Baluns and ununs are essential in RF signal chains for many applications. RF balun designs are most commonly associated with core-and-wire transformers, but can also be realized through coaxial and coupled stripline technologies. The behavior of baluns and ununs was introduced in Part 1 of this series, where we established that both these devices are designed for impedance matching purposes. The major difference between them is that baluns are designed to match impedances between balanced and unbalanced circuits, whereas ununs provide impedance matching between two unbalanced circuits.
Part 1 of our Demystifying RF Transformers series discusses the basic theory and applications of RF transformers. This article aims to provide a deeper investigation into baluns and ununs with the main focus on baluns due to their greater prevalence in real-world applications.
Common Balun Applications
The most common use for baluns is when a single-ended power amplifier is used to drive a balanced load. Examples include dipole antennas or single-ended antennas such as whips, which are needed to feed an additional front-end amplifier (see Figure 1). In the past, baluns were also widely used in the CATV industry, for example when matching between a 300Ω dipole antenna for broadcast TV with a 75Ω coaxial cable. With the development of RF Integrated Circuits (RFICs), baluns are now also widely used to improve noise immunity and common mode rejection. The growth in 5G applications has also led to tremendous demand for small, wideband baluns to interface with highly integrated radio transceivers using differential inputs and outputs.
Ununs are often used if an unbalanced feedline is driving an unbalanced antenna, and there is an impedance mismatch between the feedline and the antenna. A whip antenna with a low input impedance would benefit from an impedance transforming unun to efficiently couple a 50Ω feedline with the antenna.
Figure 1: Unbalanced to balanced circuit conversion (left) and balanced to unbalanced circuit conversion (right).
Introduction to Balun Theory and Balanced and Unbalanced Systems
Before tackling balun theory, it is important to understand the differences between balanced and unbalanced two-terminal sources and loads. In a balanced circuit, signals travel along two paths, each with equal impedance to ground. The impedance of a balanced system is defined by the impedance between the two paths, whereas in an unbalanced system, one terminal is connected to ground.
Figure 2 illustrates the difference between signal response in a balanced and unbalanced circuit. The unbalanced circuit shows the voltage established between a single line and ground. The amount of current flowing between ground and the source is equal to the current in the circuit. The balanced circuit shows a differential signal flowing where the voltage is the potential difference between the two lines. In this case, the current flowing to ground on one line is equal to the current flowing from ground on the other line.
Figure 2: Unbalanced circuit (left) and balanced circuit (right)
Common Two-Terminal Systems:
- Antennas during transmission
- Meter or test instrument with a receiver
- Antennas during reception
- Signal generators
- Meters or test instruments with a generator
Understanding the difference between common mode signals and differential signals is crucial for understanding the importance of baluns. In a balanced circuit, common mode signals are those that are equal in magnitude and polarity between the two lines. Differential signals, on the other hand, are equal in magnitude, but opposite in polarity. In general, differential signals are much more robust. This is due to the fact that differential signaling has inherent noise immunity. In a typical system, external noise is equally present on both lines of a balaced configuration and appear as a common mode signal. A differential signal is represented by the difference in voltage between the two lines. Since the common mode signal is equal on both lines, it is cancelled out.
Balun Modes & Technologies
There are two main modes of baluns: current baluns and voltage baluns (see Figure 3). Current baluns operate by forcing equal currents on both balanced lines, effectively eliminating common-mode currents. Voltage baluns force equal voltage on each balanced line; this is ultimately a better fit for impedance matching applications.
Like most transformers, baluns can be fabricated using core-and-wire transmission lines (e.g. coax), Low Temperature Co-fired Ceramic (LTCC), and Monolithic Microwave Integrated Circuit (MMIC) technologies. There are two main versions of core-and-wire baluns: isolation transformers and autotransformers, both of which are voltage baluns (see Figure 4). Above a few gigahertz, it is often necessary to use transmission line baluns to achieve desirable performance. One of the best-performing varieties of transmission line balun is the Marchand balun (see Figure 5). Many of Mini-Circuits’ LTCC and MMIC baluns use the Marchand balun topology.
Figure 3: Voltage balun (left) and current balun (right).