interview (2021)

Senad Bulja talks to us about his paper ‘Multi-layered PCB distributed filter’ Please tell me a little bit about your field of research I recently joined Tyndall National Institute (TNI), Dublin, Ireland as Principal Scientist (www.tyndall.ie), tasked with creating a world class hardware Radio Frequency (RF) group. Prior to joining TNI, I spent 10 years at Nokia Bell Labs as a Senior Scientist. My group's interests are threefold. First, we interactively engage with material scientists in order to create new functional RF and mm-wave materials that will, effectively, fuel the development of reconfigurable RF hardware. Secondly, we focus on the research and development of reconfigurable RF and mm-wave hardware, such as filters, antennas and other bespoke RF and mm-wave devices. Lastly, we focus on new Transceiver (TRX) architectures for wireless communications. Our group's aim is to address the reconfigurability of RF front ends from a holistic point of view. TNI is an excellent place to conduct this multi- and cross- disciplinary research due to a synergetic mix of skills – from material scientists to application engineers. TNI has over 600 researchers working on microelectronics and photonics for applications in communications, energy, health and agritech/environment. My research will leverage significantly from TNI's wafer fabrication capability covering microelectronics, MEMS and photonics. Can you describe the background to the work that is presented in your Electronics Letters submission? While part of Nokia Bell Labs, I led the effort that established a new family of low-profile resonators – we termed them distributed resonators, due to the fact that the frequency of operation is no longer a function of one resonant post only, but of many – in other words the frequency of operation is a function of how the resonant elements are distributed in the cavity. Due to its very low profile, the distributed resonator is very attractive for RF applications. Furthermore, its low profile allows for a better distribution of heat, which is an additional benefit. However, all our realisations of distributed filters, up to this point were centred on metal cavity filters. What is the main advance you have reported in your Letter and what is the significance of this advance? Transceiver (TRX) PCB boards tend to be fully populated by various components on their surfaces, such as chips and lumped components, however, the volume under is, usually, unused. This inspired us to think of a possibility of utilising this volume effectively. We initially thought that some kind of integrated waveguide filter solutions could be used for this purpose, however, since TRX PCB boards are usually multi-layered, consisting of PCB boards with different dielectric characteristics, it became apparent that with that approach it would be impossible to control the frequency of operation, which is of crucial importance. On the other hand, the fact that the height of individual resonant elements in a standard distributed resonator has a strong influence on the frequency of operation, naturally led us to integrate resonant elements into the multi-layered PCB TRX stack. The individual resonant elements in our Letter take the form of standard blind vias. The significance of our Letter, therefore, lies with the manifestation that unused space on TRX boards can be put to good use and that one can have control of its characteristics to realise RF hardware, by simply controlling depths of the blind vias and their relative position with respect to each other. Furthermore, our multi-layered PCB distributed filter does not require post-production tuning, which is a standard procedure for all filters. This is due to the dimensional averaging effect of the proposed structure. What challenges did you have to overcome during the research for your Letter? Possibly, our biggest challenge came from device fabrication. The structure of the device presented in our Letter is rather unusual and it required close collaboration with the PCB manufacturer, so that the device could be fabricated in a cost-effective way. During this process, the device has to be re-designed several times, but, in the end we converged to a workable solution that is also cost-effective. How much has your research field changed since you began working in it, and how do you think it will develop over the next 10 years? I have now been active in this field for nearly 20 years. The research on RF hardware reconfigurability was also present then, however, it was still in its infancy. Reconfigurability was almost an afterthought – it usually consisted of an addition of an active semi-conductor component to an RF device, such as antenna or filter, which allowed one to take control of some of the characteristics of the device, such as bandwidth and frequency of operation. However, it came at the expense of increased losses. Increased insertion losses are still a problem, however, I see an increased interest from the RF research community, particularly in the context of 5G and 6G, in the integration of reconfigurability into the materials used for RF applications. This creates interesting opportunities for the exploration of the synergies between material science and RF and mm-wave applications. The creation of such “smart” RF materials will increase the need for “smart” control of RF hardware. Here, Artificial Intelligence (AI) will be expected to play a crucial role. As such, the next decade will witness a strong interplay of material science, wireless communications and AI. It will be a very interesting time for research.