Lexicon of Balun and Transformer Configurations
RF baluns and transformers may be some of the most challenging components to select for a given system design. For starters, baluns and transformers are used for a wide variety of functions which partly determine the appropriate configuration for a given application. These functions include (but aren’t limited to):
- Providing DC isolation between circuits
- Enhancing common mode rejection
- Providing an impedance transformation for impedance matching
- Efficiently coupling between balanced and unbalanced circuits
- Inducing a voltage/current step-up or step-down
On top of their many different functions, baluns and transformers are implemented with an even wider range of circuit topologies with different properties to support the full gamut of application requirements. Mini-Circuits’ balun and transformer product line encompasses 22 standard configurations (Table 1), which can make navigating the portfolio a daunting task. This quick guide will provide a brief summary of these transformer configurations to help you more readily identify which models are suitable for your needs.
For a deeper dive into the fundamentals of RF transformers, be sure to check out our series on Demystifying Transformers.1,2,3,4,5,6
Mini-Circuits’ Balun and Transformer Repertoire
Mini-Circuits has developed hundreds of balun and transformer models over the years in a wide variety of configurations. Whether DC passing or DC isolated, single-ended or balanced/differential, magnetically-coupled or transmission line transformer, we’ve got a configuration available from DC to 24000 MHz.
Not only are there over 500 different, individual models of balun and transformer that are shoehorned into 22 different configurations, but more and more are being developed and fielded every year, either in surface mount or coaxial case styles. Despite the fact that Mini-Circuits has long been recognized as a leader in core and wire balun and transformer technology, many models are wideband MMIC balun designs up to 24 GHz, or tiny LTCC models as small as 0603. Often the design calls for 50-to-75Ω matching transformers and matching pads, and we deliver. Even suspended substrate stripline designs are within our purview.
Table 1: Qualitative and parametric comparison of the many balun transformer configurations that Mini-Circuits offers.
A Comparison of Key Parameters
Mini-Circuits has such vast experience and such an extensive product lineup that it is important to enable to designer to quickly understand any number of configurations and to be able to apply them expeditiously. As a guide to our many configurations, check out our transformer and balun product page, with the configuration in the righthand column while simultaneously examining Table 1 that follows, and which is sorted by configuration in the lefthand column. Table 1 includes a schematic illustration of each configuration and provides a brief description that will be expounded upon in subsequent sections.
DC Isolation (A, B, C, E, E1)
Table 1 shows balun and transformer configurations from A through S. Transformers with DC isolation between the primary and secondary windings allow the transfer of RF signals between sections of a circuit while disparate DC bias voltages are utilized in each section. They can also be used to separate circuitry where DC biasing is needed from circuitry where DC may be unwanted. Configurations A and B are DC isolated transformers with center taps. Configuration A has a center tap on the secondary winding, and configuration B has a center tap on both the primary and secondary windings. The center tap connection allows a DC feed and DC bias at the corresponding winding without the need for additional bias tees. These configurations can be used for single-ended or balanced lines, or to convert from single-ended to balanced signaling.
Configurations C, E, and E1 are also DC isolated transformers, but these configurations do not have center taps. Configuration C can be used for single-ended and balanced lines or as a balun. Configurations E and E1 are used as baluns, where E shows a balanced-to-single-ended implementation and E1 a single-ended-to-balanced. Configuration E1 achieves 12-18 GHz operation in LTCC technology.
Autotransformers (D, D1, L, N, P, P1)
Transformer configurations D and D1 are called autotransformers. These configurations differ from those previously mentioned in that they allow passage of DC current from the primary winding to the secondary winding. Autotransformers are used in impedance conversion applications, and while the frequency range of these configurations is limited to 2500 MHz and below, they support a wide range of impedance ratios from 0.1 to 14. The D1 configuration also has a DC voltage input port on the primary winding to allow DC feed. Configurations D and D1 only support single-ended lines.
