Duplexer
A duplexer is a three-port network that, depending on certain rules, connects one of the inputs with an output. There is no or only an insignificant frequency difference between the signals at the various inputs. The connection rules are usually structured as a time dependency. Duplexers are used to operate a transmitter and a receiver bidirectionally (in so-called duplex mode ) via a single transmission channel (for example an antenna ). They can be set up as a filter assembly or as a network with switch diodes or pin diodes .
The switching process in a duplexer can be done either directly and passively (by timing control signals) or indirectly and actively (triggered by the transmission power itself). The time dependency is usually formulated according to the rule: As long as the transmitter is working, it must be connected to the transmission channel; the own receiver must meanwhile be separated from this transmission channel. With the active switchover (triggered by the transmit power), however, there is always a small switchover delay in which the full transmit power acts on the sensitive receiver input. In the case of passive control, the control signals can be selected to be somewhat longer than the duration of the transmission signal, which prevents undesired effects on the receiver.
If signals with such different frequencies are passed through such a network that they can be separated by frequency-dependent filters, then this assembly is no longer a duplexer, but a diplexer . If the transmission power is low and there is little need for isolation between the connections of the duplexer, its function can also be carried out by three or more- port ferrite circulators which, depending on the direction, route the signals from one or more inputs to one or more outputs.
Application in communication technology
In communication technology, the transmitter and receiver are operated on the same antenna. The duplexer here is often a relay that switches the transmitter to the antenna by pressing the talk button. It is also possible to process part of the transmission power into a switching voltage that switches this relay independently of manual actuation of the talk button.
In voice radio, the transmitter and receiver can also be permanently connected to the antenna. The received frequency must be transmitted again at the same time after the amplification in the relay station . In most cases, however, it is sent in a different frequency than is received. The duplex switch is therefore not a duplexer, but a diplexer.
Application in radar technology
In radar technology in particular , the transmit / receive switches in monostatic radar devices are referred to as duplexers, which alternately switch an antenna to the transmitter and the receiver in a time division multiplex process. Mechanical switching (for example by a relay) is no longer possible due to the high switching rate from 350 Hz to over 8 kHz. In the pulse radar technology works with strong impetus services. The transmitter and receiver are only alternately connected to the antenna, but never at the same time. As a special case, this condition also occurs with an FMiCW radar .
Several different types of duplexer are used in radar technology:
- Branch duplexers, which use line resonances,
- Balanced duplexers, which use phase delays for switching, and
- pin diode duplexers that are supplied with active switching voltages.
Extreme requirements are placed on the duplexer of a radar device. It has to switch very high transmission powers to the antenna (on the order of many megawatts), whereby the switching process itself may only take a few nanoseconds. In between, it has to switch the extremely small receiving power (on the order of down to a few picowatts) to the receiver with little loss and protect this sensitive receiver input from the high transmitting power during the transmission time (no breakdown ).
Branch duplexer
The branch duplexer works with λ / 4 line sections as resonance lines. An important property of λ / 4 line sections is the resistance transformation , which is used here. A short circuit becomes an infinitely high resistance after λ / 4, ie an “open line”, the “open line” is recognized as a short circuit after λ / 4.
With the help of ignited gas discharge tubes , their conductive plasma creates short circuits in a line section (see adjacent picture). This short circuit is transformed into an infinitely high resistance according to λ / 4 and thus prevents further energy from being introduced into this line section. These gas discharge tubes are called TR tubes (Transmit-Receive-Tube: the gas discharge tube in the graphic opposite at point D in front of the receiver) and ATR tubes (Anti-Transmit-Receive-Tube: the gas discharge tube at point C). While the ATR tubes Nulloden can be the TR-tubes are fremdgetriggert or vorionisisert by a glow discharge to be ionized by the transmission pulse faster and are more likely to shut off the receiver.
Working method
At the time of transmission, both gas discharge tubes have ignited due to the high voltage of the transmission signal and thus cause a short circuit at points C and D. At a distance of a quarter of the wavelength ( λ / 4), this short circuit becomes one at points A and B. transformed almost infinite resistance. The transmission energy only remains on the way to the antenna and does not reach the sensitive reception level.
During the reception time, both gas discharge tubes went out because the echo has a very low power. The gas discharge tubes have a very high internal resistance. Now the constructive short circuit takes effect at point E: after three quarters of the wavelength, the infinite resistance is "seen" again at point B and the received energy is conducted to the receiver.
disadvantage
- the operating bandwidth is only 5% (because of the line resonance required, actually only one frequency!)
- The transmission power that can be switched through is limited (the attenuation with a TR tube is "only" 30 dB), which is why several TR tubes are sometimes used at a distance of λ / 2.
- the decoupling between the transmission and reception channels is less than with other duplexers, since so much transmission energy always has to flow in the undesired direction to the receiver in order to ignite or keep ignited the gas discharge tubes.
Balanced duplexer
Here the TR tubes are integrated in a waveguide section. The transmission energy is reflected on the short circuit that occurs during ignition and is superimposed in phase towards the antenna or out of phase towards the transmitter.
Working method
When sending, the balanced duplexer works according to the following principle:
- The transmission energy is split up in the slot coupler;
- the portion that has passed the slot experiences a phase shift of 90 °;
- both power components cause the gas discharge (TR) tube to ignite;
- The transmission energy is reflected at this extreme mismatch caused by a short circuit;
- again the energy is split up in the slot coupler;
- the portion that passes the slot experiences another phase shift of 90 °;
- both parts reflected in the direction of the transmitter now have a phase difference of 180 ° and almost cancel each other out;
- both components reflected towards the antenna are in phase and add up to full power.
During the reception time the TR tubes are extinguished and after the second slot coupler both components of the reception signal add up again in phase with the original signal strength.
advantage
The balanced duplexer is very broadband and is practically only limited by the cutoff frequencies of the waveguide.
disadvantage
- The balanced duplexer also needs some transmission energy to ignite the TR tubes. Transmission energy below this ignition threshold reaches the receiver and can cause destruction there.
- After the transmission pulse, the TR tube glows a little. During this recovery time, the radar is still blind. The transmission time and the recovery time determine the minimal possible location distance of a radar device.
Duplexer with pin diodes
Duplexers in semiconductor technology with pin diodes have become an attractive alternative due to their good barrier insulation, a fast recovery time and a long service life. pin diodes have an internal resistance that depends on their bias voltage and can therefore switch large energies. A limiting circuit with pin diodes and a negligible transmission loss limits the signal at the receiver input to a constant level. However, the pin diodes must be switched to active for a good blocking of the transmission power with low losses in the reception path. This complicates the circuit and leads to the risk of total failure if the switching voltages fail due to a defect. Therefore, in practice, several switching stages are used one after the other for safety.
advantages
- long life span
- no switching delay
- quick recovery time
disadvantage
- requires active switching voltages
- Incorrect switching can have catastrophic consequences
- high outputs to be switched require additional protection
For switching, pin diodes require a control voltage, which is usually provided by the synchronizer.