Noise figure paradox
 

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

Noise figure paradox
 

On Sat, 21 Mar 2009 22:49:43 -0700, "Sal M. Onella"
wrote:

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"?)

I would call it a "grinder."

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.

I've designed for low noise, but not for amateur applications. When I
did that design, I chose to work with something that appears to be
alien here, NEP or Noise Equivalent Power. I did this because every
circuit I know of has an input and output resistance and those were
intimately associated with Johnson noise (is this too ancient a term
even if many here are using his concept expressed by Nyquist's math?).
To this point no one seems even remotely interested in resistance (and
it would appear that the focus on a 4 or 5 degree K source of deep
space would be awash in resistor noise in an amp soaking in the
typical ambient of room temperature).

73's
Richard Clark, KB7QHC

Noise figure paradox
 

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

Noise figure paradox
 

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

Noise figure paradox
 

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

Noise figure paradox
 

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.

For a narrow band application, it is indeed possible to construct a
circuit which has lower noise temperature than the active devices. Look
up "cold fet noise source". (a quick google turns up, for instance,
patent 6439763..)



In high performance systems, I perceive a preference to not use G/T as a
metric for receive system performance.

This is hams, the preferences of which you speak? In the rest of the
microwave station world, I think G/T is a popular "one metric for all",
at least for things pointed at the sky.


Rather, hams will quote (brag) Sun

Noise figure paradox
 

Jim-NN7K wrote:


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

Depends on the frequency and things like humidity and cloud cover.

At microwave frequencies (say, 10 GHz-ish) 3-4 K is a good starting
point for dry air on a clear night.

If there's any loss in the path (e.g. from watervapor absorption) the
noise temperature comes up.

If there's anything hot in the path (e.g. clouds with liquid water) then
the noise temp comes up.

If there's something in the path (clouds) that reflects the energy from
something hot (earth) then the noise temp comes up.


This kind of thing is used to measure atmospheric moisture (look up
"water vapor radiometer")

I built a precision ground station to record an orbiting radar (on
QuikScat), and you could easily tell when it was humid or there was
cloud cover by just looking at the background noise level.
http://trs-new.jpl.nasa.gov/dspace/handle/2014/18497

Some BYU students made use of it, and have put up a nice website he
http://www.mers.byu.edu/QCGS/cgs_home.htm

Noise figure paradox
 

"Owen Duffy" wrote in message
...
"Joel Koltner" wrote in
:
But here's an interesting paradox: If I take that output with 57dB SNR
and feed it to another, identical amplifier, shouldn't the SNR at its
output now drop to 54dB?
Appealing, but wrong.

Well, correct *under a certain set of assumptions*. As with, e.g.,
manufacturer's data sheets and quiz/exam problems done in school, often these
assumptions are unstated.

In other words, I'm purposely not stating my assumptions to demonstrate how to
get yourself into trouble more readily. :-)

An real antenna might supply much less through to much much more noise.

How does an antenna at 290K supply less? I mean, ignoring how well it works
as an antenna, shouldn't it still have kTB worth of noise generated just from
the resistance in its conductors?

(If you run through the numbers,
the SNR at the output of the cascaded amplifiers is 56.94dB.)
I get 60-3.2=56.8dB.

I think that's rounding differences and my using T0=290K rather than 289K as a
reference.

It is not so much an issue of safer, is it use and mis-use, it is about
how you use NF with cascaded stages. Essentially, you convert them to T,
apply the gain effects, then T back to a NF for the combination.

Sounds safe to me.

I find noise temperatures just as if not more intuitive than noise figures,
and (to me) it's more obvious what's going on when you have a string of
amplifiers.

You might find this little calculator interesting / helpful:
http://www.vk1od.net/calc/RxSensitivityCalc.htm .

Looks nice, thanks!

---Joel

Noise figure paradox
 

"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

Noise figure paradox
 

"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