Noise figure paradox
 

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?

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?

Of course, most people know the answer is "no," but it's not necessarily
immediately obvious why this is.

The problem, to quote Wes Hayward, is that "the noise figure concept has the
drawback that it depends upon definition of a standard temperature, usually
290K." In other words, the SNR at the output of an amplifier degrades by the
noise figure *only if one can assume that the noise level going into the
amplifier is equivalent to kTB*, where T is usually taken to be 290K (...by
the guy who built the amplifier).

This assumption isn't correct in the two cascaded amplifier case. Indeed,
since the first amplifier has a gain of 20dB, in 1Hz the noise power coming
out of the amplifier is -174+20+3 = -154dBm. This is equivalent to a noise
temperature of 57533K! From this vantage point it's pretty obvious that an
amplifier with a noise figure of 3dB -- corresponding to noise temperature of
290K -- will have negligible impact on the overall noise output. (If you run
through the numbers, the SNR at the output of the cascaded amplifiers is
56.94dB.)

Personally, I think that using noise temperatures tends to be "safer" than
using noise figures, as the later can easily lead one astray if you're not
careful to make sure you know what the "standard temperature" used was.
(After all, if someone just hands you a piece of coax and says, "there's a
60dB SNR signal on line, please amplify it by 20dB and insure that the output
SNR is still 59dB," without more information there's no way to determine how
good of an amplifier you need.) But I'd like to get other peoples' opinions
on this subject... how do you think about noise figures and temperatures?

Input appreciated,
---Joel Koltner

Noise figure paradox
 

"Joel Koltner" wrote in
:

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?

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.

The amplifier has an equivalent noise temperature (Teq) of 289K.

To determine the effect of two cascaded stages of the same amplifier, Teq
of the combination =T1+T2/G1=289+289/100=318K which corresponds to NF=
3.2dB

Of course, most people know the answer is "no," but it's not
necessarily immediately obvious why this is.

The problem, to quote Wes Hayward, is that "the noise figure concept
has the drawback that it depends upon definition of a standard
temperature, usually 290K." In other words, the SNR at the output of
an amplifier degrades by the noise figure *only if one can assume that
the noise level going into the amplifier is equivalent to kTB*, where
T is usually taken to be 290K (...by the guy who built the amplifier).

If you were testing the amplifier with a standard signal generator at
room temperature, the generator does suppy 290K of noise.

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


This assumption isn't correct in the two cascaded amplifier case.
Indeed, since the first amplifier has a gain of 20dB, in 1Hz the noise
power coming out of the amplifier is -174+20+3 = -154dBm. This is
equivalent to a noise temperature of 57533K! From this vantage point
it's pretty obvious that an amplifier with a noise figure of 3dB --
corresponding to noise temperature of 290K -- will have negligible
impact on the overall noise output. (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.


Personally, I think that using noise temperatures tends to be "safer"
than using noise figures, as the later can easily lead one astray if
you're not careful to make sure you know what the "standard
temperature" used was. (After all, if someone just hands you a piece
of coax and says, "there's a 60dB SNR signal on line, please amplify
it by 20dB and insure that the output SNR is still 59dB," without
more information there's no way to determine how good of an amplifier
you need.) But I'd like to get other peoples' opinions on this
subject... how do you think about noise figures and temperatures?

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. The
equation looks ugly, but if you work in T, you can do it in your head...
well until T becomes so large you want to use dBK.

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

Owen

Noise figure paradox
 

"Joel Koltner" wrote in
:

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

I meant to flag this statement.

Does it provide enough information for you to apply it in the way you
have?

It says nothing of the absolute noise power or signal power. You seem to
assume the noise power KTB noise where T is 290K.

What if you were pointing at directive antenna at cold sky, and Tnoise
was say 10K. (As a complication, no antenna is perfect, and there would
also be some spillover noise from the hot earth, but the total might be
well under 100K.)

