On Fri, 26 Jun 2015 08:24:25 -0400, rickman wrote:

I read this post in an antenna group and I don't get how this guy is
coming up with a negative noise figure. Looks to me like he is
calculating the noise figure of a resistor, not the amplifier. Anyone
care to explain this to me?

The part that seems bogus is this...

The negative NF is defined as the amplifier noise being less than the
increase in noise due to the amplifier gain.

I thought noise figure was NF = SNRin / SNRout

Rick


Hello Group,

Approximately 8 weeks ago fellow group member Steve Ratzlaff and I
had an discussion about the noise performance of amplifiers for Non
Directional Beacon work. Steve suggested that it may be a good idea
to look at the Antenna Amplifier noise floor with a simulated antenna
inductance, rather than using a resistance equal to the amplifier
input z.

Steve's suggestion makes perfect sense when one considers that the
real and radiation resistance of loop antennas are very small in
comparison to the loops inductance.

First I retested two active antennas; Wellbrook's ALA100 and
ALA100LN in the paddock using 20m circumference loops. Loop area 21
sq.m. Both antenna use noiseless feedback. The ALA100 uses Bipolar
transistors, the ALA100LN uses JFETS.

The gain comparison on MW/LW was an increase of 2-4dB in favour of
the ALA100LN.

Unfortunately the ambient noise level was too high to see any noise
floor difference.

Next; Bench Tests ( gain and Noise Figure NF of both ALA100 amps.)
were conducted using a Marconi 2019A Sig. Gen. and WinRadio
Excalibur. A 9dB Norton amplifier in the Antenna Interface was used
to ensure that the total gain was approx. 10dB higher than the
Excalibur NF. Thus to ensure that the higher amplifier gain would
mask the receiver NF affecting the measurements. A 20uH inductor was
used to simulate the loop’s reactance.

The NF was measured using the gain method i.e. the excess noise above
the amplifier gain when the input is terminated with a resistor. The
negative NF is measured with a inductance connected to the amplifier
input.

Both ALA100s have an approx. 50 Ohm resistive input z as determined
using an Array Solutions AIM 4170C Antenna Analyser.

WinRadio Excalibur set 1kHz BW and the S Meter to RMS AVG.

-144dBm is used as reference for the 1kHz BW

That is 174 dBm/Hz, indicating about 300 K noise temperature (room
temperature). UHF people would call that 3 dB reference level and it
is not so hard to go down to 100 K (1 dB NF) in a good preamplifier.

If the noise level is below 3 dB, someone would call it a negative
NF:-).

Anyway, at VLF/LF the band noise is so huge, that I do not understand
what a low NF would help, unless the antenna is extremely small and
lossy, such a ferrite bar with -60 to -80 dB antenna efficiency and
hence similar gain.

A several meter in diameter loop will have a half decent efficiency,
so extremely low amplifier NF is seldom justified. Of course, putting
the amplifier at the antenna will help keeping interference from
entering the downlead.

On Fri, 26 Jun 2015 08:24:25 -0400, rickman wrote:

I read this post in an antenna group and I don't get how this guy is
coming up with a negative noise figure. Looks to me like he is
calculating the noise figure of a resistor, not the amplifier. Anyone
care to explain this to me?

The part that seems bogus is this...

The negative NF is defined as the amplifier noise being less than the
increase in noise due to the amplifier gain.

I thought noise figure was NF = SNRin / SNRout

Yes, it is. NF = 0dB is about as low as you can go without magic.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

In message , Jeff writes
On 26/06/2015 13:24, rickman wrote:
I read this post in an antenna group and I don't get how this guy is
coming up with a negative noise figure. Looks to me like he is
calculating the noise figure of a resistor, not the amplifier. Anyone
care to explain this to me?

The part that seems bogus is this...

The negative NF is defined as the amplifier noise being less than the
increase in noise due to the amplifier gain.

I thought noise figure was NF = SNRin / SNRout

Rick


Both definitions are correct and mean the same thing; a negative NF,
when expressed in dB, would be when the SNRout is less than the SNRin.
However, the big but is that an negative NF is not possible.

It only appears to be the case due to the fact that the OP is not
comparing like with like, the test method used is only valid if the
system impedance remains the same. You cannot compare oranges with
lemons.

Is that formula correct? If the input SNR is poor, an amplifier with a
high NF has very impact on the output SNR.

Also, are the units ratios, or are they in dB?


--
Ian

On Sat, 27 Jun 2015 14:02:49 +0100, Jeff Gave us:


Is that formula correct? If the input SNR is poor, an amplifier with a
high NF has very impact on the output SNR.

Also, are the units ratios, or are they in dB?



For a particular NF the effect on the output s/n ratio is always the
same regardless of the actual input s/n, until you get to the point
where the signal vanishes in the noise, but even then it still holds
true but you just can't see it.

The signal will go up by the gain of the amplifier, and the noise will
go up by the sum of *power* of the input noise times the gain and the
noise power of the calculated from the NF times the gain.

The noise powers being in watts calculated from the NF; in a 1Hz
Bandwidth by convention. So its dB above kTB converted to watts if you
are working with NF in dB.

So for a particular NF the added noise is always the same, therefore the
SNRin/SNRout holds, and is a standard definition of NF (not in dB).

Jeff

https://en.wikipedia.org/wiki/DBm

Look at the last four entries in the table.

On Sat, 27 Jun 2015 15:49:50 +0100, Jeff wrote:


For a particular NF the effect on the output s/n ratio is always the
same regardless of the actual input s/n, until you get to the point
where the signal vanishes in the noise, but even then it still holds
true but you just can't see it.

