Diplexers

Updated August 1, 2008

Click here to go to our main page on microwave filters

Click 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.

A diplexer 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 with duplexer, 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 Books.

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.

Stub tuner diplexer

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 arm.

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.

Lange 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!)