Passive Components

Power Dividers and Combiners

A power divider, also called splitter, divides an incoming RF-signal to a number of outputs while maintaining the input impedance. Power dividers are used in communication systems, laboratories and radar applications.

A combiner operates in the opposite direction. A number of incoming signals are combined to one output. Combiners are used for increasing power in RF amplifiers.

Resistive power dividers are typically small and can be used on wide frequency bands starting at DC. Resistive power dividers cannot handle high power, loss is high and isolation is not the best. The design is fairly simple but with the right resistors they can have good frequency response. 3-way splitters are most common, but there are also star- and delta formed splitters.

Reactive power dividers have one input and two or more outputs. They can handle higher power. The design is either Wilkinson- or toroidal design. Reactive power dividers covers a wide frequency spectra, they offer low loss and high isolation between the ports.

The Wilkinson splitter, designed by Ernest Wilkson in the 60, can divide the input signal in the same phase and amplitude to the outputs. Main advantages are very good isolation between the ports, low loss and an attractive price for the performance. They can however be limited on band width and sometimes size is the problem.

Directional Couplers and Hybrid Couplers

Directional couplers only connects signal in one directions. Power on the input is transferred to a terminated port and not to the connecter port. A coupler that divides the power equally on two ports is called a hybrid coupler.

Directional couplers with coaxial connectors or wave guides are used between a sender or an amplifier and the load in order to measure or monitor microwave power levels. A coupler provide the possibility to tap a signal on a port to do measurements without moving parts or adjustments.

Directional couplers of coaxial type are available from frequencies from a few MHz to about 50 GHz.
 
Typically they are designed for low VSWR (Voltage Standing Wave Ratio) and low loss. Directional couplers can be of single type or double type. The single version has one main signal path with a terminated path in parallel. The double typ is basically two single couplers that are connected so that both the forwarded signal and that the reflected signal can be measured.

Couplers and splitters are also available with wave guide connector or for mounting on printed circuit boards.

A common type of coupler is the 3 dB, 90° hybrid coupler with four ports, either for dividing the signal equal in amplitude with a 90° phase shift between the ports or to combine two signals with good isolation between the ports.

The rat race coupler is 3 dB, 180° hybrid coupler with four ports that either divides the input signal equal with 180° phase shift or it combines two signals.

Limiters

Limiters are used to protect sensitive components such as low noise amplifiers (LNA) from excess signal power in receivers for examples. Low noise amplifiers are optimized for low noise and cannot handle much power. Impedance matching is important for the limiters because they are located in front of low noise amplifiers since noise or distortion will be amplified thereafter.

Other important limiter parameters are:
Turn On Time:  The time it takes for the limiter to limit the signal. The faster the better.
Max input power (Peak): Maximum pulse power that the limiter can handle before breaking.
Max input power (CW): Maximum continuous power that the limiter can handle under normal conditions.
Flat leakage: Specified RF output power. If a low noise amplifiers can handle 20 dBm, the limiter should have lower flat leakage.
Spike leakage: A spike leakage is fast pulse caused by an RF pulse on the input with faster rise time than the limiter turn on time, that will leak through before the limiter limits the signal.


There are also active limiters based on PIN diodes, Schottky diodes or FETs.

Attenuators and Terminators

There are many type of attenuators for different type of applications; fixed attenuators, variable  attenuators, step antenuators, manual step attenuators, variable attenuators etcetera. Attenuators can be built up with resistors or with semiconductors such as pin diodes.
 
Fixed attenuators are built up with a resistor network. They are designed to put in the signal path to reduce emitted power and amplitude without changing the characteristics of the signal. Fixed attenuators are used in everything from simple RF measurements to calibrations and verifications. Attenuators are available in different power ranges and frequency bands. They can be connected with coaxial connectors, with wave guides or mounted directly on printed circuit boards. The attenuator can be either uni-directional or bi-directional. Attenuation is typically between 1 dB and 60 dB with certain standard values, but customized values are also possible. Power range with coaxial connectors range from 1 W to 1 kW or higher.

The fixed attenuation values of a step attenuator are decided by design of a resistor network and they manually chosen with a push button switch or a rotary switch. A variable attenuator offers stepless attenuation by utilizing varying resistance over a thick or thin film conductive substrate.

A terminator is used to absorb RF and microwave energy and can function as a load in antennas and transmitters. They are also used in circulators and couplers for impedance matching of ports that are not used. 

Terminations are available with coaxial connectors, wave guides or for mounting on printed circuit boards. The power range from 1 W to the kW area, but with waveguides they are available for even higher power.

Bias Tee

A bias tee is a passive three-port device used to supply DC currents or voltages to bias RF circuits. A signal that consists of RF and DC enters on the input. Conceptually an ideal capacitor blocks all DC signals from getting through and only allows the AC-/RF-signals to pass through to a high frequency port. An ideal inductor blocks all RF signals only letting DC signals through to a DC port. A bias tee can be used to insert DC power into an AC signal to power remote antenna amplifiers. It can also be used to for DC biasing an active unit under test.

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