances. By subjecting the proposed circuit to a tolerance analysis, simulated results suggest that the circuit can be fabricated with relaxed fabrication tolerances. These discussions are verified with the fabrication of a Ka-band waveguide diplexer, where the measured and simulated scattering parameters are in very good agreement.