4 - RF transformers

Abstract

This chapter focuses on radio-frequency (RF) transformers, which are used to convert from one impedance level to another or to provide electrical isolation between two circuits. Often, isolation and impedance conversion are both required, and a suitable transformer fulfills both these functions with minimal power loss. Examples of transformers used mainly for isolation include those used to couple in and out of data networks and pulse transformers for SCR firing. Examples used mainly for impedance conversion include interstage transformers in MOSFET VHF power amplifiers and the matching transformer between a 50 Ω feeder and a 600 Ω high frequency (HF) antenna. Such a matching transformer may also be required to match an unbalanced feeder to a balanced antenna. For low power applications, a wide range of ready-made RF transformers is available from manufacturers such as minicircuits and Toko. These usually have one winding rated for 50 Ω use, with various ratios from 1:1 up to 16:1 being available, covering frequencies up to VHF or UHF, and covering a frequency range of between 30:1 and 1000:1. With low interwinding capacitances, these transformers, often in surface mounting packages, are widely used as baluns (with one or both windings being centre-tapped).