Compact passive circuits for microwave and millimetre-wave applications

Increasing demand for 5G and broadband systems has motivated the investigation of new technologies to design more compact transceivers. Reconfigurable EBG circuits and hot-via trifilars are two of many possible areas of research which are straightforward to analyse but difficult to synthesize because of their reliance on coupled EM structures. EBG structures being periodic offer sharper roll-off rates and deeper rejections in the bandgaps. However, they are challenged by larger circuit footprint, limited options to achieve multi-band operation, and inadequate measures to counter the parasitic effects of active devices while designing reconfigurable circuits. Novel mechanisms to address these issues have been demonstrated by designing and fabricating several circuits in the PCB and MMIC technologies which shows the suitability of the proposed methods for a wide range of microwave and millimetre-wave applications.

The conventional rectangular patch loaded microstrip line based EBG structure is investigated to develop closed-form equations in an effort to tie the performance of the structure with its geometrical dimensions. A step-by-step design methodology is presented to get an initial estimate of the structure dimensions for a given specification. Comparison with EM simulations and measurement show that the proposed synthesis method provides a first-pass approximation of the physical structure dimensions with 93.8% accuracy. This conventional design is evolved to achieve miniaturization by proposing three different methods. The first makes use of transversal asymmetric loadings to achieve 29.2% compactness to the structure with the liberty to flexibly place the bandgap resonances and stop bandwidths (from 0.7% to 46%) in a wide frequency range of 2.36:1 of the conventional setup. The second method makes use of meander line segments to present a comparatively large electrical EBG which shows a 59.5% reduction in size without compromising other performance attributes. In the third, a combination of two spiral inductors is used with the capacitively coupled patch to achieve 28.3% wider stopband with 62.6% smaller structure as compared to the conventional structure. This EBG mechanism is used to demonstrate a diplexer circuit design offering sharper roll-off rates at the bandgap edges with higher isolations between the transmitter and receiver channels. Similarly, two solutions to design multi-stopband filters using EBG circuits are proposed. First utilizes a combination of etched slots and interdigital capacitive fingers incorporated into the conventional EBG structures to give rise to the second bandgap using a single reactive loading element in a unit cell. The second method uses two EBGs cascaded in a unit cell with distinct reactive loadings to generate two stopbands.

A systematic study to design reconfigurable filters using this type of EBG structures is also presented. Conventionally, these circuits offer an all-pass to bandstop response with the option to tune bandwidth only. However, this work combines the off-state capacitance of the on-chip switching devices with the leakage capacitance of the reactive load to achieve an additional bandgap. Two configurations of the multi-feature reconfigurable periodic filters are experimentally validated using on-chip measurements that support multiple response types, multi-band operation, band-switching, response switching, and frequency/bandwidth tuning capabilities. These adaptive features make these proposed circuits a suitable candidate for many modern 5G and software-defined radio applications.

To support higher drain bias current supplies while maintaining a low-loss impedance transformation from 25 Ω to 50 Ω over a broadband, the design of a conventional trifilar is evolved. The new design makes use of hot-via connections to connect the spiral centre to the bias circuitry instead of existing air bridging mechanisms. This extends the DC-carrying capacity of a conventional trifilar up to 2 A with a similar form factor. The RF performance of the proposed setup is verified by fabricating and measuring trifilar circuits on a wellestablished hot-via supported MMIC process. Then another trifilar is designed with the hot-via configuration to devise post-fabrication procedures to realize the implementation of such circuits in a process that does not support hot-via fabrication. Furthermore, experimental investigations are also carried out to shift the bypass circuitry to an off-chip setup to realize trifilar circuits with comparable performance having a 45% smaller footprint.