Diplexer
From Wikipedia, the free encyclopedia
A diplexer is a passive device that implements frequency domain multiplexing. Two ports (e.g., L and H) are multiplexed onto a third port (e.g., S). The signals on ports L and H occupy disjoint frequency bands. Consequently, the signals on L and H can coexist on port S without interfering with each other.
Typically, the signal on port L will occupy a single low frequency band and the signal on port H will occupy a higher frequency band. In that situation, the diplexer consists of a lowpass filter connecting ports L and S and high pass filter connecting ports H and S. Ideally, all the signal power on port L is transferred to the S port and vice versa. All the signal power on port H is transferred to port S and vice versa. Ideally, the separation of the signals is complete. None of the low band signal is transferred from the S port to the H port. In the real world, some power will be lost, and some signal power will leak to the wrong port.
The diplexer, being a passive device, is reciprocal. The device itself doesn't have a notion of input or output.
The diplexer is different from a passive combiner or splitter. The combiner is not frequency selective. There also a power "loss" issue - a combiner takes all the power delivered to the S port and equally divides to the A and B ports. A diplexer does not.
A diplexer multiplexes two ports onto one port. We are not limited in the number of ports to multiplex. A three-port to one-port multiplexer is also known as a triplexer.
Contents |
[edit] Common Uses
The chief advantage of a diplexer is it allows two different devices to share a common communications channel. Typically, the shared channel is a long piece of coaxial cable. Rather than run two separate cables, a single cable with diplexers at each end can be used. The plan is economic if the diplexers cost less than running the second cable.
The diplexers might be used to connect two receivers inside a building to two antennas that are some distance away using a single cable. Alternatively, the two transmitters might connect to two antennas.
A diplexer is called a duplexer when it connects a receiver and transmitter to the same antenna. In that case, the antenna is part of the multiplexed communications channel, so only one diplexer is needed. The design requirements for that duplexer are often severe: the receive and transmit frequency bands are often closely spaced and lots of isolation is required.
[edit] Industrial Applications
Diplexing is useful in reducing the number of radio antennas on a radio tower, reducing the weight and loading from wind and potential ice, as well as the necessary size of the tower itself. Diplexers must be carefully engineered: designed and tuned to prevent intermodulation and keep reflected power (VSWR) to a minimum for each input transmitter and frequency. While diplexers can combine a relatively wide bandwidth, the major limitation comes with the antenna itself, which must be sufficiently wideband to accept all of the signals being passed through it, and transfer them to the air efficiently.
One of the most massive diplexers in use is atop the Empire State Building in New York, where over a dozen FM radio stations transmit through one four-panel antenna. Another such setup is on a tower in Miami Gardens, serving the Miami and Fort Lauderdale media market.
Also many other large UHF-/VHF-transmitters use diplexers. The number of transmitters, which can share an antenna is restricted by their frequency difference. Transmitters, whose frequencies are too close together cannot be combined by a diplexer in a well way.
Diplexers are also used at mediumwave broadcasting stations. However their use is not so common in this frequency range as the corresponding wavelength varies in the mediumwave band much more with the frequency than at the FM-band and so it is more practicable to use for each frequency a separate antenna as normally mediumwave transmission sites usually broadcasts as a rule only on 1 to 4 frequencies, while FM-broadcasting sites often uses 4 and more frequencies.
Nevertheles the use of diplexers can be sensitive as back-up device. So in case of maintenance work at one antenna of a mediumwave transmission site with two antennas working on two frequencies, the other antenna can be used for broadcasting both programmes. In other cases it is not possible to build a further antenna for the second transmitter, as there is not enough space on the site available. Than the diplexer is used permanently.
At longwave broadcasting sites diplexers are normally not used as these stations broadcast usually only on one frequency. A realization of diplexers for longwave broadcasting stations may be difficult, as the bandwidth ( 9 kHz) is high in relation to the transmission frequency.
At VLF-transmitters diplexers are also not used as for this frequency range their realisation is very difficult as huge tuned loading coils are used for the antenna fed at which very high voltages occur at transmitter operation.
Diplexers are also used for non-broadcast applications such as amateur radio.
[edit] Residential
Small diplexers are also used in the home, allowing direct broadcast satellite TV signals from the dish to the receiver to piggyback on one regular coaxial cable, along with lower-frequency signals from an outdoor terrestrial TV antenna for local channels. This is useful in homes which are already pre-wired, because it eliminates the need to install extra cables. However, the pre-wired cables must be the higher-quality RG-6. The less-expensive RG-59 will not pass the higher frequency band (usually 950 to 1450 MHz) output by the satellite TV LNB.
In this case, one diplexer joins the two signals together. Another diplexer then separates the signals to the receiver of the TV set and the IRD of the DBS set-top box. More complex systems have a distribution amplifier, which allows each IRD to access multiple LNBs with different antenna polarizations. These usually have an antenna input and a diplexer, so that the antenna signal is also distributed along with the satellite.
Diplexing will not work with HDTV DBS systems, as they use the low broadcast/cable range (and the high range all the way up to 3 GHz) for the additional satellites.[citation needed] This can itself be considered a form of triplexing.
Diplexers are also commonly used to combine UHF TV and VHF TV and FM signals onto one downlead, which can then be split back into its component parts as required.
A DSL filter may also be considered a diplexer, because it splits baseband telephone audio and modulated data. However, the term is usually reserved for radio frequencies; and also such a filter may only remove the DSL from the phone side, but not the phone from the DSL side.
