""Joel Koltner" wrote in message
...
Here something I've been thinking about lately...

The idea of a noise figure N is, simply enough, how much loss in SNR is
seen going through a network (typically an amplifier) -- N =
(Si/Ni)/(So/No), expressed in dB. Say I have an antenna that I know
happens to provide an SNR of 60dB... if I feed that antenna into an
amplifier with a power gain of 100 (20dB) and a noise factor of 2 (3dB),
at the output of the amplifier my SNR will be 57dB. Easy peasy, right?


Easy peasy, but wrong!!!


You may have a 60dB SNR but that says nothing about the actual level of
noise that is applied to the input of the amplifier from the antenna.

You may be better off thinking in terms of noise power (in Watts) rather
than NF.

For example, your amplifier will add a noise power of 3dB above thermal to
the path. If your input noise power from the antenna is 20dB above thermal
then when it is summed with the amplifier's noise contribution there will
only be a very very slight increase in the overall noise power. Hence the
noise figure will only increase very slightly, and your SNR will only
degrade very slightly. (It will not be 20+3dB!!!!)


The situation is the same when you add a second amplifier, you must take the
sum of the input noise from the antenna and the amplifier noise ( in watts),
multiplied by the amplifier gain (not in dB) to give you the noise power
that is at the input of the second amp. Then you must sum in the noise power
contribution of the second amplifier.

From the above it now becomes clear that if the gain of the first amp
dilutes the noise contribution of the second amp on the overall noise level.
(unless the gain is very low and the NF of the second amp is very high).

73
Jeff