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How to reduce the "In-band noise"?

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Junior Member level 2
Feb 17, 2003
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Obverously, we can reduce the out band noise by adding band pass filter, especially we can use the elecllent filter like hight temperature superconductor filter. But how can we improve the the signal in the pass band by reduce the noise in the pass band? First, can you tell me nore information about how the noise in the pass band happen? Second, can we reduce the noise in the RF system? If not, where can improve it, as the signal down to base band then improving it by DSP technology? Thanks for your help.


Hi tungchang,

Are you talking about transmitter? if so, your "in-band noise' is defined as AM/AM and AM/PM effect. For the "out-band noise", it is defined as intermodulation distortion or spectral regrowth. Linearization technique can be applied to reduce those "noise".

You may put at the input of your system a low noise amplifier to reduce the noise factor of the entire system.

Noise in pass band is mainly due to
1) Amplification of thermal noise in the band of interest. Remember that most of the times thae gains will be around 100dB or more in RF systems.
2) Intermodulation products, Harmonics of clocks, etc contribute more noise.
3) Quantization noise during sampling if any quantization is involved.

The best way to reduce noise in pass band is
1a) You cannot do any thing about thermal noise being amplified. Just use an LNA with best possible noise figure so that no "more" noise is added.
2b) Make sure that nonlinear operations are avoided as much as possible. Also add filters to the outputs of devices so that only desired components exist.
3b) Use minimum permissible sampling frequency with maximum permissible levels.

Hope this helps.

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