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Question of the Month Answers, July 2006

August 7, 2006
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Published August 07, 2006

You be the expert: The answers to the July Question of the Month are below

Jayesh Nath from Harris Corp. has submitted this month’s question:

What is the proper usage of diplexer and duplexer?

From: Simon Zhou, Mitec Telecom Inc.

Dear Jayesh,
Diplexer is a network that splits a signal to two or more loads, dependent on frequency. Often a diplexer is used to route signals, based on frequency, to two different receivers. A diplexer can also be used to create a "matched" filter that is non-reflective outside of the intended passband. It can also be used as a bias tee, to feed your favorite active device with DC power. Duplexer is a three-port network that allows the transmitter and receiver in a radar or communications system to use the same antenna. The duplexer can be as simple as a circulator in low power applications, or it may be a radioactive gas-discharge T/R tube for megawatt radars.

From: Suranga Perera, MillenniumESP

Dear Jayesh,
Let me explain the differences of diplexer and duplexer first. A diplexer is a three-port frequency-dependent device that may be used as a separator or a combiner of signals. A duplexer is a device that allows a transmitter operating on one frequency and a receiver operating on a different frequency to share one common antenna with a minimum of interaction and degradation of the different RF signals. All products with any of the selected attributes will be returned as matches. Leaving all boxes unchecked will not limit the search criteria for this question; products with all attribute options will be returned as matches.
What is a diplexer? It is a network that splits a signal to two or more loads, dependent on frequency. It is the simplest form of a multiplexer, which can split a signal into many different signal bands. Often a diplexer is used to route signals, based on frequency, to two different receivers. A diplexer can also be used to create a "matched" filter that is non-reflective outside of the intended passband. It can also be used as a bias tee, to feed your favorite active device with DC power. Do not confuse the word diplexer with duplexer, which is the network that permits a transmitter and receiver to use the same antenna. 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 to your tweeter. There are two references on duplexers and receiver protectors that are particularly useful. The first is Introduction to Radar Systems by Merrill Skolnik. The second reference is an article written by Dick Bilotta entitled "Receiver Protectors: A Technology Update," Microwave Journal, August 1997.
Now, what is a duplexer? It is a three-port network that allows the transmitter and receiver in a radar or communications system to use the same antenna. The duplexer can be as simple as a circulator in low power applications, or it may be a radioactive gas-discharge T/R tube for megawatt radars. Important properties of a duplexer are: 1. Low loss between transmitter and antenna in transmit (less than 1 dB is desirable); 2. High isolation from transmitter to receive in transmit (as much as 80 dB for megawatt systems); 3. Low loss between antenna and receiver in receive (less than 1 dB is desirable); 4. Fast switching between the transmit and receive state, sometimes automatically switched by the transmit signal, sometimes by command signal. Receiver protector circuits, sometimes the duplexer by itself, cannot provide enough isolation to the receiver during transmit, and other components are added in from the receiver.

From: Adhikarla Jogeswara Rao, Kavveri Telecom Products Ltd.

Dear Jayesh,
A duplexer is used for the transmitting and receiving purpose. The diplexer is used either for transmitting or receiving purposes.

From: Tharaka Perera, Millennium IT

Dear Jayesh,
Let me start with briefly describing the uses of diplexer and duplexer. Duplexer combines two or more signals onto a common channel or medium to increase efficiency during transmission. When used on radar systems, a duplexer isolates the receiver from the transmitter while allowing them to share a common antenna. A duplexer must be designed for operation in the frequency band used by the receiver and transmitter, and must be able to withstand the output power of the transmitter. A duplexer must provide enough rejection of transmitter noise occurring at the receive frequency, and must be designed to operate at, or less than, the frequency separation between the transmitter and receiver. A duplexer must provide sufficient isolation to prevent receiver damages.
A diplexer (combiner) combines the RF output of two or more radio transmitters into a single output. This is very helpful in reducing the number of radio antennas on a 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 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 City, where over a dozen radio stations transmit through one four-panel antenna. Diplexers are also used to combine the audio and video carriers for television, and also for non-broadcast stations such as amateur radio. 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 extremely useful in homes which are already pre-wired, as it eliminates the need for the difficult installation of unsightly extra cables. However, the cables must be the higher-quality RG6, as the less-expensive RG59 will not pass the high intermediate frequency (usually 950 to 1450 MHz), which the LNB outputs from the feederhorn. In this case, one diplexer joins the two signals together. Another diplexer then differentiates 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. Diplexers are also commonly used to combine UHF/VHF frequency signals (TV/FM radio, for example) onto one downlead, which can then be split back into its component parts as required.