Alternatively, what if you were talking about a HF antenna and say Tnoise
was say, 30000K.

Owen

Noise figure paradox
 

Joel Koltner wrote:
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?

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?

Of course, most people know the answer is "no," but it's not necessarily
immediately obvious why this is.

The problem, to quote Wes Hayward, is that "the noise figure concept has the
drawback that it depends upon definition of a standard temperature, usually
290K." In other words, the SNR at the output of an amplifier degrades by the
noise figure *only if one can assume that the noise level going into the
amplifier is equivalent to kTB*, where T is usually taken to be 290K (...by
the guy who built the amplifier).

This assumption isn't correct in the two cascaded amplifier case. Indeed,
since the first amplifier has a gain of 20dB, in 1Hz the noise power coming
out of the amplifier is -174+20+3 = -154dBm. This is equivalent to a noise
temperature of 57533K! From this vantage point it's pretty obvious that an
amplifier with a noise figure of 3dB -- corresponding to noise temperature of
290K -- will have negligible impact on the overall noise output. (If you run
through the numbers, the SNR at the output of the cascaded amplifiers is
56.94dB.)

Personally, I think that using noise temperatures tends to be "safer" than
using noise figures, as the later can easily lead one astray if you're not
careful to make sure you know what the "standard temperature" used was.
(After all, if someone just hands you a piece of coax and says, "there's a
60dB SNR signal on line, please amplify it by 20dB and insure that the output
SNR is still 59dB," without more information there's no way to determine how
good of an amplifier you need.) But I'd like to get other peoples' opinions
on this subject... how do you think about noise figures and temperatures?

Input appreciated,

Wes Hayward's articles in the 1970s completely transformed the way we
think about the sensitivity and dynamic range of HF receivers. They mark
the point where ideas such as "noise floor", "intermodulation intercept"
and "blocking dynamic range" and "reciprocal mixing" entered mainstream
amateur radio.

Inspired by those articles, I set out to apply those same concepts to
VHF/UHF receivers... and ran into problems with the definitions of
receiver sensitivity. Like everyone else who has traveled this route, I
quickly found that the very large values of noise figure and noise
temperature, that are typical at HF, can conceal some approximations and
even misconceptions.

The approximations will probably be unimportant in HF systems where the
receiver has a high noise figure / noise temperature, and antenna noise
is usually greater still. However, the misconceptions are always
important, because they will give incorrect results for VHF/UHF
receivers. The difference at VHF/UHF is that receiver noise and antenna
noise are often quite similar, and both much lower than at HF.

I must emphasize that the fundamental concepts are the same at all
frequencies. The differences are all due to the magnitudes of the
numbers involved.

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.

For example, modern Noise Figure Analyzers have options to accept inputs
and display results in any relevant engineering units; but the internal
calculations are done entirely in terms of noise temperature because
that concept will always give the correct results.


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).
That number 290 is built into the IEEE standard definition relating
noise factor to noise temperatu

F = T/290 + 1

That equation defines what the engineering world means by "noise
factor". F and T are variables but the number 290 not; it is fixed by
definition. (Noise factor F is a dimensionless ratio; the more
commonly-seen Noise Figure is simply F converted into dB.)

What engineers do sometimes assume is that the *physical* temperature of
their hardware is 290K, because that special case does allow some
convenient simplifications. But that isn't the same as saying "my
reference temperature is 290K". At best, it is loose language - fooling
ourselves by saying something that we don't really mean. At worst, it is
a perfect example of the way that a good approximation can hide a
fundamental misconception.

An engineer working at HF wouldn't even notice what he has done. Because
he is working with very high values of noise temperature, any errors
will be negligible - in other words, he has made an excellent
engineering approximation. But the misconceptions are still there...
waiting. If that same engineer moves to work on low-noise UHF and
microwave systems, he'll fall flat on his face.

