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Click here to learn about the Tsai balun (new for November 2021)
Cllick here to learn about the Marchand balun
A balun is a form of transformer that goes from an unbalanced signal to a balanced signal. The unbalanced, or "single-ended" signal can be microstrip or stripline; the key characteristic is that one of the two conductors is clearly grounded. In a balanced signal, both conductors are equally "hot" with voltage; in reality there is always a true ground somewhere outside of the balun structure, so you almost have to think of it as three conductors. A discussion of even mode and odd mode impedances is required to understand this (sooner or later!)
What's the difference between a balun and a 180-degree hybrid (such as a rat-race coupler)? This is not a joke, it is a serious question... both provide paired, equal-amplitude outputs (or inputs) that are 180 degrees apart. There a few differences... A 180-degree hybrid includes at least one isolation resistor such that the outputs are somewhat isolated. A balun does not. Also, a balun (usually) converts unbalanced signal to a balances signal. The Tsai balun breaks this rule. On a 180 hybrid, all of the ports are unbalanced.
The word "balun" is a portmanteau, for balanced/unbalanced.
Here's a link to a contributed page where a broken mixer is pulled apart, and a very scary balun is revealed... you'll have to click into a few pages to see it!
Ideal balun model
Below is a simple transmission line balun, modeled in Agilent's ADS. The unbalanced input is port 1, and ports 2 and 3 together form the balanced port. The "CLIN" transmission line is the actual balun, it ideally has an odd mode impedance (Zo) of 25 ohms and even mode impedance (Ze) of infinity (1000 ohms is close enough to infinity to be ideal in the example). Here we've split the balanced port in half so that we can plot the amplitude characteristics and phase balance. Note that the input port is 50 ohms, and the output (balanced) ports are each 25 ohms. This is equivalent to one fifty-ohm load across the two terminals. The "MeasEqn" in the schematic provides relative data between the balanced outputs, the ideal phase across ports is 180 degrees.
Also note that in the simple balun design, a shorted stub is required to match the circuit (quarterwave at 10 GHz). Below is the response of the ideal balun. You can see that from 2 to 18 GHz, you get a 10 dB input match. In the phase plot, the ideal 180 phase response occurs at the center frequency (10 GHz). From 2 to 18 GHz the phase error is less than +/- 45 degrees. Such a circuit is what makes 2-18 GHz double-balanced mixers possible.
Marchand balun
One type of balun that is often used in microwave applications is the Marchand balun. This was first reported in December 1944 issue of Electronics (which we have a copy!), titled "Transmission line CONVERSION" (their caps, not ours), by author Nathan Marchand of Federal Radio and Telephone Laboratories. We'll post a schematic for this component soon (and probably add Marchand to the Hall of Fame!)
More coming soon, including a physical model of a suspended stripline balun.
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