Overview of microwave RF passive components
- RF passive devices are divided into linear devices and nonlinear devices.
- Linear passive devices can be divided into reciprocal and non-reciprocal.
- The linear reciprocity element only performs a linear transformation on the microwave signal without changing the frequency characteristics and meets the reciprocity principle.
Generally, the passive components we call refer to linear reciprocity components.
Power divider
Power divider is a device that divides one output signal energy into two or more outputs.
It is essentially an impedance converter.
Can the power divider be reversed to replace the combiner?
Ø when used as a synthesizer, it not only needs high isolation and low standing wave ratio, but also focuses on bearing high power.
Considering the mismatch of output ports of common cavity power divider, large standing wave; Microstrip power divider bears the characteristics of low power in reverse. We do not recommend using power divider to replace combiner.
Characteristics of cavity power divider
Ø the cavity power divider adopts high-quality alloy as the conductor and the filling medium is air.
Ø it can withstand relatively high power, up to 200W; The dielectric loss and conductor loss are basically negligible, and the insertion loss is small, which can be less than 0.1dB.
Ø however, since there is no isolation resistance and the isolation of the output port is very small, the cavity power divider cannot be used as a power synthesizer
Schematic diagram of power divider test indicators
1 port can measure the standing wave ratio; The insertion loss can be measured at ports 2 and 3. Due to the device characteristics of the cavity power divider itself, the standing wave of the output port and the isolation of the output port are not proposed as the declared value.
RF coupler
Ø coupler is an element that distributes the energy of the input signal through the coupling of electric field and magnetic field, part of which becomes the output of the coupling end, and the rest becomes the output of the output end, so as to complete the RF power distribution.
Ø the power distribution of the coupler is unequal.
Also known as unequal power divider.
Directional coupler
Ø directional couplers are often used to sample microwave signals with specified flow direction. The main purpose is to separate and isolate signals, or conversely mix different signals. When there is no internal load, directional couplers are often a four-port network
Ø directional couplers are often implemented in two ways.
Cavity RF coupler
Features: carrying high power and low loss.
Reasons:
1. The filling medium inside the cavity is air. In the transmission process, the medium dissipation caused by air medium is much lower.
2. The coupling strip is generally made of conductors with good conductivity (such as silver plating on the copper surface), and the conductor loss is basically negligible.
3. Large cavity volume and fast heat dissipation Withstand high power.
Directional Coupler 6dB 350-3800MHz
3dB Hybrid Coupler
3dB Hybrid coupler is a kind of directional coupler.
When used as a power synthesizer, the two input signals are connected to each other as isolated ports, while the coupling output and direct output ports are reciprocal If it is used as two outputs without considering the loss, the input signal power is divided into two outputs.
When used as a single port output, the other output must be connected with a matching power load to absorb the output power of the port, otherwise the transmission characteristics of the system will be seriously affected. At the same time, it will also bring additional 3dB loss, which will affect the cost and reliability of its active part for system application
Main engineering applications
It is mainly used for combining between different carriers in the same frequency band.
Due to the discreteness of the circuit and processing assembly, the isolation of the input port of the bridge coupler is relatively low, so it is not recommended to be used in the combined application between different frequency bands.
To sum up, in the application of different frequency combining, except that the adjacent carrier frequency in the same frequency band (such as the adjacent carrier frequency in the GSM downlink frequency band) can only use 3dB Hybrid Coupler instead of duplex / multiplex combiner, it is recommended to give priority to duplex / multiplex combiner in use, so as to improve the performance index of the system and increase the reliability.
broadband hybrid coupler
RF attenuator
The attenuator is a two port reciprocal element
The most commonly used attenuator is the absorption attenuator
Ø Coaxial Attenuators are usually used in engineering, which are composed of “π” or “t” attenuation networks.
Coaxial Attenuators usually have fixed and variable attenuation.
The attenuator is mainly used to control the transmission energy of microwave signal and consume excess energy in the detection system, so as to expand the dynamic range of signal measurement, such as power meter, spectrum analyzer, amplifier, receiver, etc.
