Chapter 12 - Transmission lines and printed circuit boards as filters

Abstract

Quarter-wavelength lines of either short- or open-circuit termination can be used to pass some frequencies and to stop others. This chapter provides a description of how transmission lines and printed circuit boards are used to produce filters. One application of this is to allow a radio carrier signal into a receiver from an antenna while preventing internal signals, from the receiver, from radiating back to the antenna. Connecting a short-circuit quarter wavelength line across the antenna input will short circuit low-frequency signals but not interfere with signals at the quarter-wavelength frequency. Transmission lines of less than a quarter wavelength at the passband cutoff frequency can be used to replace inductors and capacitors. The design process starts by producing a conventional lumped element filter design. Short-circuit lines then replace inductors and open-circuit lines replace capacitors. At the stopband frequency, each of these short- and open-circuit lines is a quarter wavelength long. .