Duplexer

A duplexer is a filter assembly, the certain rules depending connects two inputs to an output.

These duplexers extreme requirements:

• It must be connected to the antenna very high transmission power;
• It must be connected to the receiver loss extremely small reception services;
• The receiver must be protected from the high transmission power ( no strike );

• 4.1 Functioning
• 4.2 advantage
• 4.3 disadvantages

• 5.1 advantages
• 5.2 disadvantages

Application in radio

In the radio transmitter and receiver are constantly connected to the antenna. Because the received frequency must be simultaneously radiated by the gain in the relay station again For details please look here under duplexer

Application in radar

In the radar operating with pulses. The transmitter and receiver are turned on so it alternately to the antenna, but never simultaneously. Especially in the radar art, the transmit-receive switch is in mono -static radars are referred to as a duplexer, an antenna switch alternately in time division multiplex method to the transmitter and the receiver.

In the radar technique, several different types were used

• Ferrite circulators,
• Branch duplexer which line resonances exploit
• Balanced duplexer which utilize phase delay times for switching, and
• PIN diode duplexer, which are provided with active switching voltages.

The switchover may last only a few nanoseconds.

Branch duplexer

The Branch duplexer works with λ/4- line sections as resonant lines. Important characteristic of λ/4- line sections is the resistance transformation. A short is by λ / 4 to an infinitely high resistance, so an "open line ", "open line " is detected by λ / 4 as a short-circuit.

With the aid of gas discharge tubes short circuits are generated in a line in the section of ignition. This short circuit is transformed by λ / 4 in an infinitely high resistance and thus prevents further energy is introduced into this line section. These Gas Discharge Tubes are as TR- tubes (Transmit -Receive - tube: the gas discharge tube in the image opposite at point D in front of the receiver) (: the gas discharge tube at point C Anti- Transmit -Receive - Tube) called and ATR tubes.

Operation

At the time of sending both gas discharge tubes have ignited because of the high voltage of the transmitted signal and cause a short circuit at the points C and D. At a distance of a quarter wavelength ( λ / 4) is the short-circuit at the points A and B to a almost infinite resistance transformed. The transmit power remains the only way to antenna and does not reach the sensitive receiver stage.

During the reception time both gas discharge tubes are off, because the echo has a very low power. The gas discharge tubes have a very high internal resistance. Now acts of constructive short circuit at point E: after a three-quarter of the wavelength at point B again the infinite resistance "seen" and the reception energy is transmitted to the receiver.

Disadvantages

• The operating bandwidth is only 5% (due to the required line resonance actually only a single frequency! )
• By switchable transmission power is limited ( the damping with a TR tube is "only" 30 dB), therefore more TR tubes at intervals of λ / 2 are sometimes used.
• The isolation between the transmitting and receiving channel is less than other duplexers as much must always transmit energy in the undesired direction of flow to the receiver, in order to ignite and keep the ignited gas discharge tubes.

Balanced duplexer

Here, the TR- tube are integrated in a waveguide section. On the resulting short-circuit in the ignition, the transmission energy is reflected and phase towards the antenna, or out of phase superimposed in direction of the transmitter.

Operation

During transmission of the Balanced duplexer works on the following principle:

• The transmission energy is distributed in the slot couplers;
• The fraction which has passed through the slit, undergoes a phase shift of 90 °;
• Both performance shares bringing the gas discharge (TR ) tube for firing;
• To this extreme by a short circuit mismatch the transmission energy is reflected;
• Again, the energy splits in the slot couplers;
• The portion that passes through the slit, undergoes a further 90 ° phase shift;
• Both shares reflected in the direction transmitter now have a phase difference of 180 ° and lift almost on;
• Both shares reflected in the direction antenna are in phase and add up to full power.

During the reception time the TR tubes are off and both portions of the received signal are added together after the second slot coupler back in phase with the original signal strength.

Advantage

The Balanced duplexer is very wide band and is only limited by the cut-off frequencies of the waveguide.

Disadvantages

• Even the Balanced duplexer needs some transmission energy to ignite the TR tubes. Transmit energy below this firing threshold reaches the receiver and can cause damage there.
• Following the excitation pulse, the TR tube lights for something else. During this recovery time, the radar is still blind.

PIN diode duplexer

Duplexer in semiconductor technology with pin diodes were an attractive alternative due to good isolation, fast recovery time and long life. PIN diodes have a dependent of its bias internal resistance and can also switch large amounts of energy. A limiter circuit with PIN diodes, and a negligible insertion loss limits the signal at the receiver input at a constant level. However, the pin diodes for a good block the transmission power must be activated with low losses in the receive path. This complicates the circuit, and results in the risk of a total loss when the switching voltages as a result fail of a defect. Therefore, several switching stages consecutively used in practice for safety.

Benefits

• High life
• No switching delay
• Fast recovery time

Disadvantages

• Requires active switching voltages
• Switching errors can have disastrous consequences
• High to be switched services require additional protection

PIN diodes need for switching a control voltage, which is usually provided by the synchronizer.