J. Indian Inst. Sci., May–June 2006, 86, 257–264 © Indian Institute of Science. *Present address: Department of Electronics and Computer Engineering, Indian Institute of Technology (IIT)- Roorkee, Roorkee 247 667, India; Ph: 91-1332-285591, Fax: 91-1332-285368 Novel designs of high-temperature superconducting bandpass filters for future digital communication services SRIKANTA PAL* Department of Engineering Science, University of Oxford, Parks Road, Oxford, OX1 3PJ, United Kingdom. email: pal_srikanta@yahoo.co.uk Received on November 21, 2005; Revised on May 2, 2006. Abstract The design and experimental results of a 5-, 10- and 23-pole lumped-element HTS bandpass filters for the appli- cations in digital terrestrial television and other digital communication services is reported here. These filters are designed around 800 MHz centre frequency, 7.6 MHz bandwidth using end-coupled lumped element resonators on yttrium-barium-copper-oxide (YBCO) or thallium-barium-calcium-copper-oxide (TBCCO) thin films in a 2 inches lanthanum aluminate (LaAlO3) substrate. A new layout profile of a complex 23-pole filter for 70 dB out- of-band rejection 0.4 MHz away from the passband edges is designed, fabricated and tested in an integrated RF- cryocooler measurement system. The design of 23-pole filter was very compact and a significant amount of com- puter time was elapsed to electromagnetically simulate the structure-which was finally done by parts. These lumped element filters were tuned in liquid nitrogen using small pieces of lanthanum aluminate substrates and PTFE rods. The measured frequency responses of 5- and 10-pole filter are quite encouraging. In this investiga- tion, a very useful conclusion is drawn that a compact lumped element design in twinned substrates like lantha- num aluminate is highly susceptible to the local variations in the film parameters and this effect becomes more and more severe as we increase the order of the lumped element filters. Keywords: Electromagnetic simulations, method of moments, high-temperature superconductivity, microstrip filters. 1. Introduction Highly selective, minimum insertion loss, narrowband high-temperature superconducting (HTS) microstrip filters have huge potential for future wireless communications. As an ex- ample, for the application of digital terrestrial television, there is a requirement of HTS fil- ters at around 800 MHz with 7.6 MHz bandwidth, 70 dB rejection loss at 0.4 MHz away from the band edges. In order to achieve that requirement it was calculated that such a filter should be of Chebychev type of 23rd order. This performance is impossible to achieve in conventional conductor technology. Based on this example requirement, a number of design strategies such as the use of lumped element end-coupled bandpass filters, open-square loop-elliptic filters, spiral mean- der filters were investigated and assessed [1–15]. For realizing such a filter in microstrip form, the starting point was to identify a simple structure, which has narrow bandwidth and