So-- Which is the most relevant noise measurement? Noise Figure-
or Noise Temperature? If one is better than another at a given
frequency, than another, and then the other is better at greater freqs,
WHY? (and, keeping in mind the FIRST stage establishes the Noise
figure,IF it's gain is enough to overcome the next stage's noise
figure) , then why is this a consideration?
Finally, as temperature is free space must approach absolute zero,
but, considering space "noise from stars, ect", what is it REAL
absolute Noise Temp of the (cold) sky? Inquiring minds want to know!

Jim NN7K


Owen Duffy wrote:
Hello Ian,

Ian White GM3SEK wrote in
:

...
To cut the story short, noise temperature is the only concept that
will always give correct results. As Owen points out, some of the
numbers are large and ugly - but the important thing is that they are
correct. The results can easily be converted back into a more
comfortable format... and those results will likewise be correct.

I make the observation that hams *like* Noise Figure, the the roll up of
a system component's Noise Figure into whole of system impact is often
(very often) not done well.

I was explaining to a local EME enthusiast that a certain two stage 1296
LNA that represents NF=0.51dB when the FET specs give NF=0.78dB for the
first FET alone, is very creative. When the effects of input circuit loss
and roll up of the second stage noise is included, it is unlikely that
such a preamp would have a guaranteed NF better an 0.9dB.

In high performance systems, I perceive a preference to not use G/T as a
metric for receive system performance. Rather, hams will quote (brag) Sun
noise rise (Sun/ColdSky ratio) without statement of the solar flux at the
time, or the time (from which solar flux can be estimated from historical
records), or if they do quote solar flux, it will be the 10.7cm flux
which cannot be reliably extrapolated to the relevant ham band.

The 'science' is often obscured by shallow discussions about whether LNA
Noise Figure is more important than Gain.

Owen