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RF - LNA Basics
1. Concept and Function of LNA
LNA, or low noise amplifier, is a key part of the RF receiver. It can amplify weak signals received by the antenna, improve the receiver sensitivity and dynamic range, and reduce interference. In the receiving circuit, the signal will be attenuated by components such as the mixer, and weak signals are easily masked by noise. LNA can amplify the signal under the influence of low noise, which is convenient for subsequent processing.
Nowadays, wireless communication tools have higher requirements for system receiving sensitivity. The system receiving sensitivity is closely related to the noise factor (NF), and the noise of the first-stage amplifier plays a decisive role in the entire receiver, so LNA is a key means to improve the sensitivity of the system.
2. Main indicators of LNA
Take the LNA in the routing device as an example, let's look at several key parameters:
- Gain: It is the ratio of the average power transmitted to the load to the maximum available power of the signal source. LNA is designed according to the best noise matching, and the relevant gain at this time is 2-4dB less than the maximum gain.
- Noise factor (NF): It is the ratio of the input signal-to-noise ratio of the amplifier to the output signal-to-noise ratio. The NF calculation method of single-stage and multi-stage amplifiers is different. In engineering applications, tubes with high gain and low noise coefficient are usually selected. However, the indicators in the specification book are for reference only and need to be tested in practice.
- Stability: The absolute stability of the amplifier RF needs to meet specific conditions, which is measured by the stability discrimination coefficient K. K greater than 1 is a stable state. In engineering, devices with K greater than 1 should be selected, and the final test value K must also be greater than 1 during design.
With the improvement of integrated circuit integration, WIFI6 and WIFI7 basically no longer use single LNA devices, but commonly use integrated modules. But we still need to study LNA in depth to better solve the problem.
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