[TenTec] Orion, not noisy

Sinisa Hristov shristov at ptt.yu
Thu Nov 30 10:14:19 EST 2006


Hi Ken


Quoting from http://en.wikipedia.org/wiki/Noise_figure

"...noise figure (NF) is the ratio of the output noise power of a device
 to the portion thereof attributable to thermal noise..."


The output noise can be represented as:

  output noise = gain * (thermal noise + excess noise)

where

  thermal noise is a physical constant constant (for a given temperature and BW)

  excess noise = added noise of an "imperfect receiver",
                 referenced to the input

Now

  noise figure = (thermal noise + excess noise) / thermal noise
  (usually expressed in dB)


Therefore, the audible noise is affected by two variables:

 * the first is "excess noise" and the receiver is
   clearly better if it has less of it;

  * the second is mere gain and it cannot change the signal to noise ratio.


Now consider the following two receivers:

  * RX A having a lot of excess noise and a very low gain;
    the output noise will be low, but the signal to noise ratio
    will be degraded seriously due to the large excess noise;

  * RX B having almost no excess noise and a very high gain;
    the output noise will be high, but the signal to noise ratio
    will be degraded very little.


73,

Sinisa



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