"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.

---Joel

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

Thanks for the clarifications, Ian. (OK, really, thanks for pointing out the
numerous errors I made. :-) )

"Ian White GM3SEK" wrote in message
...
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.

It's interesting to me that, when I was in school, all the noise
figure/temperature stuff was done without Friis's name ever coming up...
whereas his name was prominently mentioned when discussing the path lose
relations (based on distances, antenna gains, etc.)

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)

Speaking of interesting things, I've always thought that you RSGB guys tend to
produce books/articles/etc. at a rather higher technical level, on average,
than the ARRL does. The first time I was at Dayton and stopped by a booth
that George Dobbs was manning with various QRP kits and RSGB books, I must
have dropped $100. :-)

---Joel

"Joel Koltner" wrote in
:

"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.

Joel, you misunderstood my calc.

The 289K was the internal noise of the DUT with NF=3.00000dB. The reference
was (and must be) 290K.

Owen

"Owen Duffy" wrote in message
...
Joel, you misunderstood my calc.

Yeah, Ian pointed that out to me. My apologies...

""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

On Fri, 20 Mar 2009 19:46:53 -0700, "Joel Koltner"
wrote:

Say I have an antenna that I know happens to provide an SNR
of 60dB...

I've been following this saga for a while now, and I note no one seems
nonplused by the statement above. For as much that has been unsaid,
there must be a flood of presumptions that flowed from this detail.

73's
Richard Clark, KB7QHC

Richard Clark wrote in
:

On Fri, 20 Mar 2009 19:46:53 -0700, "Joel Koltner"
wrote:

Say I have an antenna that I know happens to provide an SNR
of 60dB...

I've been following this saga for a while now, and I note no one seems
nonplused by the statement above. For as much that has been unsaid,
there must be a flood of presumptions that flowed from this detail.

Indeed. I addressed some in my second posting, perhaps you missed it?

Owen

On Sun, 22 Mar 2009 06:34:03 GMT, Owen Duffy wrote:

Richard Clark wrote in
:

On Fri, 20 Mar 2009 19:46:53 -0700, "Joel Koltner"
wrote:

Say I have an antenna that I know happens to provide an SNR
of 60dB...

I've been following this saga for a while now, and I note no one seems
nonplused by the statement above. For as much that has been unsaid,
there must be a flood of presumptions that flowed from this detail.

Indeed. I addressed some in my second posting, perhaps you missed it?

Owen

Hi Owen,

I did note:
On Sat, 21 Mar 2009 03:25:39 GMT, Owen Duffy wrote:
I get 60-3.2=56.8dB.

Which appears to embrace this oddity of characterization.

And, as you offer, you say:
On Sat, 21 Mar 2009 03:43:21 GMT, Owen Duffy wrote:
It says nothing of the absolute noise power or signal power. You seem to
assume the noise power KTB noise where T is 290K.

Which still leaves an astonishing characterization accepted, if only
to seemingly fulfill a presumption.

Perhaps I should more blunt, but the quote I lifted only speaks to two
things: an antenna, and a claim for its signal to noise ratio.

60 dB ??????????????

This isn't credible leaving the gate, and how it is then used as a
source to expand the discussion is bewildering beyond compare.

The topic heading as being a paradox is certainly apt, however.

73's
Richard Clark, KB7QHC

Thanks all, very informative-- as this old geezer learned of noise
figure/factor , in the early 60's. and about the time Satelite TV
appeared started seeing reference to noise temp, but was never
too worried about the difference- just curious. and -as I
check this group every couple-3 days, and usually only down
load the most recent 35 pages- must have missed the original
postings. Always wondered if compairing apples to apples,
or to oranges! Now I know ! Again , TNX & 73 Jim NN7K

Richard Clark wrote:
On Sun, 22 Mar 2009 06:34:03 GMT, Owen Duffy wrote:

Richard Clark wrote in
:

On Fri, 20 Mar 2009 19:46:53 -0700, "Joel Koltner"
wrote:

Say I have an antenna that I know happens to provide an SNR
of 60dB...
I've been following this saga for a while now, and I note no one seems
nonplused by the statement above. For as much that has been unsaid,
there must be a flood of presumptions that flowed from this detail.
Indeed. I addressed some in my second posting, perhaps you missed it?

Owen