Diplexer vs Duplexer vs Multiplexer: Telling the Three Apart
A diplexer splits two well-separated bands, a duplexer splits the TX and RX bands of one FDD system, and a multiplexer handles three or more bands. Count the bands on the common port, then ask whether they are the TX and RX of one system.
For practitioners
Three names, one idea
Diplexer, duplexer and multiplexer all describe a filter network with one common port and several band-limited ports. The names differ by one syllable, and people swap them freely. That causes real mistakes when a block diagram is read quickly.
The difference lies in what is being separated:
- A diplexer separates or combines two different frequency ranges at one common port.
- A duplexer separates the transmit and receive bands of one FDD system at a shared antenna port.
- A multiplexer extends the idea to three or more bands on a common port.
So the prefix counts bands, and "duplexer" adds a purpose: TX and RX of one system. The rest of this article shows what that means for the filters inside each box.
The diplexer: two bands, two ports
A diplexer routes each of its two bands to its own port. The bands are usually well separated. They are not necessarily the transmit and receive bands of one system. Typical examples are a low band and a high band that share one cable or one antenna feed.
The usual construction is a low-pass filter and a high-pass filter whose inputs are tied to the common port. The low-pass output is the low band port. The high-pass output is the high band port.
For an nth-order Butterworth filter with cutoff fc, the attenuation is:
The high-pass curve is the low-pass curve mirrored about f/fc = 1. At f = fc both formulas give 10 log10(2) = 3.01 dB. That is why a low-pass and high-pass pair with the same cutoff crosses at the cutoff, with each filter about 3 dB down.
The figure shows this for n = 3. Both curves start at the same common port. The low band port stays flat to the left of the crossing. The high band port is flat to the right. They meet at the marked point, f/fc = 1, at 3.01 dB.
Worked example: where does a signal go?
Take the third-order pair above and a signal at f = 0.5 fc.
Low-pass path:
High-pass path:
The signal leaves almost untouched through the low band port, and it is cut by about 18 dB at the high band port. An attenuation of 18.1 dB is a power ratio of 65, so only about 1.5 % of the power reaches the wrong port.
Notice also that for these ideal responses the two power ratios add to one at every frequency. At fc each port gets half the power, which is the 3.01 dB figure. A diplexer works because the bands are far enough from fc that one path passes and the other rejects. It does not need a sharp filter edge.
The duplexer: TX and RX of one system
An FDD system transmits and receives at the same time on two different bands. They are separated by a fixed duplex spacing. A duplexer sits at the shared antenna port and sends the transmit signal from the transmitter to the antenna and the received signal from the antenna to the receiver.
The key difference from the diplexer is the closeness of the bands. TX and RX are often close together. A gentle low-pass and high-pass pair cannot separate them, because the crossing region would sit right on top of one of the bands. The duplexer therefore uses sharp bandpass filters, one centered on the TX band and one on the RX band.
The demand is also asymmetric in practice. The transmit signal is strong and the receive signal is weak. The receive path must reject the transmitter's output, and the transmit path must not load the antenna with the receive filter. These requirements push the design toward steep skirts and high rejection in the other band.
Topologically the duplexer is still a three-port network: antenna, TX and RX. What makes it a duplexer is the purpose, serving one FDD system, and the filter type that purpose forces.
The multiplexer: three or more bands
A multiplexer is the same idea with more branches. One common port feeds three or more filters, and each filter passes one band to its own port. A diplexer is the two-band case.
Adding bands makes the design harder, because every filter loads the common port at the frequencies of all the other filters. The branches must be designed together, not as separate parts. In a handset this is the kind of structure that appears when several bands share one antenna feed. For the system-level reason bands multiply, see the other front-end articles on RF View.
A rule for reading a block diagram
Use two questions in this order:
- How many bands are on the common port? Two means a diplexer or a duplexer. Three or more means a multiplexer.
- If there are two, are they the TX and RX of one system? If yes, it is a duplexer. If no, it is a diplexer.
A label on a diagram is not proof. If a box is called a duplexer but its two ports are a low band and a high band of different systems, treat it as a diplexer. The function decides the name, not the drawing.
The ports give a second clue. A box with an antenna port and TX and RX ports on the other side is almost certainly a duplexer. A box with "low band" and "high band" outputs is a diplexer. A box with band 1, band 2, band 3 and more is a multiplexer.
In plain words · for students and beginners
Diplexers, duplexers and multiplexers in plain words
A picture of the three
Think of a road junction where one road splits into branches. The common port is the single road. The other ports are the branches. A filter at each branch lets only certain traffic through, and the traffic is signals at different frequencies.
The figure below shows the three boxes side by side. Each has a common port on the left.
- The diplexer has two exits, low band and high band, far apart.
- The duplexer has an antenna port and TX and RX exits close together, for one FDD system.
- The multiplexer has three or more exits.
The analogy breaks down in one place. Road traffic takes one exit and stays there. A signal at the crossover frequency of a diplexer is split between both ports, about half the power each, because the filters overlap there.
The same example in simple steps
- Build a low-pass filter and a high-pass filter with the same cutoff frequency, called fc. Tie their inputs together. That is your diplexer.
- Send in a signal at half of fc.
- The low-pass filter barely touches it: a loss of 0.07 dB.
- The high-pass filter blocks it: a loss of 18.1 dB, which is about 65 times less power.
- The signal therefore comes out of the low band port.
- A signal exactly at fc loses 3.01 dB in each filter, so it splits roughly in half.
Mistakes beginners make
- Using "duplexer" for any three-port filter. A duplexer is specifically the TX and RX of one FDD system.
- Assuming a duplexer can be built from a simple low-pass and high-pass pair. TX and RX are often close together, so it needs sharp bandpass filters.
- Forgetting that "-3 dB" at the crossing is a power split, not a failure. Each filter is meant to be 3 dB down at fc.
- Counting the antenna as a band. The antenna is the common port. Count the bands that sit behind it.
Remember
- Count the bands on the common port: two is a diplexer or duplexer, three or more is a multiplexer.
- With two bands, ask whether they are the TX and RX of one FDD system. Yes means duplexer, no means diplexer.
- A low-pass and high-pass pair with the same cutoff crosses at about 3 dB, and a diplexer relies on the bands being far apart.
Related pages on RF View
Frequently asked questions
What is the difference between a diplexer and a duplexer?
A diplexer separates or combines two frequency ranges, usually well apart, and sends each to its own port. A duplexer separates the transmit and receive bands of one FDD system at a shared antenna port, and those bands are often close together.
Is a duplexer a type of diplexer?
Both are three-port filter networks with one common port, so the topology looks alike. The difference is the job: a duplexer serves the TX and RX bands of one FDD system and needs sharp bandpass filters, while a diplexer only has to split two bands that are usually far apart.
What is a multiplexer in RF?
A multiplexer extends the diplexer idea to three or more bands on one common port. Each band gets its own filtered port.
Why do a diplexer's low-pass and high-pass filters cross at -3 dB?
A low-pass and high-pass pair with the same cutoff frequency crosses at that cutoff. For a Butterworth response each filter attenuates by about 3 dB there.