RF Transformer Types: Core and Wire, Transmission Line, LTCC, MMIC

This article covers RF transformer basics and RF transformer types. It describes the working principle and construction of different types of RF Transformers, namely Core and Wire, Transmission Line, LTCC, and MMIC. The article also explores the functions and applications of RF Transformers.

What is a Transformer?

A transformer is a device featuring two or more conductive paths linked by a mutual magnetic field. When AC current flows through one conductive path, it generates a varying magnetic field (‘H’). This varying ‘H’ induces current in the adjacent conductive path. The induced current is proportional to the ratio of magnetic coupling between the two conductive paths.

An RF Transformer operates at radio frequencies. It typically comprises two or more distinct wires wrapped around a magnetic core or an air core. They are often described by the ratio of the number of windings or turns. The figure above depicts the schematic of an ideal transformer.

RF Transformers are constructed using various manufacturing techniques and diverse materials. Depending on these factors, an RF transformer can operate at low or high frequencies and can be narrowband or wideband.

RF Transformer Functions and Applications

RF Transformers serve several important functions:

• Impedance Transformation: Facilitating impedance matching in various applications.
• Voltage/Current Step Up/Down: Increasing or decreasing voltage or current levels as needed.
• DC Isolation: Providing electrical isolation between circuits.
• Common Mode Rejection: Enhancing the rejection of common-mode signals.
• High Frequency Filtering: Helping to filter out high-frequency signals from a single line.

Different Types of RF Transformers | Core and Wire, Transmission Line, LTCC, MMIC

Based on various topologies, technologies, and materials employed in their manufacture, RF transformers come in several types. They are available in various package types with a wide range of characteristics.

Core and Wire RF Transformer

• Construction: These transformers are manufactured by wrapping conductive wires (e.g., insulated copper wires) around a magnetic core (e.g., toroid).
• Windings: The number of secondary windings can vary, with options for one or more windings, with or without a center tap.
• Advantages: Smaller core and wire RF transformers can operate at higher frequencies compared to larger ones, owing to the nature of inductive coupling between the core and wire.
• Disadvantages: Smaller size transformers may exhibit higher resistive losses, leading to a higher insertion loss at lower frequencies.

Transmission Line RF Transformer

• Construction: This type consists of transmission lines between two mismatched loads or may involve multiple transmission lines in a complex arrangement.
• Basic Configuration: Includes two lines. One conductor line is interfaced from the generator to the load, while the second line is interfaced at the output of the first line and connected to GND.
• Common Version: Quarter-wave transmission line transformer.
• Advantages: Offers a wider operating bandwidth (BW) than core and wire RF transformer types.

LTCC RF Transformer

• LTCC Definition: LTCC stands for Low-Temperature Co-fired Ceramic.
• Construction: These are multilayer components built using a ceramic substrate.
• Functionality: Functions as transmission lines to achieve signal conversion (single-ended to balanced) and is also used for impedance transformation.
• Advantages: Relies on capacitive coupling, enabling operation at higher frequencies compared to ferromagnetic transformers. LTCC technology delivers small and rugged transformers suitable for high-reliability applications.
• Disadvantages: Performance can degrade at lower frequencies.

MMIC RF Transformer

• Construction: MMIC-based RF transformers are fabricated using 2D substrates with precision-layered planar metallization.
• Configuration: Typically made using spiral inductors printed on a substrate in a two-transmission line configuration. The lines are parallel in this setup.
• Fabrication: Constructed using a GaAs (Gallium Arsenide) integrated passive device process.
• Advantages: Outstanding repeatability, higher performance, and excellent thermal efficiency can be achieved with a precision lithography process.

RF Transformer Manufacturers

The following table lists some popular manufacturers of RF Transformers: