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RF Transformer Types-Core and wire, Transmission line, LTCC, MMIC

This article covers RF Transformer basics and RF Transformer types. It describes working and construction of different types of RF Transformers viz. Core and wire, Transmission line, LTCC and MMIC. The article describes functions and applications of RF Transformer.

What is Transformer?

The transformer is a device with two or more conductive paths linked by mutual magnetic field. When AC current is passed through one conductive path, varying magnetic field ('H') is produced. This varying H induces current in the adjacent conductive path. This current is proportional to ratio of magnetic coupling between the two conductive paths.

Ideal Transformer schematic

RF Transformer operates at radio frequencies. It consists of two or more distinct wires wrapped around magnetic core or air core. They are described as ratio of number of windings or turns. The figure depicts schematic of an ideal transformer. RF Transformers are constructed using various manufacturing techniques and using diverse materials. Based on this, RF transformer can operate at low frequency or high frequency and it can be narrow band or wide band.

RF Transformer Functions and applications

Following are the functions of an RF Transformer.
• It helps in impedance transformation for impedance matching applications.
• It can be used to step up/down the voltage or current.
• It offers DC isolation between the circuits.
• It enhances common mode rejection.
• It helps in filtering high frequency signals from single line.

Different types of RF Transformers | Core and wire, Transmission line, LTCC, MMIC

Based on various topologies, technologies and materials used in manufacturing, RF transformers are of following types. They are available in various package types with range of characteristics.

Core and wire RF Transformer

• They are manufactured by wrapping conductive wires (e.g. insulated copper wires) around magnetic core (e.g. toroid).
• The number of secondary windings may be one or many and with or without center tapped.
• Advangages: Small dimension based core and wire RF transformer operate at higher frequency compare to larger one due to nature of inductive coupling between core and wire.
• Disadvangages: Smaller size transformers will have higher resistive losses and consecutively higher insertion loss at lower frequencies.

Transmission Line RF Transformer

• This RF transformer type consists of transmission lines between two mismatched loads or it may have multiple transmission lines in complex arrangement.
• Basic configuration: It includes two lines. One conductor line is interfaced from generator to load where as second line is interfaced at the output of first line and load to GND.
• Common version: Quarter wave transmission line transformer.
• Advantages: It offers wider operating BW (i.e. Bandwidth) than core and wire RF transformer type.

LTCC RF Transformer

• LTCC stands for Low Temperature Co-fired Ceramic.
• They are multilayer components constructed using ceramic substrate.
• It functions as transmission lines in order to achieve signal conversion (single ended to balanced). It is also used for impedance transformation.
• Advantages: It relies on capacitive coupling and hence it can operate at higher frequencies compare to ferromagnetic transformer. LTCC technology delivers small and rugged transformers which can be used for high reliability applications.
• Disadvantages: It's performance degrades at lower frequencies.

MMIC RF Transformer

• MMIC based rf transformers are fabricated using 2D substrates with precision layered planar metallization.
• Configuration: They are made using spiral inductors printed on substrate in two transmission line configuration. The lines are parallel in this configuration.
• They are constructed using GaAs integrated passive device process.
• Advantages: Outstanding repeatability, higher performance and excellent thermal efficiency can be achieved with precision lithography process.

RF Transformer Manufacturers

Following table mentions popular manufacturers of RF Transformer.

Manufacturers
Coilcraft
AVX Corporation
Mini Circuits
Johanson Technology
Murata
MiniRF
RFMW
Pulse Electronics

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