[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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