Like the passive element, the source of the source of the RF circuit has many the same characteristics as the source components in the low -frequency simulation system. However, some devices are unique to RF design. In addition, different semiconductor technology is usually used to ensure that RF components maintain sufficient performance at a very high frequency.

Amplifier
The amplifier circuit usually revolves around a computing amplifier in low -frequency and high -frequency simulation design. In the RF system, there are two basic types of amplifiers: power amplifier and low noise amplifier. The former is used to increase the power level of the RF signal before launch, and the latter is used to amplify the (usually very small) signal received by the antenna.

Power amplifier
The power amplifier (PA) is used to increase the power level of the signal before the signal is sent to an antenna. A similar situation is also found in the audio circuit: the amplitude of the audio signal may be completely sufficient in terms of voltage, but the power amplifier is required to provide a large amount of current to the speaker coil. In the field of audio, the larger the current, the greater the power, and the greater the volume. The higher the power in RF technology, the larger the coverage.

Low noise amplifier
Many non -RF applications require low noise magnification, but the specific phrase of "low noise amplifier" is common in the RF environment. In fact, we usually hear the abbreviation of this term, that is, LNA.

The receiving signal passed by antenna can be a non -frequently low order, and it is drowned in the noise. This signal needs to be enlarged to further process, but it is also important to minimize the further decrease of the signal -to -noise ratio. Therefore, a low noise amplifier aims to provide high voltage gains and contribute minimal noise.

LNA's noise performance is quantified by "noise coefficient" (NF), which corresponds to the SNR performance decrease (DB) caused by the amplifier. Therefore, NF = 0 DB of the ideal amplifier, as the noise performance decreases, NF increases.

mixer
Another basic RF component is a mixer. This name may cause misunderstandings; radio frequency mixer does not combine signals, just like a mixer. Instead, the RF mixer receives two input frequencies and generates a third output frequency by multiplication. In other words, the hybrid execution frequency translation.

The mixer allows the signal to shift to higher or lower frequencies by maintaining signal details. For example, the (ie, the modulation) baseband signal of the information can be moved to a high frequency suitable for wireless transmission, and the transmitted signal will retain important modulation details in the baseband signal.

Lock -up ring
The actual production of the cycle signal is closer to the field of non -passive elements, but source elements are used to manipulate these cyclical signals. Locking loop (PLL) is actually a system composed of multiple sub -components, including at least one phase test device, a low -pass filter, a voltage -controlled oscillator (VCO), and a frequency division. Make multiple output frequency.

Combining the PLL with the high -precision temperature compensation oscillator can convey the high -precision but fixed; the constant reference frequency to a highly accurate system variable output frequency. The oscillator combined with the PLL is called a synthesizer, which can generate a series of frequency components.

The ability to regulate the frequency of oscillator is very important in RF design. In order to avoid interference, a specific system may need to work on different channels, so the frequency of the oscillating circuit must be adjusted. In addition, the frequency interval between adjacent channels may be relatively small, so adjustment must be accurate.

Data converter
Although the modulus converter (ADC) and the digital modular converter (DAC) are not standard devices in the context of historical RF engineering, they must be realized that they are becoming increasingly important in many RF systems. The ADC and DAC enable the RF system to benefit from the special functions provided by digital signal processing technology, as well as general flexibility and convenience brought by software -based solutions.

The term "software definition radio" (SDR) refers to the wireless communication system that relies on software to implement the important part of the RF signal chain. Data converters are key components in such systems. For example, DAC can be used to directly generate baseband waveforms, and ADC can be used to digitize the received baseband waveforms (then further analysis in digital signal processors).

SDR will increase the complexity of design, but it also provides some advantages, especially in some applications.

RF semiconductor
Silicon is still the main material in semiconductor manufacturing. However, other materials are more suitable for high signal frequencies in RF systems. The three alternative materials used in RF semiconductors are nitride (GAN), GaaS, and SIGE (SIGE). Dedicated semiconductor technology makes it possible to manufacture sufficient performance at extremely high frequencies (ie, above 100 GHz).

Integrated circuit
Like low -frequency devices, the basic active components in the RF integrated circuit are transistors. However, so far, we use the word "component" to refer to devices that may consist of many transistors. The reason for understanding this is very important: designing high -performance and high -frequency RF devices is very challenging, not within the skill range of many RF engineers. The focus of the actual RF project is to combine these components to complete the function circuit, and then deal with various complex problems that occur.

Summary
The active devices used for the RF system may provide special functions or standard functions, but the ability to maintain performance under high frequency is stronger.

RF amplifier is usually divided into power amplifier (PA) or low noise amplifier (LNA). The former provides power gain for transmission, and the latter provides high voltage gain and low noise coefficient.

The RF mixer transforms the frequency by multiplying the two input signals.
Locking loop (PLL) can combine with the oscillator to generate frequency synthesizers.

ADC and DAC are important components in some RF devices. They are becoming more and more common in modern wireless systems, and they are essential in software definition radio.

SIGE, GaaS, and GAN are special semiconductor materials that are better than silicon in high -performance RF applications.

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