RF Balun

RF transformers provide the very useful functions of an impedance transformation and a voltage step-up or step-down depending on the transformer design. RF transformers may have multiple conductive pathways, each with a unique coupling ratio, which can be used to realize complex impedance or voltage transformations. Common uses for RF transformers are to efficiently connect balanced and unbalanced circuits (baluns), DC current injection, to provide DC isolation, improve common mode rejection, impedance matching, and voltage/current transformation. RF transformers, and hence baluns, can be fabricated with a variety of methods, from hand wound core-and-wire components all the way to compact monolithic microwave integrated circuits (MMICs).

The technology used to fabricate an RF Transformer/balun is dependent on the frequency and performance requirements of the RF transformer/balun, with practical limits that make some manufacturing methods preferred at various frequency regimes and performance levels. Due to these practical limitations and the fundamental operation of RF transformers, these components have a lower and upper cutoff frequency. Hence, practical RF transformer bandwidth is limited by the component’s own inductive, capacitive, and resistive features.

The frequency translation capabilities of mixers is only possible due to the nonlinear nature of what is called a mixer circuit. Many nonlinear circuits can be used as mixers, but not all perform well for any given application. The most common mixer devices are made using gallium arsenide (GaAs) transistors, Schottky diodes, silicon field effect transistors (FETs), and complementary metal oxide semiconductor (CMOS) transistors.

The two main uses for RF transformers in a signal chain is to match two disparate impedances, and to effectively connect balanced and unbalanced circuits. Without impedance matching between two circuits, there could be substantial reflections leading to a high VSWR and a significant loss in efficiency that may also lead to thermal management issues if the signal power passing through the poorly matched node is high enough. To achieve maximum RF power transfer, two circuits need to present impedances with inverted reactive impedance components and matched real impedance components. RF transformers do have a limit to their impedance ratio that is a function of the typology and fabrication technology, which may mean that some RF transformers are better suited to different impedance matching applications.

In the other case, when one side of the circuit is balanced (differential) and the other side is unbalanced (single-ended), then simply connecting these circuits would result in grounding one of the differential sides of the balanced circuit. An RF transformer balun, the most common type, can be used to make the conversion from balanced to unbalanced, symmetrically, without introducing significant loss or an impedance mismatch. This is why baluns are commonly found at antenna ports where an RF antenna or circuit may be balanced or unbalanced, and a conversion is required. Introducing an RF transformer may come with the tradeoff of limiting a systems bandwidth, or requiring a higher performance RF transformer to meet the desired bandwidth requirements.

Important RF Transformer/Balun Performance Parameters

• Impedance ratio
• Insertion loss
• Mid-band loss
• 3dB bandwidth
• Return loss
• Amplitude balance (unbalance)
• Phase balance (unbalance)
• Common-mode rejection ratio (CMRR)
• Max DC current carrying capacity
• Max RF power
• Max Curie temperature
• Operating temperature

Learn more about Fairview Microwave’s Surface Mount Baluns