20171209sdr看书笔记

Setting the RF Gain

• RF gain setting
  • RTL AGC
    RTL AGC turns on the internal dongle automatic gain control to algorithm. （RTL AGC开启内部适配器自动增益控制算法） It is almost never used as it tends to introduce a lot of inwanted noise.
  • Tuner AGC
    Tuner AGC uses the RTL-SDR drivers AGC settings and finally,the gain slider can be used to manually set the gain.
  • What is AGC?
    Auto Gain Control(AGC).AGC为自动增益控制,它的作用是当信号源较强时,使其增益自动降低;当信号较弱时,又使其增益自动增高,从而保证了强弱信号的均匀性。
• Goal
  The goal when setting the gain is to try and get the signal to noise ratio(SNR)as high as possible.We can calculate the SNR by subtracting(减去) the peak signal right from the height of the noise floor(噪音基底).When increasing the gian,there will come a point at which the noise floor begins to rise faster than the signal strength rises.This is the point to stop increasing the gain.

Setting the PPM correction

• PPM(parts per million)
  百万分之一。跟在频率后面一般代表标称频率的偏差,如标称频率10MHz的晶振,频率偏差1ppm就是10Hz
• Reasons
  High quality oscillators(振荡器，振子) have a low PPM value.As the RTL-SDR dongles are mass produced with low quality 28.8MHz crystal oscillators(晶体振荡器),the tuned frequency can be out by approximately +-100PPM.
• Ways
  • know the true frequency offset you may encounter.
  • Adjust the PPM offset value box accordingly so that the signal lines up exactly with the red tuning line.

SDR SHARP Plugins

see details in the book.