to go to our mixer page
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!)
The word balun is a portmanteau!
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!
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 (Ze) of 25 ohms and even mode impedance (Zo) of infinity
(1000 ohms is close enough to infinity to be ideal in the example).
Thanks for the correction, TLK! 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.
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.