Does this matter to he average radio amateur? Yes, it does, for many of
us have multiband transceivers with coverage from HF through to UHF -
exactly the range of applications where those pitfalls await the unwary.
The engineers who design those radios need to have their concepts
straight; so do the people who write equipment reviews; and if we want
to make intelligent buying decisions, so do we all.



--

73 from Ian GM3SEK

Noise figure paradox
 

""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

Noise figure paradox
 

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.

In high performance systems, I perceive a preference to not use G/T as a
metric for receive system performance. Rather, hams will quote (brag) Sun
noise rise (Sun/ColdSky ratio) without statement of the solar flux at the
time, or the time (from which solar flux can be estimated from historical
records), or if they do quote solar flux, it will be the 10.7cm flux
which cannot be reliably extrapolated to the relevant ham band.

The 'science' is often obscured by shallow discussions about whether LNA
Noise Figure is more important than Gain.

Owen

Noise figure paradox
 

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


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.

In high performance systems, I perceive a preference to not use G/T as a
metric for receive system performance. Rather, hams will quote (brag) Sun
noise rise (Sun/ColdSky ratio) without statement of the solar flux at the
time, or the time (from which solar flux can be estimated from historical
records), or if they do quote solar flux, it will be the 10.7cm flux
which cannot be reliably extrapolated to the relevant ham band.

The 'science' is often obscured by shallow discussions about whether LNA
Noise Figure is more important than Gain.

Owen

Noise figure paradox
 

Jim-NN7K . wrote in
:



So-- Which is the most relevant noise measurement? Noise Figure-
or Noise Temperature? If one is better than another at a given

As both Ian and I mentioned, Noise Figure is based on the degradation in
S/N ratio assuming that the source contributes 290K thermal or Johnson
noise (KTB noise) from the equivalent source resistance.

This if fine for describing the operation of a receiver when driven by a
standard signal generator.

The radiation resistance component of the equivalent source impedance of
an antenna is not a source of KTB noise, but is a source of received
noise power from various sources, and the level varies with many factors
including frequency and time.

Expressing a receive system performance as a Noise Figure assumes an
external or 'ambient' noise component that is of little application
relevance.

Expressing a receive system performance as an equivalent Noise
Temperature expresses only the receiver's internal noise, which is a
limited perspective from an application point of view. However,
comparison of the system's internal noise with the external noise gives
insight into the S/N degradation due to the system.

Both measures contain sufficient information, just that you have to
transform NF to obtain Teq which is the more direct input to calculation
of system S/N, or exploration of cascaded stages for example. Because of
this, NF is sometimes misinterpreted as to its direct signifcance.

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?

The first stage is very important in determining system noise
temperature, but in high performance stations, so are the losses in the
feed system, switching etc. The contribution of later stages should not
be considered insignificant until calculated.

Often, the LNA runs with so much gain that the transceiver AGC reduces
gain sufficiently to degrade transceiver noise temperature to perhaps
30,000K (NF=20dB). Consider a 0.5dB NF 35dB gain LNA (T=35K, Gain=3,000),
then it rolls 30,000/3000=10K into the system noise temperature which may
be significant depending on the external noise level.

Even worse is the scenario where an OM installs a 20dB attenuator between
LNA and transceiver to 'correct' S meter readings. In that case, a 5dB NF
receiver with 20dB attenuator has NF=25dB, T=90,000K, so it rolls
90,000/3000=30K into the otherwise same system... but this is done!

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!

IIRC the coldest part of the sky in the 5 - 10GHz region is around 4K. As
I mentioned in an earlier post, practical antennas capture significant
energy in their sidelobes, so the total noise input power might be well
in excess of 4K.

The more interesting question is the background when pointing in the
desired direction (eg the moon for EME), and how much sidelobe noise is
received.

Owen

Noise figure paradox
 

"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.)

The noise figure of the first stage strongly influences the total system
noise figure, hence the oft-seen placement of a low noise preamp close to
the antenna.

