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

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

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

"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.

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

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

"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

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

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

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