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