Introduction to RF Baluns
A balun, meaning balanced-unbalanced, is a three-port device that converts differential signals to single-ended signals, and vice versa. Hence, a balun presents a 180° phase shift between the signals in the balanced end, unlike a power divider with no phase shift between the splitting ports. This functionality allows for baluns to take the input, or output, of a transmission line and convert that into a single-ended signal referenced to ground instead of the reverse current in a transmission line. For an antenna, this means that a single ended antenna can be fed into a coaxial transmission line, but there are many more modern applications for baluns. The growing applications for baluns has led to a diverse range of types of baluns and their uses.
There are several aspects of a balun behavior to worth keeping in mind:
A balun splits the power entering the single-ended port into the differential ports, or combines the differential ports power into the single-ended port, reciprocally.
The differential, or balanced ports, will present equal and opposite phase (180° phase shift) power, but the two outputs are not necessarily matched.
The unbalanced, or single-ended port, is matched to the input transmission impedance, but the outputs of the device may not be the same impedance as the input.
The differential ports may demonstrate different return loss for differential and common mode signals, as the outputs may not be isolated.
There are also several key characteristics of real balun devices to consider:
Different balun technologies, designs, and methods of construction enable the balun to operate over certain frequency ranges.
The balanced ports of a balun, being nonideal, demonstrate some level of phase and power imbalance. These are known as phase balance, and amplitude balance.
The ability for a balun to attenuate identical signals injected into the balanced ports, known as common mode rejection ratio (CMRR), is an important figure in determining the quality of a balun. Common mode signals, such as some noise sources and interference, could otherwise be transferred to the unbalanced port. This may be undesirable for some applications.
The impedance ratio of a balun is a comparison of the unbalanced port impedance to the balanced port impedance. Depending on the load, different impedance ratios may be desirable.
Like virtually all RF devices, there insertion loss and return loss parameters for baluns. This is a measure of how much energy passes through the device and how much energy is reflected from the device, measured per port and it varies with frequency. Different applications will favor
The measure to what degree the differential ports are isolated is known as balanced port isolation. Zero, or high, isolation may or may not be desirable depending upon the balun application.
The balanced port may not be DC or ground isolated from the balanced port, which also may be a consideration.
Signals often experience different transit times through RF devices and interconnect, a phenomenon known as group delay. The flatness of the group delay can also be important for wideband baluns, as it may impact distortion.
Magnetic baluns tend to operate well below 10 MHz, and capacitive baluns may operate well from 1 GHz to much higher frequencies. The type of balun influence the frequency range and the range of other operational parameters.