From: SM Hassan, Virginia Tech

Dear Jayesh,
RF devices can be highly sensitive to their output matching. As an example, if the IF port in the mixer is not properly terminated, signals can be reflected back in the IF port and can cause undesired spurious products to be generated. Since the traditional filters are reflective in the stopband (for example, they do not provide a constant impedence as a function of frequency), their use following a mixer should be avoided. Diplexer filters minimize input reflections by offering a constant impedence as a function of frequency. The simple design for a diplexer would be based on a low pass and high pass filter in parallel. The stopband filter is terminated in a matched load while the output is taken from the passband filter. The diplexer is mostly used just after the mixer in a RF circuit to avoid the spurious signals mixing. Duplexer is a device that allows a transmitter operating on one frequency and a receiver operating on a different frequency to share one common antenna with a minimum of interaction and degradation of the different RF signals. Radio receivers can be damaged if high level RF signals, like those directly from a transmitter output, is applied to the receiver antenna. Additionally, receivers may become 'desensitized' and not receive weak signals when high noise levels or another signal near the receive frequency is present at the receivers antenna input. From a technical point of view, the circuit construction of the diplexer and duplexer is almost the same. The only difference is their usage.

From: Todd Nichols, Harris MCD

Dear Jayesh,
These two terms are often confused in our industry. Both have the common root "plex", which means "divided into a number of parts"; "multiplex" means "many parts". Both also have suffixes - "du-" and "di-" - that mean "two", thus adding to the confusion. A diplexer combines/splits two different signals onto/from the same transmission path. Two coupled filters are commonly used to make a diplexer. Examples of diplexers are units that allow VHF and UHF radios to share a dual-band antenna, or that allow the 800 and 1900 MHz paths in a dual-band handset to share a dual-band antenna. These devices that separate/combine signals widely spaced in frequency is a diplexer.
A duplexer also combines/splits two different signals onto/from the same transmission path, but it is usually associated with a tranceiver. A circulator is commonly used as a duplexer. An easy way to remember this definition is to think about a transceiver which has separate, but closely spaced, transmit and receive bands with a fixed frequency offset between the two. This offset is called the duplex distance. Thus, a device that separates/combines signals closely spaced in frequency is a duplexer. These definitions based on frequency spacing unambiguously separate the two devices and their functions.

From: Mark Fallica, Hittite Microwave Corp.

Dear Jayesh,
A duplexer is a component that allows a radio to employ a single antenna for both transmit (Tx) and receive (Rx) functions. A diplexer is a type of duplexer that separates the Tx and Rx signals based on their frequency at the antenna. In addition to the diplexer, there are many types of duplexers, including: a) isolator, Tx and Rx signals are separated by their direction to/from the antenna, b) Ortho Mode-T, Tx and Rx signals are separated by their polarization in the antenna and c) T/R switch that separates the Tx and Rx signals at the antenna in the time domain.

From: Nikolay Ilkov, Synergy Microwave Research

Dear Jayesh,
Duplexer is a device allowing a transmitter and a receiver to share the same antenna and providing a good isolation between them (in order to protect the receiver). Diplexer is a sort of diplex filter - a device used to separate and combine signals with different frequencies (in frequency mixers, for example).

From: Jiri Polivka, Spacek Labs Inc.

Dear Jayesh,
Diplexer is used in frequency-division systems. It comprises band-pass and band-reject filters (sometimes a circulator) to separate transmitted and received signals going to and from antenna to receiver, and from a transmitter. Duplexer is a switching device used in time-division systems like radar. It allows a powerful pulse to get from a transmitter to an antenna while blocking the receiver from overload. After transmitting the pulse, receiver input is connected to the antenna to receive returned echoes.

