Joel Koltner wrote:
"Ian White GM3SEK" wrote in message
...
An important misconception is about the role of "290K" as a reference
temperature. Contrary to what is stated above, this is *not* a designer
option ("usually 290K", implying that some other value could be chosen).
Well, Owen was using 289K and Wes says, "the noise figure concept has the
drawback that it depends upon definition of a standard temperature, usually
290K." Hence, while I certainly accept that "the IEEE standard definition" is
290K, it seems to me that it's a bit of wishful thinking to suggest that no
one has ever used a different reference temperature in their work.
Owen was responding to the following statement made by you:
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,
To which Owen replied:
The amplifier has an equivalent noise temperature (Teq) of 289K.
A noise factor of 2 is not exactly equal to a noise figure of 3dB.
If the amplifier has a noise factor of exactly 2, then its noise
temperature would be exactly 290K, because F = 1 + (T/290).
But if it has a noise figure of exactly 3dB, then by the same definition
its noise temperature would be 288.626etc K which rounds to 289K.
So Owen was not "using 289K" as an alternative reference temperature. He
was simply giving the correct answer to one of your two alternative
questions :-)
As for Wes's statement, I'm afraid that even in 1975 when originally
published, it was no longer correct for a US source to describe the
reference temperature for the definition of noise factor as "usually"
290K. Strike out the "usually".
All of these concepts originate from a classic 1944 IRE paper by Friis,
which recognized that noise factor and noise temperature must be related
by some arbitrary value of reference temperature - and that very same
paper suggests 290K. However, this was an arbitrary choice; at least in
principle, others were free to choose a different temperature, and I
think that is how the word "usually" crept in.
But in practice 290K gained widespread acceptance and by 1975 it had
already been formally adopted by the IEEE. From that point forward, the
standard reference temperature became 290K - and no other.
--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek