Resonant Tunnelling Diodes for Millimetre and Sub-Millimetre Wave Mixing Applications

The primary intention of this research work was to evaluate a topology for a sub-harmonic down conversion mixer exploiting the fourth harmonic of a LO signal. Designs were evaluated by simulation at 640GHz and 320GHz with the aim of exploring the potential of a RTD based down-converter at 640GHz, in the 580-750GHz atmospheric window, with an intermediate frequency signal in the range around 2GHz by mixing with the fourth harmonic of a 159.5GHz LO signal. Related design studies were undertaken at 320GHz which gave a simulated single side band (SSB) conversion loss of 5.7dB, and with a LO power requirement of less than -9.5dBm which vindicated the principle, as far as the design stage is concerned, of using RTDs as the non-linear mixing element, where the layer design can be tailored to favour very low pump powers.
The other, related, target of the current PhD work was to also explore the potential for high LO drive level mixers and their up-conversion efficiencies using the same novel devices, i.e. RTDs, but with a different layer design, better suited to support high pump powers in this instance. For achieving the latter goal, two different sub-harmonic up-conversion mixers employing a single RTD and using the second harmonic of an LO signal were designed and evaluated at two different frequencies. The first mixer design was aimed at 180 GHz providing -7.5dBm of output power while the second one should work at 110GHz showing output power in the range of -4dBm, and was used to initially evaluate the approach and which could, in principle, be later fabricated and measured.
All these down and up-conversion mixers were carefully designed using ADS and HFSS and evaluated using two different technologies, microstrip and Grounded Coplanar Waveguide (GCPW), and both compared with a nearest Schottky diode based approaches, and also their physical mask was produced in anticipation of a later fabrication stage.