Noise exists objectively. The noise which disturbs and threatens useful signals is the main obstacle to the receiving system to reach the maximum sensitivities, trying to reduce noises is extremely important. In order to diminish the external noise (e.g. man-made noise), rules which are related to the noisy environment protection and frequency limitation must be carried out strictly, using highly stable frequency and high quality electrical apparatus, or enhance the shielding and electrical source filtration of the high frequency apparatus; In order to diminish the internal noise, usually a low noise magnifying machine which can work at low temperatures is used as the first magnifier of the receiving system, or reduce the consumption of the feeder line. External noise data is a important environmental parameter of the wireless system.
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