From: Zhaolong Li, University of Montreal

Dear Jayesh,
A duplexer is a switch that alternately connects the receiver or transmitter to the antenna (if both transmitter and receiver share the same antenna). For example, in a pulsed radar system, the duplexer, in the form of a switch, is used to connect the transmitter to the antenna to transmit the high power pulse signal. After the pulse is sent, the duplexer is aligned to connect the receiver to the antenna to "listen to" the echo of a target. PIN diodes are used to design switches often. A diplexer is a coupling system that allows two or more transmitters to operate simultaneously or separately from the same antenna; it is a two path multiplexer. Generally, it is used in telecommunication systems to seperate many frequencies in one channel into seperate channels of narrowband frequencies. Quite often, it is designed using filter theory and made of a group of filters.

From: Thomas Perkins, BAE Systems

Dear Jayesh,
These two terms are often confused. They are not interchangeable. A diplexer separates (filters) two different bands. A duplexer separates two frequencies (or signals) within the same band, such as in a ham radio repeater. They typically are three port devices, where the input or output is common. The common port is often connected to an antenna. Duplexers usually require highly selective band-pass filters. Diplexers consist of simpler low pass and high pass circuits.

From: Gregory Popp, Automated Cargo Transport Systems

Dear Jayesh,
A duplexer allows the sharing of a single antenna by both a transmitter and a receiver operating in the same frequency vicinity. The duplexer, an in-band device, must be capable of preventing the transmitter's energy from entering the receiver's front end and causing desensitization - a feat that requires careful design and tuning. One major use of a duplexer is in a repeater system where the TX and the RX frequencies are offset. Diplexers, also known as combiners, are used to combine signals onto a feedwire, such as UHF and VHF television signals. The diplexer can also be used in reverse as a splitter to separate the combined signals. The diplexer, however, does not have the high isolation between ports that is required of a duplexer.

From: Carlo Madè, ABF Elettronica

Dear Jayesh,
The proper usage is on radio link. It combines two different frequencies (RX and TX), which have to work simultaneously on the same port (antenna).

From: Mike Stricker, SDES Inc.

Dear Jayesh,
A duplexer is something that allows two signals to share a common device and maintain isolation between the two signals. An example might be a transmitter and a receiver sharing a common antenna and operating at the same time. These are frequently used in repeater applications. A diplexer is a special filter configuration that allows an incoming signal to be split into bands and maintain a relatively flat impedance versus frequency load for the source. An example might be to have a low pass filter and a high pass filter. These are frequently used in the front end of TVs.

From: Mujfarmiya Shaikh, Cranes Software International Ltd.

Dear Jayesh,
A duplexer is a device that separates two different frequency bands. It normally requires only low pass and high pass circuits. A duplexer is a device that separates two frequencies within the same band. It requires more selective circuits to perform separation. The best example to understand the usage is to check/study the block diagram of any "Dual Band Mobile Phone." For example, in an AMPS (CDMA)/US-PCS dual band mobile, just after the antenna diplexer is used to separate bands of AMPS and PCS systems, and after that within the AMPS and PCS band duplexer, is used to separate two frequencies in order to be interfaced with the trans and receive portion of the circuit.

From: Elochukwu John Adisionu, Jonel Engineering Co.

Dear Jayesh,
Diplexer and duplexer are important devices with vast applications in telecommunication and printing realms. The application of both devices is inevitable because they check problems created by reduction in frequency spectrum and the consequent generation of out-of-band frequencies due to increased wireless communications. These problems are checked by combining filters (high pass and low pass) to produce a device known as an antenna diplexer. In a nut shell, applications of diplexer can be summarized as: 1. Permission of multi radio transmitters to share a single antenna; 2. Combination and separation of off-air signals from or onto a single feed line, as observed in a TV-satellite diplexer; 3. Combination of VHF and UHF signals in TV to boost efficiency; 4. Combination of audio and video carriers for TV to maximize performance. Applications of duplexer can be summarized as: 1. Add-on accessory attached to a printer, this device creates room for double-sided simultaneous printing; 2. Combination of multi signals onto a single channel to create room for upsurge in transmission efficiency.

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