Sal M. Onella wrote:

"Jim-NN7K" . wrote in message
.. .


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?

In my experience, the community seems to dictate the terminology. (If you
buy a big, long sandwich for lunch, is it a "hero," a "sub" or a "hoagie"?)

More to the point, when selecting an LNA for C-band satellite, you will
almost always see the noise temperature in the specs. However, for Ku-band,
the LNA noise figure is usually spec'ed.

As was pointed out, they are directly convertible. Go a little less than
halfway downpage at
http://www.microwaves101.com/encyclo...oisefigure.cfm
and see the graph of noise temperature versus noise figure. (This web page
also provides illustrations of what's already been presented here.)


You're quite correct. It's the same underlying physics and theory in
every case, but each user community chooses the approach that it finds
most useful.

For example, audio/LF designers tend to deal in noise voltages and also
need to think about source and load resistances. RF designers think more
in terms of noise power, noise factor (ratio) and noise figure (dB); and
since performance tends to be specified and measured in a 50-ohm system,
it often isn't necessary to know the individual source and load
impedances.

The alternative for RF designers is to think in terms of noise
temperatures. For individual devices such as LNAs, NF and noise
temperature are virtually interchangeable (and the difference in usage
between C-band and Ku-band is purely historical). However, noise
temperature is more appropriate for analysis of complete receiving
*systems* that must include the antenna noise temperature as another
important variable.

There are no paradoxes and no conflicts here, only alternative ways of
looking at the same physical phenomena. That vision only falls apart if
one of the alternative viewpoints contains unaware approximations or
errors.



--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

"Ian White GM3SEK" wrote in message
...
For example, audio/LF designers tend to deal in noise voltages and also need
to think about source and load resistances. RF designers think more in terms
of noise power, noise factor (ratio) and noise figure (dB); and since
performance tends to be specified and measured in a 50-ohm system, it often
isn't necessary to know the individual source and load impedances.

These days using a regular old op-amp as an HF amplifier can often be
attractive, although when you go through the math you find out that it's very
difficult to obtain a low enough noise op-amp such that it has a noise figure
less than about 10dB (and even obtianing 20dB requires some care -- you can
easily end up with 40dB if you're not careful!). Texas Instruments has a
good application note on this: focus.ti.com/lit/an/slyt094/slyt094.pdf .

Hence op-amps are pretty much out for LNAs, but can be quite useful by the
time you're hitting an IF and already have some reasonable amount of gain
ahead.

---Joel