"Owen Duffy" wrote in message
...
"Joel Koltner" wrote in
:
But here's an interesting paradox: If I take that output with 57dB SNR
and feed it to another, identical amplifier, shouldn't the SNR at its
output now drop to 54dB?
Appealing, but wrong.

Well, correct *under a certain set of assumptions*. As with, e.g.,
manufacturer's data sheets and quiz/exam problems done in school, often these
assumptions are unstated.

In other words, I'm purposely not stating my assumptions to demonstrate how to
get yourself into trouble more readily. :-)

An real antenna might supply much less through to much much more noise.

How does an antenna at 290K supply less? I mean, ignoring how well it works
as an antenna, shouldn't it still have kTB worth of noise generated just from
the resistance in its conductors?

(If you run through the numbers,
the SNR at the output of the cascaded amplifiers is 56.94dB.)
I get 60-3.2=56.8dB.

I think that's rounding differences and my using T0=290K rather than 289K as a
reference.

It is not so much an issue of safer, is it use and mis-use, it is about
how you use NF with cascaded stages. Essentially, you convert them to T,
apply the gain effects, then T back to a NF for the combination.

Sounds safe to me.

I find noise temperatures just as if not more intuitive than noise figures,
and (to me) it's more obvious what's going on when you have a string of
amplifiers.

You might find this little calculator interesting / helpful:
http://www.vk1od.net/calc/RxSensitivityCalc.htm .

Looks nice, thanks!

---Joel