Configurations L, N, P and P1 are also autotransformers. L is a back-to-back version of D, N is a quintessential balanced-to-balanced autotransformer and P is a blend of an autotransformer primary with a transmission line transformer secondary. P1 has a transmission line transformer primary with a step-up secondary section and can perform single-ended-to-balanced operation, which cannot be performed by L.
Transmission Line Transformers (F, G, K)
Transmission line transformer configurations G and K are capable of providing very wide bandwidths. Configuration G may be used in single-ended, balanced or balun applications, while K is not suitable for single-ended-to-single-ended uses. Both configurations are DC passing, supporting DC bias on both the primary and secondary. RF coupling is through the magnetic core at low frequencies and through capacitive coupling at higher frequencies.
While G is typically wound bifilar in core and wire implementations, configuration F is wound trifilar, and can be arranged as G, or as K. If the external connections are made using two windings in series as the secondary, F can take the form of the 1:4 Guanella transformer described in the following section (see H). Since the trifilar-wound F transformer uses three open windings, it can be used in single-ended or differential applications or as a balun. This configuration also provides DC isolation.
Guanella Transformer (H)
Configuration H is known as a Guanella transformer. This type of transmission line transformer can achieve bandwidths in the 10 to 4500 MHz range and impedance ratios of 2 or 4. It can be used for single-ended, balanced or balun applications and is a DC passing configuration. The Guanella configuration also achieves excellent power handling. Mini-Circuits models using this topology currently achieve maximum power ratings up to 10W. This topology is commonly used as a balun transformer in push-pull amplifier applications. The Guanella transformer has been around for over 80 years.
Marchand Balun (J, R)
The Marchand balun configuration is used in microwave applications due to its outstanding high-frequency performance characteristics. J and R configurations offer bandwidths ranging from about 200 MHz up to 24 GHz. This balun design is also over 80 years old. Note that the topology provides DC isolation between the primary and secondary. While J is the quintessential Marchand balun and has the outboard ends of the secondary winding grounded, R enables the DC bias on the secondary to pass to subsequent stages by floating its potential above ground with a capacitor.
Impedance Matching Transformers (M, M1, Q)
Impedance matching transformers are often used for interfacing between 50Ω and 75Ω characteristic impedance environments. These transformers are sometimes used in test setups where a 75Ω DUT needs to be measured with standard 50Ω instrumentation. While M is simply an RF choke that can feed DC bias, M1 adds the transmission line transformer of G to achieve impedance matching. Configuration Q is often times referred to as an isolator. While it, too, is capable of impedance matching, there is often a low-pass filter characteristic to these isolators, enabling DC up to the cutoff frequency to pass while filtering out high frequencies.
Trick Question (S)
Now that your appetite is whet for transformer configurations, the trick question is to determine what configuration S is. After reviewing all these configurations, the reader will naturally try to fashion S into a balun or transformer, but it is in effect a diplexer. Mini-Circuits manufactures hundreds of diplexers using technologies from LTCC to suspended substrate stripline and for a wide variety of applications.
Be a Quick Study
This quick guide should help get you started deciding which RF transformer configurations are right for your needs. If you need more specific guidance on product selection, Mini-Circuits’ applications engineers are always here to support you.
References
- Demystifying RF Transformers: A Primer on the Theory, Technologies and Applications | 2019-10-10 | Microwave Journal
- Demystifying RF Transformers | 2020-07-24 | Microwave Journal
- Demystifying RF Transformers | 2020-07-29 | Microwave Journal
- Demystifying RF Transformers | 2020-08-11 | Microwave Journal
- Demystifying Transformers: Baluns and Ununs – Mini-Circuits Blog
- RF Balun Transformers Demystified – Mini-Circuits Blog
- Frequently Asked Questions About RF Transformers – Mini-Circuits Blog
- RF Transformer Fundamentals – Mini-Circuits Blog
- RF Transformers – Mini-Circuits Blog
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Courtesy of Mini-Circuits
