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From this paper:
"3) The radiation resistance of free space is 鈭(渭0 /蔚0). What is the rms noise voltage at the terminals of an antenna which will compete, say, with an FM radio signal?"
Is that supposed to imply that free space generates thermal noise??? - Omegatron 18:36, Apr 15, 2005 (UTC)
- You are confusing radiation resistance with the impedance of free space. Radiation resistance is a property of the antenna geometry, and is a measure of the power lost from the transmitter because it is radiated as electromagnetic waves. The impedance of free space describes the ratio of electrostatic to magnetic field in the propagating wave.
I see a number of problems with the discussion of radio communications on this page. First of all, it is NOT true that the noise received by a receiver listening to a radio channel is kTB. It can be much higher OR lower than this. If we exclude external sources of noise (like lightning and the sun), then the dominant noise source is the receiver itself. Internal receiver noise is usually rated by Noise_figure, which, for a receiver, is the ratio of Pout/G*Pin, where Pin is the thermal noise from a resistor at 290K attached to the input of the receiver and Pout is the noise output power of the receiver (usually can consider just the first few stages) and G is the receiver gain. So, a perfect receiver would have a noise figure of 1 (0 dB). A receiver which introduced internal noise equal to thermal noise at 290K (which is what is implied in this article as being always true) would be 2 (3 dB).
UHF and microwave receivers regularly achieve noise figures of less than 1 dB (1.25).
In real use, the receiver is not attached to a resistor as its source but to an antenna with the proper feedpoint impedance. Because the resistive part of the feedpoint impedance is not a power loss but a power transformation (from an EM wave propagating in free space to a signal in the wires going to the receiver), it does not produce Johnson noise.
So, in summary, if you attach an antenna to a receiver, point it away from all radio sources (free space), then you will see simply the receiver internal noise (plus the cosmic background radiation, but that is almost surely much less than the receiver internal noise), and this internal noise may be much less than kTB. If no-one responds to this, I will change the article to reflect this, along with links to other relevant articles such as Noise_figure Sbreheny ( talk) 06:10, 24 December 2007 (UTC)
It seems to me the Preceivernoise equation should represent a closed system at equilibrium. If true, then the antennas temperature is equal to its surrounding temperature. Therefore, the amount of thermal noise received by the antenna due to blackbody radiation is relative to the antennas temperature. Correct me if I'm wrong, but power noise (KTB) is applicable to antennas; e.g., a 10MHz-BW 50-ohm (radiation resistance) antenna at 300K should receive K*300*10MHz = 4E-14 watts. Paul Lowrance, April 13 2008. 鈥擯receding unsigned comment added by 72.25.73.160 ( talk) 16:27, 13 April 2008 (UTC)
Hi! Thanks for your interest in this topic. An antenna may not "see" surroundings as you expect, though. For example, a directive antenna pointed up into the sky is almost unaffected by the ground. The blackbody radiation it sees is just the cosmic background at 3K. Therefore, you cannot assume that the noise temperature of the antenna is the same as its physical temperature. There will be a contribution from its physical temperature, but that is determined by the amount of ohmic resistance in the antenna, NOT its entire feedpoint impedance. If the antenna is resonant (impedance is all real), for example, and has a 50 ohm radiation resistance and a 1 ohm ohmic resistance, then the noise power at the output will be attenuated because you have a 1 ohm noise source driving an unmatched load, namely 100 ohms (antenna plus receiver)) Sbreheny ( talk) 06:04, 20 May 2008 (UTC)
