TO BLOCK UNWANTED frequencies, it is common to place a very-high-Q high-temperature-semiconductor (HTS) filter before the low-noise amplifier (LNA) of a radio telescope's front end. A miniaturized HTS four-pole filter for the RF interference mitigation of the 900-MHz cellular band in radio telescopes was recently presented by Alonso Corona-Chavez from Mexico's National Institute for Astrophysics, Optics, and Electronics; Ignacio Llamas-Garro from Spain's Technical University of Catalonia; and Michael J. Lancaster from the University of Birmingham in the UK.
To achieve a quasi-elliptic response, capacitive cross coupling is added between the first and last nonadjacent resonators to introduce two transmission zeroes at the edges of the rejected band. The notch filter targets RF interference mitigation centered at 859 MHz with 8.1-percent fractional bandwidth for the migration of the 900-MHz cellular band. Miniaturized half-wavelength resonators are coupled to the main transmission line. See "A High Temperature Superconducting Quasi-Elliptic Notch Filter for Radioastronomy," Microwave And Optical Technology Letters, January 2010, p. 88.