Circulator

A circulator is a device or a circuit for the separation of signaling directions. We distinguish:

• Passive components of the high and very high frequency technology with usually three links (ports ) that function much like a Faraday rotator
• Active circuits of operational amplifiers ( so-called active circulators ) for low signal frequencies; Such circulators can have different numbers of inputs and outputs.

A signal which is fed into one of the ports is forwarded to the next port. At an open port, it is forwarded unchanged in a short-circuit port, the sign of the signal voltage is inverted. If the connection is completed correct impedance, the signal is not routed to the next port. The signals are quasi forwarded "in the circle ", hence the name of circulator.

• 2.1 Electrical Properties
• 2.2 Principle of operation 2.2.1 Row: Decomposition of a ferrite circulator

Active circulators

In electronics, there is a circulator of a plurality of identically constructed with an operational amplifier stages, each with a port. On a port that is not connected, the input signal of a stage is passed on unchanged to the output. At a specified port to ground, the voltage of the signal is inverted. The port is connected with a resistor R to the amount with respect to earth, the voltage of the signal on R is present and the signal is not forwarded to the next port.

Operation

In addition, the function of the shown in Figure 3- port circulator is described. Since the outputs of the individual stages being connected to the inputs of the following stage is considered that:

If the voltage U1 docking in port 1 to ground at terminal 2 the resistor R = Rg connects to ground and the terminal 3 leaves open the operation can be shown. The output voltage of the operational amplifier Ua2 N2 is in this case zero. The stage of the operational amplifier N3 has a gain of 1, since the port is not connected, which Ua3 = Ue3 = Ua2 is. The amplifier stage N1 functions as an electrometer amplifier 2 with the reinforcement, which means that the output voltage Vo1 has a value of 2 × U1. The terminal 2 falls on the resistor R from the half of the output voltage Vo1, which corresponds to the amount of U 1.

For the calculation of intermediate values ​​of these extremes the control node is applied to the N- and P- inputs of the operational amplifier. This results in the following equations:

Structure with current sources

From the equations shown above is obtained by elimination of the input and output voltages, the following equations:

From these equations it is evident that the currents depend on the voltages. Consequently, a circulator may also be constructed from the voltage-controlled current source with a differential input. These to build especially from CC operational amplifiers, which are particularly suitable for this purpose. The figure shows the structure of a voltage-controlled current source is illustrated. The current Ia at the output is given by the equation

Application

An example of active circulators is a telephone hybrid circuit. This consists of a circulator with three ports ( which is chosen independent of the cable type used ) with the Zirkulatorwiderstand Rg is completed. Dating from the microphone signal is routed to the exchange, but does not get into the speaker (handset ). Conversely, the light coming from the switch signal is transmitted to the speaker, however, does not reach the microphone. The crosstalk attenuation is determined here mainly on the matching tolerance of the termination resistors.

Passive circulators

Circulators high frequency technology are manufactured with Faraday rotators of ferrites in waveguide or stripline technology. For stripline technology is, for example, an annular conductor loop or circular area surrounded by a soft magnetic ferrite material. Arranged perpendicular to it is the yoke of a permanent magnet biased ferrite. On annular or circular conductors are mounted at an angle of 120 ° to each other three ports (ports) that serve the coupling and decoupling of the signals.

Such circulators are often designed as a flat, about 30 ... 50 mm round or rectangular components which, provided with three coaxial sockets are usually housed in RF - tight enclosures. There are also miniature circulators for installation in printed stripline circuits. Waveguide circulators are used for very high frequencies and power levels, eg of radars used.

Electrical properties

The transmission loss of passive RF circulators is usually well below 1 dB while the attenuation in the reverse direction - assuming a correct adjustment - is more than 20 dB. The desired function is frequency dependent, that is, a circulator can be used only within the specified frequency range. The bandwidth is, for example, 10%. In waveguide type, about 1 MW CW RF power ( CW ) or above 50 MW can be transmitted pulsed.

Ideal scattering matrix:

Principle of operation

The function of the circulator is that the energy at the input ( port 1 ) is first divided into two equal parts, but the ferrite obtained by a different propagation velocity. At Port 3 both halves are in anti-phase signal, so they cancel each other out. At the Port 2 signal both halves are in phase, so they add up to the complete signal.

The symmetrical construction of the ferrite, it is possible always to be determined by the choice of a connection defined way direction. If an antenna is to port 3, the transmit power is always routed from port 2 to the antenna, while the echo signals always find the way to the receiver at port 1.

The behavior of a circulator is non-reciprocal, ie transmitting from Port 1 to Port 2 is not corresponding to the transmission in the reverse direction. This is achieved by the use of materials (ferrite in the DC magnetic field) whose permeability depends on the field direction. The behavior of the ferrite is anisotropic and can be described as a skew-symmetric tensor ( Polder tensor ). A role of the electron spin and the precession of the atoms within the ferrite in an applied magnetic field.

Row: Decomposition of a ferrite circulator

Under ... the body of the circulator of two strong permanent magnet is surrounded ...

Under the magnet ... nor ever a ferrite layer and in the middle of the line structure with the terminals.

Special shapes

If at one of the three ports of a ferrite circulator with a terminating resistor (dummy) firmly connected, so signals to the remaining two ports can be forwarded only in one direction. In the other direction the signals are strongly attenuated. Due to this property, this embodiment insulator is called. Application: For the suppression of reflected waves in the antenna line of radio or microwave transmitters. A missing or incorrectly connected antenna would otherwise lead to an unfavorable mismatch of the transmitter.

On the same principle of the Faraday rotator, there are insulators for optical wavelengths which make it possible to decouple signals polarization-dependent.

Optical circulators and isolators are used in communication engineering in WDM systems, fiber amplifiers or in the OTDR technique to approximately minimize the crosstalk.

Application

Circulators are often used in radars as a duplexer, ie sent to decouple received signals. In the radio technology and radar technology ferrite circulators are used.

For example, an incoming from the antenna signal at port 1 is transmitted to a receiver port 2. However, the RF power of a transmitter on the port 3, only to the antenna (port 1) pass. The transmit power at port 3 can not come back to the receiver at Port 2 - the receiver is protected and it is lost no signal power. In this way the same antenna can be used simultaneously for transmission and reception. Prerequisite for the correct function is an impedance- correct connection of all three ports.