I think that you a mixing things up a bit. Radiation is, as I'm sure you know, not the only heat transfer method. Therefore, you can have an antenna which is neither getting hotter or colder but yet "sees" a much colder temperature via radiation. The rest of the thermal input is coming from conduction and convection. The reason why you don't freeze instantly when you step outside is because of the warm blanket of air (as you say) but it is NOT because of radiation from the air, but from convection and conduction. I agree that an antenna pointing at the sky is rarely going to see anything close to 3K, but that is not because of the atmosphere - it is because of cosmic RF sources. The atmosphere's attenuation to some frequencies (for example, around 400MHz) is very low. Since the attenuation is low, this necessarily also means that the noise radiation at that frequency is also low. This is precisely why the "Dew effect" you mention happens. Objects which "see" the ground stay a bit warmer than those which "see" the sky. So, it really is not all that complicated to determine the noise output of an antenna which is pointed at a quiet portion of the sky, in a frequency range where atmospheric attenuation is negligible. It would be equal to the component received from space plus the contribution from the ohmic losses in the antenna. This is often much lower than a noise temperature of 300K (otherwise it wouldn't make sense to make preamps with very low noise figure). Sbreheny ( talk) 04:35, 26 May 2008 (UTC)
Hi again Paul. The sky's temperature is frequency dependent. I'm sure that in the IR spectrum, it is not 3K. However, at RF frequencies, it is. Check out [1] top of page 5. Since antennas do not receive thermal noise in the IR spectrum, I was focusing on the RF sky noise temperature. As for the first part, I honestly do not know the technical definition of equilibrium (I had thought it meant "not changing temperature" but perhaps I am wrong). My main concern is that antennas are very rarely surrounded by materials which are true blackbodies. Therefore, antennas will usually NOT output noise equal to the equation you give. You may be strictly speaking correct for thermal equilibrium, but since that situation is very rare with real antennas, I don't think it should be given as a typical example in this article. Sbreheny ( talk) 05:33, 28 May 2008 (UTC)
There are two statements in this section
"The root mean square (rms) of the voltage, , is given by..."
"The root mean square (rms) of the voltage, , is given by..."
I understand the difference in the formulas (the former is per sqrt(Hz) ), but the terminology is confusing. -- agr 11:31, 17 October 2006 (UTC)
"Although the rms value for thermal noise is well defined, the instantaneous value can only be defined in terms of probability. The instantaneous amplitude of thermal noise has a Gaussian, or normal, distribution." (p. 203)
"The crest factor of a waveform is defined as the ratio of the peak to the rms value." ". . . a crest value of approximately 4 is used for thermal noise." (p. 204) [2]鈥擯receding unsigned comment added by 70.18.4.75 ( talk 鈥 contribs)
Is the formula for the Johnson-Nyquist noise on capacitors valid for both RC circuits in series and in parallel? 66.31.1.215 18:21, 16 February 2007 (UTC)
Thanks for the explanation. I'm sorry though, but I do not understand your answer completely. If I understand you correctly, you say that the effective resistance is the same for both RC circuits. So that would mean you should use the square root of (4kTRf), with that effective resistor as R, right? But is this then, the same as the square root of (kT/C)? Also, I thought Norton and Thevenin only work for resistor circuits, not for RC circuits.
Ok, I understand it better now. So only if the Norton and/or Thevenin equivalence hold, are they the same and does the R of the RC not contribute to the noise, right?
This article seems to imply that noise from Caps is equivalent to noise from Resistors. I'm not sure that this is true, given the earlier teachings on the benefits (noise) of switched-capacitor filters. Maybe an explanation of the paradox is necessary. Petersk 16:41, 12 April 2007 (UTC)
KTC noise is not fundamental noise. The source of Thermal noise across a capacitor comes from resistance. The source of KTC noise is easy to understand from .noise Spice analysis. Regardless of the parallel resistance across the cap (it could theoretically be infinite, or 1T ohm, 1p Ohm, etc.), the noise comes from parallel resistance across the capacitor. Kent H. Lundberg properly describes KTC noise, Please see page 10: http://web.mit.edu/klund/www/papers/UNP_noise.pdf 鈥擯receding unsigned comment added by 72.25.73.160 ( talk) 01:58, 18 April 2008 (UTC)
Also the KTC noise equation, Vn = sqrt(K T / C), is accurate for pure white noise with infinite bandwidth. For example, consider an amazing 220uF cap that has 100T ohm parallel internal resistance. According to the KTC equation the cap noise should be 4.3nV rms, but in reality such KTC noise would be 4.1nV rms due to the limitation of thermal noise bandwidth, which is ~ 7 THz. With a parallel resistance of 500T ohms the KTC noise is 3.4nV rms. 鈥擯receding unsigned comment added by 72.25.73.160 ( talk) 14:37, 18 April 2008 (UTC)
Mike Engelhardt, the creator of LTspice, gave permission to post his email, as follows.
