to go to our main page on microwave filters
here to go to our page on duplexers
What's a diplexer? It's a three-port
network that splits incoming signals from a common port into two
paths (sometimes called "channels"), dependent on frequency.
A diplexer is the simplest form of a
multiplexer, which can split signals from one common port into
many different paths. The incoming signals must be offset in frequency
by an appreciable percentage so that filters
can do their job sorting them out.
could be used to route signals to two different receivers, based
on frequency. Or it could be used to create a "matched"
filter that is non-reflective outside of the intended passband.
It could also be used as a bias tee, to feed your favorite active
device with DC power.
Don't confuse the word diplexer
which is the three-port network that permits a transmitter and receiver
to use the same antenna, at essentially the same frequency. Duplexers
are often used in radar, because the transmitter
and receiver commonly share an antenna, and the returned signal
doesn't vary much from the transmitted signal in frequency (just
shifted slightly by the Doppler effect).
Diplexers are employed more
often in communications, seldom in radar. But never say never,
as Chris points out! Wideband multifunction radars can use diplexers
to split received signals to different receive chains based on frequency.
Examples of this appear in Modern Radar Systems by Hamish
Meikle which is available on Google
A diplexer is yet another example
of a microwave concept with an audio analogy. In audio, a "crossover
network" is used to route bass signals to your sub-woofer and
woofer, and treble signals to your tweeter.
Below is a very simple example
of a diplexer that requires that a signal at 2.0 GHz be split from
a signal at 2.2 GHz, modeled on Agilent's ADS. It was a college
homework problem, not an example of how someone might actually attempt
to do this. One requirement was that only a pair of open stubs be
used on each arm.
Try to think of the diplexer
in terms of what each arm must block. In order to block 2.0 GHz,
quarterwave open stubs (E=90) are used in the right arm. In order
to block 2.2 GHz, quarterwave stubs at 2.2 GHz are used in the left
Refer to our section on quarterwave
tricks. Here you will learn that two mismatches of equal magnitude
can be made to cancel each other if placed approximately one-quarter-wave
apart by rule of thumb. This
diplexer application stretches our rule of thumb, you will see that
the "quarter-wave spacing" between the left arm is 16
degrees, and on the right arm it is 152 degrees... hey, we said
approximately, right? You always have to "tune" the length
of line between the mismatches, and when the mismatches are extreme
like they are here, you get results that seem far from 90 degrees.
In the response below you will
see that that port 3 blocks the 2.2 GHz signal and passes the 2
GHz signal, and port 2 does the opposite. The problem with this
circuit is that because the two frequencies are so close together,
the bandwidth is very poor. In real life you would do this with
a pair of higher-order bandpass filters, maybe lumped element, but
more likely coupled line, if you have the room.
coupler bias tee
You can use a pair
of Lange couplers, cascaded back-to-back, to make a diplexer for
use as a bias tee. Referring to the figure below, port 1 is the
input port, port 2 is the DC port and port three is the RF port.
From 6 to 18 GHz you have less than 1 dB RF loss from port 1 to
3. Note that the impedance match at port 1 is excellent, from DC
to 18 GHz.
An interesting point
was raised on our message board recently. If you follow the ADS
symbol for a Lange (shown below), you don't see that both sets
of diagonal ports are DC-connected. What's missing in the picture
are the wirebonds. Look on our Lange
coupler page and you'll get a better look at a real Lange at
the bottom of the page (the wires are hard to see, but they're there!)