[SI-LIST] Re: balun theory
- From: "Ingraham, Andrew" <a.ingraham@xxxxxxxx>
- To: "Si" <si-list@xxxxxxxxxxxxx>
- Date: Sat, 10 Jul 2004 09:26:33 -0400
One way to make a 1:1 balun, is with just a common-mode choke that blocks
common-mode currents. This can be done by wrapping a pair of wires around a
common core, or with a transformer lying "sideways" like what Paul Levin
described, or putting ferrite beads around both conductors of the
transmission line. It doesn't really force a balanced condition on one
side, but it allows it; it really converts unbalanced to floating, which you
may treat as balanced or unbalanced or in-between, as you desire.
4:1 baluns may have a third winding, and then there is a point that is
normally grounded, thus forcing the balanced side to be truly balanced and
not just floating.
Another device with similar bal-un characteristics is a transformer, hooked
up in the normal way (primary in, secondary out). But these may not be
called baluns. If the secondary has a center tap, and you ground it, it
forces that side to be balanced; otherwise it is just floating and could be
connected to either a balanced or unbalanced load (like the 1:1 common-mode
choke balun above). Transformers can be wound to have any impedance ratio,
whereas baluns generally can have only certain discrete ratios (1:1, 4:1,
etc.).
There may be a number of ways to make a balun out of coupled transmission
lines. Some might be broadband while others are narrowband.
If two transmission lines are tightly coupled, then the forward signal
applied between the two on one end, is also what comes out, after the
t-line's delay, between the terminals on the other end. This signal between
the two t-lines lines is the odd-mode or differential signal, and you would
want the odd-mode impedance to match your devices. Now if the even-mode
impedance is made very high (which is basically what the choke balun behaves
like), then it tends to allow the common-mode signals on the two ends to be
anything (i.e., uncoupled), which means that the signal on one end can be
unbalanced while the other end can be balanced or floating. Think of it as
allowing the common-mode voltages on the input and output to be anything,
while blocking common-mode current. What I described would be a wideband
balun since it's not designed to be frequency dependent (i.e., doesn't
require lengths to be a quarter wavelength or anything like that).
Regards,
Andy
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