Paul,
Yes, I fully agree with you that kTC noise is due to the resistance(even though the switch resistor specifics don't matter). This is basically lot and parcel that reactances generate no noise to the extent that they have no losses. That is why SPICE is correct not to include *any* noise whatsoever from a capacitor. There simply is none if the capacitor has no loss(resistance mechanism). BTW, LTspice will include noise from a capacitor because it supports series and parallel losses in the cap and those do generate noise. So while LTspice has noise coming from capacitors, its due to the losses in the capacitor, not the capacitance itself.
Now the problem with using SPICE or LTspice with kTC noise is that LTspice is doing a noise computation of the linear circuit. But you can't see any kTC noise in a linear circuit. It is the result of the resistance of the switch as it opens. It occurs during the non-linear operation. It isn't that LTspice is incorrect, it just that it doesn't do non-linear noise analysis.
Feel free to post the above as a quote of private communication with me.
Regards,
--Mike
The main problem with arguments containing a resistance of zero is that the "zero" is a limit and can mean different things depending on the nature of the limit process. After reading this discussion, I talked to one of the Authors of the Sarpeshkar et. al. paper, and he told me that the derivation shown in that paper was intended as a short cut only, but had triggered intensive and almost religious discussions in the research community that have not been decided yet. The "filtered noise" view is more productive in the design of switched-capacitor filters, while the thermodynamic-equilibrium view is more productive in the design of optosensors, especially when one goes on to finding ways how to prevent the sensor from reaching thermal equilibrium by different methods (c.f. the work of Boyd Fowler).
Now Wikipedia is an encyclopedia, it is not the place to decide between correct and incorrect, but it is most certainly the place to write about a disagreement that is going on in the research community. I therefore suggest to rewrite the kTC section as follows: (1) kT/C noise viewn as filtered Johnson Nyquist noise, (1b) how this is applied to explain noise in SC circuits, (2) kT/C noise viewn as a thermal equilibrium state of a system with one degree of freedom, (2b) how this is applied in photo sensorics (c.f. Boyd Fowler), (3) physical cause under debate since Sarpeskar's paper.
Target: not longer than it is now. I also suggest to remove the table with the number of electrons; those who know what this is about really don't need that table; those who would need that table cannot make any use of it.
Dick: I apologize for saying "factual error" in my edit; I promise to not do such a rushed edit again on a Wikipedia article.
Dick, Paul, I'd welcome comments! Hanspi ( talk) 08:42, 9 September 2009 (UTC)
So resistors emit white light? What's the connection to blackbody radiation? 鈥 Omegatron聽( talk) 00:34, 4 May 2008 (UTC)
I quote "The power spectral density, or voltage variance (mean square) per hertz of bandwidth, is given by
\bar v_{n}^2 = 4 k_B T R
where kB is Boltzmann's constant in joules per kelvin, T is the resistor's absolute temperature in kelvins, and R is the resistor value in ohms." Where does 'Hz' come from? [k_B in Joules/Kelvin]x[T in Kelvin]x[R in ohms] = [J][ohms] I can't find anything that leads to 'per Hertz or 1/time. Looking other books in transmission medium noise, they describe another apparently inconsistent result (No = 4*Kb*T) and also verbally express the units as Joules/Hertz!? What happened to [Ohms' in those cases? Loucosta ( talk) 17:16, 15 June 2008 (UTC)loucosta
"The cause of Johnson noise is blackbody radiation within the conductor. This explains
why the noise power is independent of the resistance 鈥 it only depends on the
temperature. However the resistance does affect the observed voltage. The actual charges
in the conductor move so as to try to nullify the electrical fluctuations caused by the
blackbody radiation. But the resistance to their movement limits the nullification, so a net
random time varying potential can be observed across the conductor."
http://www.claysturner.com/dsp/Johnson-Nyquist%20Noise.pdf 鈥擯receding
unsigned comment added by
71.167.60.17 (
talk)
06:13, 3 November 2009 (UTC)
The previous comment is correct - Johnson noise definitely is blackbody emission. To be specific, it is a 1D blackbody in the low frequency limit (hf << kT, also known as the Rayleigh-Jeans limit). To check this, take Planck's law, which is the classical result for a 3D blackbody, multiply by an area of lambda squared (recalling that lambda = the speed of light divided by the frequency), and expand the exponential to first order in the limit hf << kT. This gives the familiar Johnson noise result. Tls60 ( talk) 12:39, 8 September 2011 (UTC)
Do very large resistances have very large noise voltages?