The signal will go up by the gain of the amplifier, and the noise will
go up by the sum of *power* of the input noise times the gain and the
noise power of the calculated from the NF times the gain.

The noise powers being in watts calculated from the NF; in a 1Hz
Bandwidth by convention. So its dB above kTB converted to watts if you
are working with NF in dB.

So for a particular NF the added noise is always the same, therefore the
SNRin/SNRout holds, and is a standard definition of NF (not in dB).

Jeff

https://en.wikipedia.org/wiki/DBm

Look at the last four entries in the table.


..and your point is???

....between its shoulders.

On Sat, 27 Jun 2015 12:50:39 -0400, krw Gave us:

On Sat, 27 Jun 2015 15:49:50 +0100, Jeff wrote:


For a particular NF the effect on the output s/n ratio is always the
same regardless of the actual input s/n, until you get to the point
where the signal vanishes in the noise, but even then it still holds
true but you just can't see it.

The signal will go up by the gain of the amplifier, and the noise will
go up by the sum of *power* of the input noise times the gain and the
noise power of the calculated from the NF times the gain.

The noise powers being in watts calculated from the NF; in a 1Hz
Bandwidth by convention. So its dB above kTB converted to watts if you
are working with NF in dB.

So for a particular NF the added noise is always the same, therefore the
SNRin/SNRout holds, and is a standard definition of NF (not in dB).

Jeff

https://en.wikipedia.org/wiki/DBm

Look at the last four entries in the table.


..and your point is???

...between its shoulders.

krw is a pointless jackass, despite what some have said.

http://www.imdb.com/title/tt0067595/

On Sat, 27 Jun 2015 12:50:39 -0400 krw wrote in Message
id: :

On Sat, 27 Jun 2015 15:49:50 +0100, Jeff wrote:


For a particular NF the effect on the output s/n ratio is always the
same regardless of the actual input s/n, until you get to the point
where the signal vanishes in the noise, but even then it still holds
true but you just can't see it.

The signal will go up by the gain of the amplifier, and the noise will
go up by the sum of *power* of the input noise times the gain and the
noise power of the calculated from the NF times the gain.

The noise powers being in watts calculated from the NF; in a 1Hz
Bandwidth by convention. So its dB above kTB converted to watts if you
are working with NF in dB.

So for a particular NF the added noise is always the same, therefore the
SNRin/SNRout holds, and is a standard definition of NF (not in dB).

Jeff

https://en.wikipedia.org/wiki/DBm

Look at the last four entries in the table.


..and your point is???

...between its shoulders.

....and under his comb-over.

On Sun, 28 Jun 2015 06:48:13 -0400, JW Gave us:

...and under his comb-over.

Full head of hair here. Sounds like you looked in the mirror,
asswipe.

On 6/27/2015 4:07 AM, Jeff wrote:
On 26/06/2015 13:24, rickman wrote:
I read this post in an antenna group and I don't get how this guy is
coming up with a negative noise figure. Looks to me like he is
calculating the noise figure of a resistor, not the amplifier. Anyone
care to explain this to me?

The part that seems bogus is this...

The negative NF is defined as the amplifier noise being less than the
increase in noise due to the amplifier gain.

I thought noise figure was NF = SNRin / SNRout

Rick


Both definitions are correct and mean the same thing; a negative NF,
when expressed in dB, would be when the SNRout is less than the SNRin.
However, the big but is that an negative NF is not possible.

I don't think both definitions mean the same thing. If the amplifier
adds *any* noise it increases the NF above zero by the conventional
definition. The only way the NF can be negative is if the amplifier
removes noise from the input, or in other words, increases the SNR.

What he seems to be suggesting is that NF is the ratio of the signal
noise to the amplifier noise.


It only appears to be the case due to the fact that the OP is not
comparing like with like, the test method used is only valid if the
system impedance remains the same. You cannot compare oranges with lemons.



--

Rick

On Sat, 27 Jun 2015 13:43:16 +0100, Jeff Gave us:

On 27/06/2015 13:26, rickman wrote:
On 6/27/2015 4:07 AM, Jeff wrote:
On 26/06/2015 13:24, rickman wrote:
I read this post in an antenna group and I don't get how this guy is
coming up with a negative noise figure. Looks to me like he is
calculating the noise figure of a resistor, not the amplifier. Anyone
care to explain this to me?

The part that seems bogus is this...

The negative NF is defined as the amplifier noise being less than the
increase in noise due to the amplifier gain.

I thought noise figure was NF = SNRin / SNRout

Rick


Both definitions are correct and mean the same thing; a negative NF,
when expressed in dB, would be when the SNRout is less than the SNRin.
However, the big but is that an negative NF is not possible.

I don't think both definitions mean the same thing. If the amplifier
adds *any* noise it increases the NF above zero by the conventional
definition. The only way the NF can be negative is if the amplifier
removes noise from the input, or in other words, increases the SNR.


Yes that is correct, but the definitions are also correct. The flaw in
the negative noise figure argument is that it is not possible to have a
better SNRout than SNRin *for the same system conditions*.

The apparent negative noise figure only come about by comparing the NF
of the amp in a 50ohm system with the output from a system with
something different on the input.

The test method used is also very prone to measurement errors for low
noise figures.

Jeff

To me, NF refers to "noise floor".

Lets see him go below that.

GPS received signals are among the lowest "power" signals we currently
grab. They sit just above the noise floor.