On Sat, 27 Jun 2015 09:19:23 -0400, DecadentLinuxUserNumeroUno
wrote:

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.

And you believe everything that your government claims ?

The GPS DSSS signal is more than 1 MHz wide, so you could claim -30 dB
SNR. However, after despreading, the signal is only 1 kHz wide and the
data rate is only 50 bit/s wide. Thus, the SNR should be calculated at
25-50 Hz bandwidths, giving quite positive SNR.

On Sat, 27 Jun 2015 09:19:23 -0400, DecadentLinuxUserNumeroUno
wrote:

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".

It probably would to an utter imbecile such as you.


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.

On Mon, 29 Jun 2015 09:34:52 +0100, Pomegranate *******
Gave us:

It probably would to an utter imbecile such as you.

Relating to you and your invasion of the group, it does mean noise
factor, just not electronics related. Just like *you* are not
electronics related or educated.

On Mon, 29 Jun 2015 05:42:57 -0400, DecadentLinuxUserNumeroUno
wrote:

On Mon, 29 Jun 2015 09:34:52 +0100, Pomegranate *******
Gave us:

It probably would to an utter imbecile such as you.

Relating to you and your invasion of the group, it does mean noise
factor, just not electronics related. Just like *you* are not
electronics related or educated.

C'mon DikeyGurl, remind me of how you came to be named DimBulb. I
could do with a good laugh!

On Sat, 27 Jun 2015 15:49:12 +0100, Jeff Gave us:


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.


It might to you, but in this context it means either Noise Factor or
Noise Figure.

Of course you can go below the Noise Floor, and in some circumstances
and modes the signal is receivable and decodable.

30dB below the noise floor....

http://www.bentongue.com/xtalset/1nlxtlsd/1nlxtlsd.html

The answer to all your needs. Less is more. That Chef's Hat
conglomeration is overkill.

In message , Jeff writes

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.


It might to you, but in this context it means either Noise Factor or
Noise Figure.

But you have to be careful, as "noise factor" is a numerical ratio, and
"noise figure" is in dB.

Of course you can go below the Noise Floor, and in some circumstances
and modes the signal is receivable and decodable.

In the analogue cable TV world, the noise figure (in dB) can be looked
at as the amount of noise power that (say) a real-world amplifier
notionally has at its input in excess of that which would be generated
from a perfect resistor as its source impedance.

As a rule-of-thumb, in a 4MHz vision bandwidth, a perfect 75 ohm
resistor generates -59dBmV. [Subtract around 48dB if you want dBmW.]

The output of a noiseless amplifier would be -59dBmV + G, where G is the
gain in DB.

The output of a real-world amplifier would be -59dBmV + NF + G, where N
is the noise figure.

One method of measuring the noise figure is first to feed the amplifier
first from a resistive source, and measure the output noise level. Next,
feed the amplifier from a source containing a known amount of noise, and
note the increase of output noise. The noise figure can then be
calculated.

In practice, the noisy source is usually a calibrated noise meter*. The
first reading is taken with the noise meter set at zero additional noise
output, and then the noise output is increased until the amplifier
output level rises by 3dB. This means that the noise meter is now
contributing the same amount of noise as the amplifier, and the noise
figure can be read directly from its output display. [This conveniently
saves having to do any further calculations.]

*Usually, a noise meter has a calibrated output meter or other display,
and this indicates the level of its noise output in a stated bandwidth -
both as an absolute level, and as the equivalent in dB with respect to
the basic minimum absolute level. In the cable TV world, the minimum
would be -59dBmV (probably shown in microvolts) in a 4MHz bandwidth, or
0dB. If, to increase the amplifier output level by 3dB, the noise meter
output had to be turned up to -49dBmV / 10dB, its noise figure would, of
course, be 10dB.


--
Ian

In message , Jeff writes
On 27/06/2015 17:08, Ian Jackson wrote:
In message , Jeff writes

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.


It might to you, but in this context it means either Noise Factor or
Noise Figure.

But you have to be careful, as "noise factor" is a numerical ratio, and
"noise figure" is in dB.

Of course you can go below the Noise Floor, and in some circumstances
and modes the signal is receivable and decodable.

In the analogue cable TV world, the noise figure (in dB) can be looked
at as the amount of noise power that (say) a real-world amplifier
notionally has at its input in excess of that which would be generated
from a perfect resistor as its source impedance.

As a rule-of-thumb, in a 4MHz vision bandwidth, a perfect 75 ohm
resistor generates -59dBmV. [Subtract around 48dB if you want dBmW.]

The output of a noiseless amplifier would be -59dBmV + G, where G is the
gain in DB.

The output of a real-world amplifier would be -59dBmV + NF + G, where N
is the noise figure.

One method of measuring the noise figure is first to feed the amplifier
first from a resistive source, and measure the output noise level. Next,
feed the amplifier from a source containing a known amount of noise, and
note the increase of output noise. The noise figure can then be calculated.

In practice, the noisy source is usually a calibrated noise meter*. The
first reading is taken with the noise meter set at zero additional noise
output, and then the noise output is increased until the amplifier
output level rises by 3dB. This means that the noise meter is now
contributing the same amount of noise as the amplifier, and the noise
figure can be read directly from its output display. [This conveniently
saves having to do any further calculations.]

*Usually, a noise meter has a calibrated output meter or other display,
and this indicates the level of its noise output in a stated bandwidth -
both as an absolute level, and as the equivalent in dB with respect to
the basic minimum absolute level. In the cable TV world, the minimum
would be -59dBmV (probably shown in microvolts) in a 4MHz bandwidth, or
0dB. If, to increase the amplifier output level by 3dB, the noise meter
output had to be turned up to -49dBmV / 10dB, its noise figure would, of
course, be 10dB.



Great way if you have a R&S SKTU!!

Indeed it is. The '3dB rise' method is essentially a good dodge for
engineers to avoid having to do any hard sums.

The normal way these days is the Y-factor method and uses a switchable
noise source with a fixed known and calibrated Excess Noise Ratio
(ENR). The noise power from the device is measured with the source on
and off and the NF calculated from that ratio. That is how Noise figure
test sets normally work.

Which is sort-of what I said in the middle of my ramblings.

It should be relatively easy to conjure-up your own noise measuring
machine by using an old-fashioned, high-gain, rather noisy, wideband
amplifier as the noise source, and follow it with a switched (or
calibrated variable) attenuator. If you know the amplifier noise figure,
and its gain, you know how much output noise it will produce - although
it would help if you can get a friendly guru to check.

For high noise levels and low attenuator settings, the noise is
essentially inversely proportional to the attenuator setting, but if
there's low noise and a lot of attenuation, the noise output becomes
asymptotic to the basic noise floor of the noise generated in the
attenuator itself (ie no matter how much attenuation you switch in, the
noise doesn't get any lower).

--
Ian

On 27.6.15 15:43, Jeff wrote:
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

The whole discussion has a strong scent of golden speaker leads of
the audio fans. Just substitute Litz for the gloden leads / connectors.

Is the whole project for the new crystal sets?

--

-TV