The resistivity of air is about 1013 惟. So for a short distance, a typical resistance is about 10 P惟. Does this then produce 2 V of thermal noise? What about long distances? 鈥擯receding unsigned comment added by 71.167.60.17 ( talk) 05:57, 3 November 2009 (UTC)
What am I missing here? As previously stated the Johnson-Nyquist noise at room temperature in a 6MHz bandwidth is -106dBm. With a receiver noise figure of 3dB the receiver noise power would then be -103dBm. With a receiver noise figure of 1dB the noise power would be -105dBm. Where do you get -112dBm?
"For example a 6 MHz wide channel such as a television channel received signal would compete with the tiny amount of power generated by room temperature in the input stages of the receiver, which, for a TV receiver with a noise figure of 3 dB would be 鈭106 dBm, or one fortieth of a picowatt. For a TV with a noise figure of 1 dB, the noise power would be 鈭112 dBm." -- Golden Eternity ( talk) 21:13, 28 May 2010 (UTC)
From the "Noise voltage and power" section, the following statement is confusing: "For the general case, the above definition applies to". There is not a definition above the sentence (it is the first section). If it's referring to shot noise, it should just say the "For the general case, the above definition of shot noise applies to" to remain clear. 鈥擯receding unsigned comment added by 70.133.83.60 ( talk) 21:06, 16 June 2010 (UTC)
I think that the date (1928) given for Johnson's first thermal noise measurements might be incorrect here - though his detailed Physical Review paper was published (back-to-back with Nyquist's theory paper) in 1928, he published a Nature paper (Nature 119, p. 50, Jan. 8, 1927) in early January 1927 summarizing that work, and an abstract for a paper presented by Johnson at an American Physical Society Annual Meeting (held December 28-30, 1926) is published in a conference proceedings in a February 1927 Physical Review issue (Phys. Rev. 29, p. 367, Feb. 1927).
I suspect that since I had to dig this all up, this could be running afoul of WP:NOR, but it's clear to me that Johnson must have discovered thermal noise no later than December 1926, earlier than the 1928 date given. Does anyone disagree that the February 1927 abstract publication - for a December 1926 conference - is sufficient evidence? Should I change it to 1926 (with the reference given)? 00:54, 8 September 2011 (UTC) 鈥 Preceding unsigned comment added by Lambda(T) ( talk 鈥 contribs)
This does not follow from "the equation above" (which equation?). It is for the case of a matched load. Very misleading. 鈥 Preceding unsigned comment added by Peter1c ( talk 鈥 contribs) 12:19, 8 May 2013 (UTC)
The discution I start here reffers to the following statemant found on the page:
"A resistor in a short circuit dissipates a noise power of" (this line follows a formula of power).
Now I want to put the question -> into what type of power/energy is transformed that power quantity (4*k*T*deltaF).
The word 'dissipation' reffer generaly at transforming work (in various forms, including electrical work) into heat (a type of energy that rises the temperature of things (or changes thier phase -> melding, vaporising)).
The noise power exist all the time and it cannot "dissipate" into heat because otherwise a resistor will heat spontaneosely.
So I suggest that the words: "a circuit disipates a noise power" is not suitable because the noise power does not transform into anything (it does not dissipates, it stays into the device in the same form all the time).
A more intuitive phrase would be: "the circuit present at the terminals a noise power of ... ". 鈥 Preceding unsigned comment added by 84.247.79.142 ( talk) 15:51, 4 September 2014 (UTC)
I think it would be nice to have a separate page called " reset noise" for capacitor reset noise, any objections?
Reasons: One way to derive the thermal reset noise on a capacitor (mean square voltage of kTC) is using Johnson noise, but that's not the only way to derive it. Even basic statistical mechanical arguments work so there is no need for justifying kTC noise with fluctuation-dissipation theorems. Another motivation for the split is that Johnson noise is a dynamically changing noise whereas reset noise is a static fluctuation.
