Noise figure paradox
 

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

Noise figure paradox
 

Perhaps , might be related to the old dB Rnco Standard, in analog
microwave paths (for setting Muting (squelch)) with a 30 dB S/N ratio,
at a specified freq slot or channel in the bandwidth (think 1.8 MHz )?

Think gave close to 52 dB s/n ratio at the lowest frequency in the
baseband (order wire) . Jim NN7K




Joel Koltner wrote:
"Owen Duffy" wrote in message
...
"Joel Koltner" wrote in
60dB+ isn't unheard of for hilltop-to-hilltop microwave links though,
is it? And one might obtain 50dB with regular TV antennas if they have
a good line-of-sight to the transmitter and there aren't significant
reflections, right?
It doesn't solve the problem.

I thought Richard's main problem was that 60dB is (relatively) unheard of in
wireless systems. I agree with you 100% that not enough information was given
to determine the absolute signal or noise levels.

Noise figure paradox
 

Joel Koltner wrote:
Radiation resistance is a virtual resistance and does not contribute
thermal or Johnson noise.

Certainly, agreed.

But the loss resistance of the antenna itself is still contributing
kTB, right?

If I take a small loop of wire that has, say, a 100 milliohms of
resistance, it still generates kTB watts of thermal noise power. Why
isn't this a "problem?"


Because that noise power has a source impedance of 100 milliohms, which
is dramatically mismatched to the input impedance of a normal receiver.
This is explained in Wes Hayward's full-length textbook, 'Introduction
to Radio Frequency Design' (now re-published by ARRL).


--

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

Noise figure paradox
 

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

Noise figure paradox
 

On Mon, 23 Mar 2009 10:11:31 -0700, "Joel Koltner"
wrote:

"Richard Clark" wrote in message
.. .
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 ??????????????

Originally I almost added something like, "(assume you're standing next to the
transmitter)" :-)

60dB+ isn't unheard of for hilltop-to-hilltop microwave links though, is it?
And one might obtain 50dB with regular TV antennas if they have a good
line-of-sight to the transmitter and there aren't significant reflections,
right?

This is comparing elephants to oranges. You haven't specified
anything that is noise related, you said nothing about antennas (exept
what might be presumed from vague associations), and receive and power
levels are wholly missing. As dB is a ratiometric relationship, you
have offered nothing to validate the ratio.

Hilltop-to-hilltop microwave links can be designed for a 60 dB snr
(one cannot call it gain, certainly); or 60 db directivity; however
hilltop-to-hilltop microwave links do not automagically qualify as
coming with that directivity if they are too close!

So, you came up with 60 dB, what was the noise level in? what was the
noise level out? What is the source of the noise in? What are you
loading the 1,000,000 * (S+N) into?

73's
Richard Clark, KB7QHC

Noise figure paradox
 

"Joel Koltner" wrote in
:

....
But the loss resistance of the antenna itself is still contributing
kTB, right?

Yes... but summing the contributions isn't trivial.

An alternative view is to consider the contribution of conductor loss and
other losses in the antenna structure and feed, and treat the system as
an ideal (lossless antenna) with a specified 'feed loss'.

My observation is that convention is the use the antenna connector or w/g
flange as a reference point for such calcs. It may even be laid down in
standards... but I am not sure. Someone else may know?

Notwithstanding that convention, I note the VK3UM tools seem to make
their reference point a point on the space side of the antenna. That
would give rise to a slighly different G/T figure.


If I take a small loop of wire that has, say, a 100 milliohms of
resistance, it still generates kTB watts of thermal noise power. Why
isn't this a "problem?"

I don't know what you mean by "problem". I have explained above that it
should be accounted for, and a method.
....
A discussion of noise sounds like a good topic for a ham fair...
technically there's little more complex than algebra (i.e., it's
accessible to pretty much everyone), but plenty of room for
misapplication.

I haven't been to ham fairs in your country, but here there are mostly
focussed on exhanging junk (selling the junk bought at the last fair, and
buying some different junk to sell at the next fair).

Noise is dealt with pretty well in text books, but text books aren't as
popular as mags.

Complicating this in the real world is that receivers aren't perfectly
linear, and measurements in a shielded room often have limited relevance
to real life performance where the 'noise' due to intermodulation
distortion is a significant issue... especially with a trend to avoiding
front end loss (noise) by ditching front end selectivity.

Noise is an interesting topic.

I have just discovered an Agilent AN which discusses uncertainty in noise
measurement. I am about to compare it to my proposition of a statistical
estimate of noise measurement (sampling) uncertainty, see
http://www.vk1od.net/measurement/noise/nmu.htm .

Owen

Noise figure paradox
 

Hi Owen,

"Owen Duffy" wrote in message
...
Yes... but summing the contributions isn't trivial.

OK.

I don't know what you mean by "problem". I have explained above that it
should be accounted for, and a method.

By "problem" I mean "the noise contribution from the loss resistance of the
antenna is routinely ignored." -- Presumably because the background EM noise
(coming in through the antenna's radiation resistance) often far exceeds it.

I haven't been to ham fairs in your country, but here there are mostly
focussed on exhanging junk (selling the junk bought at the last fair, and
buying some different junk to sell at the next fair).

:-)

The larger ham fairs often have some reasonably "meaty" technical seminars
(antenna design and modeling in, e.g., EZNEC is popular). Somewhat more
focused conventions (e.g., Microwave Update) often end up with a fair amount
of technical information as well.

But yes, there's always plenty of junk to be exchanged and junk food to be
consumed.

eBay has diminished the number of true "deals" left at ham fairs, but they do
still exist... including such relevant items as phase noise meters, LNAs, RF
generators.

---Joel

Noise figure paradox
 

Ha... look at this: http://www.microwaveupdate.org/prgmactivities.php

"Noise figure testing w/probable network analysis"

There you go. Come on over from Oz, Owen, we'd love to have you!

:-)

---Joel

Noise figure paradox
 

Joel Koltner wrote:
Hi Richard,

"Richard Clark" wrote in message
...
This is comparing elephants to oranges.

Not intentionally; I misunderstood your objections.

The whole point of the exercise was that just starting with an SNR doesn't
provide enough information to do anything useful relating to noise figures,
although I didn't realize when I posted it that specifying "an antenna" is way
too vague.

So, you came up with 60 dB, what was the noise level in?

To be consistent with what I was trying to concoct, the noise level would have
been kTB with T=290K.

Here's a question for you: What's the noise output power of your
run-of-the-mill RF signal generator (e.g., an HP 8594A/B/C)? I'm thinking the
noise output power is *well* in excess of kTB (where T is the room temperature
you're operating the generator in)?



This can be answered by looking at the specs for the generator.

For example, an Agilent N5181 looks like the noise floor is around
-160dBc/Hz well away from the carrier (e.g. 10MHz). That's probably
representative of the overall noise floor with the carrier at some level
like 0dBm. If we take that level, then it's 14 dB above kTB of -174 dBm/Hz

Noise figure paradox
 

"Joel Koltner" wrote in
:

....
Here's a question for you: What's the noise output power of your
run-of-the-mill RF signal generator (e.g., an HP 8594A/B/C)? I'm
thinking the noise output power is *well* in excess of kTB (where T is
the room temperature you're operating the generator in)?

Most SSGs will have a large attenuation at the output, and as the output
level is reduced (attenuation increased), the noise power density away from
the carrier approaches kT/Hz.

Owen