Similar to digital and low-frequency circuits, RF circuits are also divided into passive devices and active devices. RF circuits that can complete their functions without power are called passive circuits, while RF circuits that require an external power source to operate are called active circuits. According to this definition, then this source is the power supply.
This passive circuit includes common resistors, capacitors, and inductors, as well as various functional devices such as power splitters, couplers, ringers, filters, antennas, etc.
Resistance, inductance, and capacitance
The three most basic components in a circuit are resistors, inductors, and capacitors. Whether it’s a digital circuit or a radio frequency circuit, these three components all play a crucial role,
In digital or low-frequency circuits, resistors, inductors, and capacitors exhibit pure resistance, inductance, or capacitance characteristics, while in RF circuits, any component has a specific frequency band in which it operates. Beyond this frequency band, its characteristics will also change.
For RF resistors, we usually use the following diagram to equivalent their distribution parameter characteristics. Pay attention to the pin inductance and lead capacitance, and their frequency characteristics are also shown in the following figure: as the frequency increases, their capacitance characteristics will become more and more obvious, and of course, the inductance characteristics will also be prominent. At a certain frequency, the lead inductance and capacitance will resonate, and after this resonance point, the electrical inductance characteristics will become the main characteristic.
Of course, inductance and capacitance also have similar characteristic variation curves. In RF circuit design, it is important to understand the operating frequency of the selected components to avoid unnecessary trouble to the design.
Another major category of passive devices is functional devices composed of inductors, capacitors, resistors, or transmission line structures.
Power divider
The function it implements is the separation, equal or unequal division of signals.
Power splitters have a wide range of applications in RF circuits, especially in antenna feed networks. The most commonly used power divider is the Wilkson power divider, which can achieve the function of one minute N through appropriate cascading, as shown in the following figure:
Coupler
In RF circuit design, we not only need to decompose and synthesize the signal, but sometimes we prefer the signal to be distributed according to a certain phase and power. At this time, directional couplers are a good choice. The schematic diagram of the directional coupler is shown in the following figure. This four port device is usually divided into input port, output port, coupling port, and isolation port.
The following figure is the structural diagram of a directional coupler
Annular/Isolatorular devices are often used as isolators and have a wide range of applications in RF circuits. An isolator, also known as a unidirectional device, is a device that transmits electromagnetic waves in a unidirectional manner. It is a non reciprocal dual port microwave ferrite device. When electromagnetic waves propagate in a forward direction, it can feed all power to the load, causing significant attenuation of reflected waves from the load. This unidirectional transmission characteristic can be used to isolate the impact of load changes on the signal source. The main requirements for the performance of isolation devices are: small forward attenuation (generally not exceeding 0.5-1dB), large reverse isolation (generally greater than 20-30dB), small voltage standing wave ratio (generally not greater than 1.10-1.25), a certain frequency band width, and provisions should also be made for withstanding power and operating temperature.
Filter/Multiplexer
As a cleaner of RF channels, filters play an indispensable role in RF circuit design, whether it is image suppression, or the filtering of stray and harmonic waves, they are inseparable from filters. Whether it is the current dielectric waveguide filter or the current normalized surface/bulk acoustic wave filter.
Antenna
Antennas, as the eyes of RF circuits, are crucial in the entire wireless system. We will explain antennas in more detail later.
There are many passive devices, such as attenuators, phase shifters, and so on, in addition to the ones mentioned earlier.
Post time: Jul-19-2023