One possible problem is that presently " thermal noise" redirects to this article.-- Nanite ( talk) 11:13, 15 June 2015 (UTC)
I have changed the rather common and derogatory, off-handed remark that Johnson was the "first to measure" to "first to discover and measure."
Johnson did discover it in 1926 (it had clearly not been "predicted" by Nyquist in 1928, as is frequently claimed) it was formulated by Nyquist as a direct result of its discovery/observation by Johnson, who had isolated it sufficiently to understand that it was an intrinsic thermal noise (by comparison, one cannot make the same eponymous claim about Brownian motion, which was completely misunderstood by it's discoverer). Wikibearwithme ( talk) 03:19, 16 September 2016 (UTC)
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this is wrong
corrected
values in the table are 6.2 dB to low 26.85掳C is a funny room temperature.
If no one objects I will correct the formula.
Someone did not know to handle dBm 聽?驴 But at dBm are also problems, there is a value stated 0.4 dB different to this here (and also wrong).
See https://www.sengpielaudio.com and many other sources.
-- AK45500 ( talk) 21:44, 8 December 2020 (UTC)
in the best case (a matched load)
Googled "thermal noise power formula"
[3]I found the sengpielaudio calculator. They calculate dBu and dBv, neither of which is dBm.
I also found
[4] Thermal noise in a 50 惟 system at room temperature is -174 dBm / Hz.
[5] calculator produces -173.8277942 dBm
[6] Eq. 2.121 verifies formula for power P = kTB
[7] Verifies Thermal noise power equation
[8] -173.9 dBm鈦凥z
Here is an old but good one from the IEEE: "the available thermal noise power, from a resistor, a lossy network, a lossy dielectric, or an antenna is always kTB" [9]
Constant314 ( talk) 21:02, 10 December 2020 (UTC)
and some printed sources
"Noise power at 300 K = -173.83 dBm/Hz" [1]:鈥260鈥
"Noise power = kTB" [2]:鈥621鈥
"P = kTB" [3]:鈥203鈥 Constant314 ( talk) 22:03, 10 December 2020 (UTC)
I and many engineers have memorized the following helpful fact when doing noise analysis. I would like to add the following to the article. Before I do that I would like to hear any comments on the wording and format.
Memory aid
The noise voltage of a 1k惟 resister at room temperature is 4 nV per root hertz.
Constant314 ( talk) 14:19, 27 June 2024 (UTC)
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From this paper:
"3) The radiation resistance of free space is 鈭(渭0 /蔚0). What is the rms noise voltage at the terminals of an antenna which will compete, say, with an FM radio signal?"
Is that supposed to imply that free space generates thermal noise??? - Omegatron 18:36, Apr 15, 2005 (UTC)
- You are confusing radiation resistance with the impedance of free space. Radiation resistance is a property of the antenna geometry, and is a measure of the power lost from the transmitter because it is radiated as electromagnetic waves. The impedance of free space describes the ratio of electrostatic to magnetic field in the propagating wave.
I see a number of problems with the discussion of radio communications on this page. First of all, it is NOT true that the noise received by a receiver listening to a radio channel is kTB. It can be much higher OR lower than this. If we exclude external sources of noise (like lightning and the sun), then the dominant noise source is the receiver itself. Internal receiver noise is usually rated by Noise_figure, which, for a receiver, is the ratio of Pout/G*Pin, where Pin is the thermal noise from a resistor at 290K attached to the input of the receiver and Pout is the noise output power of the receiver (usually can consider just the first few stages) and G is the receiver gain. So, a perfect receiver would have a noise figure of 1 (0 dB). A receiver which introduced internal noise equal to thermal noise at 290K (which is what is implied in this article as being always true) would be 2 (3 dB).
UHF and microwave receivers regularly achieve noise figures of less than 1 dB (1.25).
In real use, the receiver is not attached to a resistor as its source but to an antenna with the proper feedpoint impedance. Because the resistive part of the feedpoint impedance is not a power loss but a power transformation (from an EM wave propagating in free space to a signal in the wires going to the receiver), it does not produce Johnson noise.
