信号和结果范围的接收是发射机（输出）功率，接收机灵敏度和噪声的结果。 发射机功率：你能喊多大声？ 接收机灵敏度：你的听力有多好？
- 功率越大越好 低灵敏度比较好
- 单位以dbm来衡量 (for example +10dBm) 单位以dbm衡量（比如-93dbm)什么是噪声？
噪音可能会中断信号接收！
电池寿命以及移动电话和基站之间的距离受前端放大器本底噪声的影响。
- 共同的噪音来源热噪声：
- Thermal Noise: As a result of thermal excitation, the electrons in a resistor have an inherent random motion which results in random voltage appearing across the resistor terminals. Flat power spectral density.
- Shot Noise (Poisson Noise): The current that flows across a p-n junction consists of carriers having discrete charges. Charges crossing the junction will give rise to fluctuations of the average current which are proportional to the DC bias current. Flat power spectral density.
- Flicker Noise (1/f): Low frequency noise, typically from semiconductor devices. The spectral power density is inversely proportional to frequency. Not a factor at microwave frequencies. (Modelithics will discuss the relationship between 1/f and thermal noise)
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