Talk:Noise figure

message: "Attenuators have a noise factor F equal to their attenuation ratio L"  I just can't see how this can be correct, for example an attenuator made of capacitors (voltage divider) will not introduce any new noise so the SNRin and SNRout will be the same, therfore the noise figure of such an attenuator will be zero, I am sure that there are many other examples where the NF is zero or irrelevant.  A resistive attenuator's NF would be a function of the resistors used not the level of attenuation.  — Preceding unsigned comment added by 114.34.217.17 (talk) 04:31, 27 March 2014 (UTC)
replies: text: The signal gets attenuated by L but the thermal noise stays at the same level; consequently, the NF gets worse by the attenuation factor. Glrx (talk) 03:35, 22 December 2015 (UTC)

text: I think "attenuator" implies that the excess power is absorbed. In other words, input and output are matched. The reactive circuit you describe does not really attenuate; it reflects the excess power (it presents a mismatch). In a matched attenuator it does not matter what the internal resistors are, only the system impedance matters when calculating noise. And you will find that the statement that F=attenuation is correct in that case. Mayrayday (talk) 17:05, 8 February 2024 (UTC)
thread_title:  Noise figure in attenuators 

message: The article currently says:
...in the case of satellite communications systems, where the antenna is pointed out into cold space, the antenna effective temperature is often colder than 290 K. In these cases a 2 dB improvement in receiver noise figure will result in more than a 2 dB improvement in the output signal to noise ratio.
From the definition of NF, I don't see how this can be true:
:\mathrm{SNR_{out,dB}} = \mathrm{SNR_{in,dB}} - NF
The receiver doesn't know anything about the signal source. What am I missing? GyroMagician (talk) 10:29, 19 March 2010 (UTC)
thread_title:  Satellite communication systems 

message: Hello MrOllie. I'm curious - why did you remove the external links to Analog? I'm not saying you were right or wrong, just curious to know your reasoning. GyroMagician (talk) 17:30, 11 August 2011 (UTC)
replies: text: I was thinning links to analog.com out (especially the ones that include fluff pieces on Analog employees and/or products) because I recently noticed that they have been systematically added by a single purpose editor for a fairly long period of time. - MrOllie (talk) 17:47, 11 August 2011 (UTC)
replies: text:  I agree with the removal of the ADC Noise Ref; it doesn't extend the article. I don't know about the log amp one; I can see reasons to keep or delete. I'm not sure a discussion of log amps belongs in this article. Glrx (talk) 03:44, 12 August 2011 (UTC)
replies: text:  Yes, that seems fair. I asked because I met an editor a while ago who was convinced that all company design guides, etc, were advertising (I think it related to an Agilent document describing S-parameters). Anyway, I'm happy to find you're more sensible MrO ;-) The log amp article seems useful, but I'd agree this is the wrong place for it. GyroMagician (talk) 18:35, 15 August 2011 (UTC)
replies: text:  For reference, the log amp link: http://www.analog.com/library/analogDialogue/archives/42-06/noise_figure.html Glrx (talk) 15:00, 16 August 2011 (UTC)
thread_title:  External Links 

message: The whole part about noise temperature doesn't really make sense in my opinion. Te and T0 are both called "noise temperature", so it's not clear what's the difference between these is.   — Preceding unsigned comment added by 2A02:908:F324:7780:F830:11A7:9F6D:DA19 (talk) 18:49, 6 June 2016 (UTC)
replies: text: Yes, the article uses the term standard noise temperature, T0 as short hand for standard temperature at which noise is specified, T0.  Noise temperature is just another way to specify noise power.  Saying there is a standard noise temperature is saying that there is a standard noise power, which is nonsense. Constant314 (talk) 19:01, 6 June 2016 (UTC)
thread_title:  Definition 

message: Could someone better than I add a discussion of the concepts of single- and double-sided noise figure? Thanks, Marcas.oduinn (talk) 10:08, 28 June 2022 (UTC)
thread_title:  Single-sided vs double-sided NF 

message: After spending some time trying to untangle the confusion around noise factor I have to say that I do not agree with the basic and often repeated statement that the Friis definition is F=SNRi/SNRo. This statement is incomplete. Look at equation (4) in the Friis paper, and the text right above it. 
:The noise figure F of the network is defined as the ratio of the available signal-to-noise ratio at the signal-generator terminals to the available signal to noise ratio at its output terminals. Thus F=(Sg/KTB)/(S/N)   (4)
Unfortunately, Friis does not state it clearly in mathematical terms by itself, or in the text by itself, but it is clear from matching equation (4) up with the text just above it: Ng=kTB. Furthermore, T has been defined to be T0=290K by IEEE (Friis suggested it, but did not require it).
Hence the often re-printed statement of F=SNRi/SNRo is incomplete, we must also state Ni=kT0B. And when we do that, F becomes a number that describes the residual noise source in the network (a power in watts), as stated by equation (7). The power of that output referred residual noise source is (F-1)GkT0B watts. Equivalently, the input referred residual noise source is (F-1)kT0B.
Here is how to determine F of a given network: 
:Apply a signal that has a noise component of kTB (e.g. a highly attenuated signal, where thermal noise of the matched input termination is dominant). Divide the measured (or known) SNR of that input signal with the measured SNR of the output signal.
Here is how to use F:
:1) compute the residual (additive) output referred noise power Noa=(F-1)GkTB
:2) compute the impact that additive noise has on your signal. Here you can plug in your actual signal without restrictions. But you need to know actual power, SNR (a relative measure) is not sufficient. No=NiG+Noa
:Note that G (the network power gain) can be substituted for So/Si (or S/Sg in the Friis formula), and hence we really do not need a signal, only noise.
:If, and only if the input signal of the circuit has a noise component of kT0B, the noise factor can be directly used to determine the noise at the output, i.e. No=NiF=kT0BF.
A generalized form of the Friis equation can be derived, which allows for an arbitrary input noise Ni. Note that these equations reduce to the Friis equation when substituting Ni=kT0B.
It is unclear why Friis chose to represent an addition by a factor. We can speculate that the reason is two-fold: Friis' paper was about radio receivers, where the input signal is highly attenuated and hence the input noise is defined by kTB, and the convenience of using dB. An addition in linear terms (which additive noise is) has no equivalent operation in dB. An addition in dB is a multiplication in linear terms. However, an addition of two fixed summands can be represented by a factor (the two fixed summands being Ni=kT0B and the network additive noise Na). Alas, this leads to confusion for any application where the input noise is not kT0B. Mayrayday (talk) 16:59, 15 March 2024 (UTC)
thread_title:  Confusion around Noise Figure