So, in summary, if you attach an antenna to a receiver, point it away from all radio sources (free space), then you will see simply the receiver internal noise (plus the cosmic background radiation, but that is almost surely much less than the receiver internal noise), and this internal noise may be much less than kTB. If no-one responds to this, I will change the article to reflect this, along with links to other relevant articles such as Noise_figure Sbreheny ( talk) 06:10, 24 December 2007 (UTC)
It seems to me the Preceivernoise equation should represent a closed system at equilibrium. If true, then the antennas temperature is equal to its surrounding temperature. Therefore, the amount of thermal noise received by the antenna due to blackbody radiation is relative to the antennas temperature. Correct me if I'm wrong, but power noise (KTB) is applicable to antennas; e.g., a 10MHz-BW 50-ohm (radiation resistance) antenna at 300K should receive K*300*10MHz = 4E-14 watts. Paul Lowrance, April 13 2008. 鈥擯receding unsigned comment added by 72.25.73.160 ( talk) 16:27, 13 April 2008 (UTC)
Hi! Thanks for your interest in this topic. An antenna may not "see" surroundings as you expect, though. For example, a directive antenna pointed up into the sky is almost unaffected by the ground. The blackbody radiation it sees is just the cosmic background at 3K. Therefore, you cannot assume that the noise temperature of the antenna is the same as its physical temperature. There will be a contribution from its physical temperature, but that is determined by the amount of ohmic resistance in the antenna, NOT its entire feedpoint impedance. If the antenna is resonant (impedance is all real), for example, and has a 50 ohm radiation resistance and a 1 ohm ohmic resistance, then the noise power at the output will be attenuated because you have a 1 ohm noise source driving an unmatched load, namely 100 ohms (antenna plus receiver)) Sbreheny ( talk) 06:04, 20 May 2008 (UTC)
I think that you a mixing things up a bit. Radiation is, as I'm sure you know, not the only heat transfer method. Therefore, you can have an antenna which is neither getting hotter or colder but yet "sees" a much colder temperature via radiation. The rest of the thermal input is coming from conduction and convection. The reason why you don't freeze instantly when you step outside is because of the warm blanket of air (as you say) but it is NOT because of radiation from the air, but from convection and conduction. I agree that an antenna pointing at the sky is rarely going to see anything close to 3K, but that is not because of the atmosphere - it is because of cosmic RF sources. The atmosphere's attenuation to some frequencies (for example, around 400MHz) is very low. Since the attenuation is low, this necessarily also means that the noise radiation at that frequency is also low. This is precisely why the "Dew effect" you mention happens. Objects which "see" the ground stay a bit warmer than those which "see" the sky. So, it really is not all that complicated to determine the noise output of an antenna which is pointed at a quiet portion of the sky, in a frequency range where atmospheric attenuation is negligible. It would be equal to the component received from space plus the contribution from the ohmic losses in the antenna. This is often much lower than a noise temperature of 300K (otherwise it wouldn't make sense to make preamps with very low noise figure). Sbreheny ( talk) 04:35, 26 May 2008 (UTC)
Hi again Paul. The sky's temperature is frequency dependent. I'm sure that in the IR spectrum, it is not 3K. However, at RF frequencies, it is. Check out [1] top of page 5. Since antennas do not receive thermal noise in the IR spectrum, I was focusing on the RF sky noise temperature. As for the first part, I honestly do not know the technical definition of equilibrium (I had thought it meant "not changing temperature" but perhaps I am wrong). My main concern is that antennas are very rarely surrounded by materials which are true blackbodies. Therefore, antennas will usually NOT output noise equal to the equation you give. You may be strictly speaking correct for thermal equilibrium, but since that situation is very rare with real antennas, I don't think it should be given as a typical example in this article. Sbreheny ( talk) 05:33, 28 May 2008 (UTC)
There are two statements in this section
"The root mean square (rms) of the voltage, , is given by..."
"The root mean square (rms) of the voltage, , is given by..."
