# Antenna Amplifier Noise Figure

Started by June 26, 2015
```>> 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???

Jeff
```
```On Sat, 27 Jun 2015 15:49:12 +0100, Jeff <jeff@ukra.com> 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 <mmmd5e\$2vg\$1@speranza.aioe.org>, Jeff <jeff@ukra.com> 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
```
```On Sat, 27 Jun 2015 15:49:50 +0100, Jeff <jeff@ukra.com> 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 <krw@nowhere.com> Gave us:

>On Sat, 27 Jun 2015 15:49:50 +0100, Jeff <jeff@ukra.com> 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 11:51:19 -0400
DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> wrote:

>  30dB below the noise floor....

Only because of the processing gain in the system, noise is always
related to the bandwidth.

--

Brian Morrison

"I am not young enough to know everything"
Oscar Wilde

```
```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

```
```On Sat, 27 Jun 2015 09:19:23 -0400, DecadentLinuxUserNumeroUno
<DLU1@DecadentLinuxUser.org> wrote:

>On Sat, 27 Jun 2015 13:43:16 +0100, Jeff <jeff@ukra.com> 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 12:50:39 -0400 krw <krw@nowhere.com> wrote in Message
id: <t1ltoadb52mc20cgg0seuqc5k1l82b15us@4ax.com>:

>On Sat, 27 Jun 2015 15:49:50 +0100, Jeff <jeff@ukra.com> 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 27/06/2015 17:08, Ian Jackson wrote:
> In message <mmmd5e\$2vg\$1@speranza.aioe.org>, Jeff <jeff@ukra.com> 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!!

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.

Jeff
```