Noise figure paradox
 

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 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

Noise figure paradox
 

"Richard Clark" wrote in message
...
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.

It would have been much better off for me to state that, "Say I have a signal
generator that I know happens to provide an SNR of 60dB." I knew that
background radition temperatures were high, but not that even the quietest
parts of the spectrum are 4,000K!

Noise figure paradox
 

"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?

Noise figure paradox
 

Jim Lux wrote in
:

Owen Duffy wrote:
....
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.

Yes Jim, an omission on my part... hams tend not to use G/T... but yes,
the rest of the world recognises the value of G/T as a single metric,
especially for space comms.

I suppose one complication of G/T for EME is that the noise varies with
moon position and local elevation... but one thinks that a range of G/T
figures could be expressed to characterise a station's capability.

G/T for a Sun transit at high elevation would be a most useful metric for
assessing a station against state of the art.

Owen

Noise figure paradox
 

"Joel Koltner" wrote in
:

"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?

An antenna's feedpoint impedance comprises radiation resistance and loss
resistance.

Radiation resistance is a virtual resistance and does not contribute
thermal or Johnson noise.

It is a common mistake to consider that an antenna always includes 290K
due to kTB in its radiation resistance. If that were the case, we would
never have need for receivers with Teq much less than 290K! Mind you, if
a directive antenna points at hot earth, then external noise will never
be much less than 290K, so the requirements for terrestrial shots will be
different to space shots.

Attenuation gives rise to noise, and feed system loss is no exception.

An antenna does receive noise power from its environment, lets call it
external noise, and that needs to be factored into a receive system for
an overall figure of merit.

The ratio Gain/Temperature is antenna gain divided by total equivalent
noise temperature (internal and external) all referred to a common
reference point (usually the antenna connector or w/g flange). It is an
overall figure of merit, and if the power flux density (or field
strength) at the receive antenna is known, then S/N can be calculated
from that and G/T.

Hams tend to not use G/T.


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

The 289 was not the reference, it was the result of using 3.00000dB NF (I
know you stated Noise Factor =2, but I used your rounded NF=3dB value).
....
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.

That was my point. Dealing with K is like dealing with power (P=kTB).


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

Looks nice, thanks!

Thanks.

There is a related calculator for deterimining the level of ambient noise
when receiver noise figure is known, see
http://www.vk1od.net/calc/anc.htm .

By and large, although lots of hams express an interest in weak signal
working, they aren't very interested in noise... which is a key parameter
determining whether a signal can be copied. I have asked scores of weak
signal enthusiasts their ambient noise level, and to date, only one has
answered (though not in absolute terms, but nevertheless had an
appreciation of the issue).

Owen

Noise figure paradox
 

"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

Noise figure paradox
 

"Joel Koltner" wrote in
:

"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?

It doesn't solve the problem.

You still haven't given enough information to determine the absolute
level of either signal or noise, and you need that to consider the impact
of the DUT's internal noise (which you know in absolute terms).

Owen

Noise figure paradox
 

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

Sounds like what Walter Maxwell has been saying
for decades.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com
"Government 'help' to business is just as disastrous as government
persecution..." Ayn Rand

Noise figure paradox
 

"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
 

"Owen Duffy" wrote in message
...
An antenna's feedpoint impedance comprises radiation resistance and loss
resistance.

OK.

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?"

Hams tend to not use G/T.

If doesn't seem like "receiver factor" (input intercept point/noise figure)
has caught on much either.

By and large, although lots of hams express an interest in weak signal
working, they aren't very interested in noise... which is a key parameter
determining whether a signal can be copied.

I realized awhile back that noise is the primary factor that limits how far
you can transmit a signal and still recovery it successfully. (Granted, these
days it's often phase noise in oscillators rather than the noise figures in
amplifiers that determines this, but still.)

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'm learning a lot here...

---Joel

Noise figure paradox
 

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)?

---Joel

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