I understand the difference in the formulas (the former is per sqrt(Hz) ), but the terminology is confusing. -- agr 11:31, 17 October 2006 (UTC)
"Although the rms value for thermal noise is well defined, the instantaneous value can only be defined in terms of probability. The instantaneous amplitude of thermal noise has a Gaussian, or normal, distribution." (p. 203)
"The crest factor of a waveform is defined as the ratio of the peak to the rms value." ". . . a crest value of approximately 4 is used for thermal noise." (p. 204) [2]鈥擯receding unsigned comment added by 70.18.4.75 ( talk 鈥 contribs)
Is the formula for the Johnson-Nyquist noise on capacitors valid for both RC circuits in series and in parallel? 66.31.1.215 18:21, 16 February 2007 (UTC)
Thanks for the explanation. I'm sorry though, but I do not understand your answer completely. If I understand you correctly, you say that the effective resistance is the same for both RC circuits. So that would mean you should use the square root of (4kTRf), with that effective resistor as R, right? But is this then, the same as the square root of (kT/C)? Also, I thought Norton and Thevenin only work for resistor circuits, not for RC circuits.
Ok, I understand it better now. So only if the Norton and/or Thevenin equivalence hold, are they the same and does the R of the RC not contribute to the noise, right?
This article seems to imply that noise from Caps is equivalent to noise from Resistors. I'm not sure that this is true, given the earlier teachings on the benefits (noise) of switched-capacitor filters. Maybe an explanation of the paradox is necessary. Petersk 16:41, 12 April 2007 (UTC)
KTC noise is not fundamental noise. The source of Thermal noise across a capacitor comes from resistance. The source of KTC noise is easy to understand from .noise Spice analysis. Regardless of the parallel resistance across the cap (it could theoretically be infinite, or 1T ohm, 1p Ohm, etc.), the noise comes from parallel resistance across the capacitor. Kent H. Lundberg properly describes KTC noise, Please see page 10: http://web.mit.edu/klund/www/papers/UNP_noise.pdf 鈥擯receding unsigned comment added by 72.25.73.160 ( talk) 01:58, 18 April 2008 (UTC)
Also the KTC noise equation, Vn = sqrt(K T / C), is accurate for pure white noise with infinite bandwidth. For example, consider an amazing 220uF cap that has 100T ohm parallel internal resistance. According to the KTC equation the cap noise should be 4.3nV rms, but in reality such KTC noise would be 4.1nV rms due to the limitation of thermal noise bandwidth, which is ~ 7 THz. With a parallel resistance of 500T ohms the KTC noise is 3.4nV rms. 鈥擯receding unsigned comment added by 72.25.73.160 ( talk) 14:37, 18 April 2008 (UTC)
Mike Engelhardt, the creator of LTspice, gave permission to post his email, as follows.
Paul,
Yes, I fully agree with you that kTC noise is due to the resistance(even though the switch resistor specifics don't matter). This is basically lot and parcel that reactances generate no noise to the extent that they have no losses. That is why SPICE is correct not to include *any* noise whatsoever from a capacitor. There simply is none if the capacitor has no loss(resistance mechanism). BTW, LTspice will include noise from a capacitor because it supports series and parallel losses in the cap and those do generate noise. So while LTspice has noise coming from capacitors, its due to the losses in the capacitor, not the capacitance itself.
Now the problem with using SPICE or LTspice with kTC noise is that LTspice is doing a noise computation of the linear circuit. But you can't see any kTC noise in a linear circuit. It is the result of the resistance of the switch as it opens. It occurs during the non-linear operation. It isn't that LTspice is incorrect, it just that it doesn't do non-linear noise analysis.
Feel free to post the above as a quote of private communication with me.
Regards,
--Mike
The main problem with arguments containing a resistance of zero is that the "zero" is a limit and can mean different things depending on the nature of the limit process. After reading this discussion, I talked to one of the Authors of the Sarpeshkar et. al. paper, and he told me that the derivation shown in that paper was intended as a short cut only, but had triggered intensive and almost religious discussions in the research community that have not been decided yet. The "filtered noise" view is more productive in the design of switched-capacitor filters, while the thermodynamic-equilibrium view is more productive in the design of optosensors, especially when one goes on to finding ways how to prevent the sensor from reaching thermal equilibrium by different methods (c.f. the work of Boyd Fowler).
Now Wikipedia is an encyclopedia, it is not the place to decide between correct and incorrect, but it is most certainly the place to write about a disagreement that is going on in the research community. I therefore suggest to rewrite the kTC section as follows: (1) kT/C noise viewn as filtered Johnson Nyquist noise, (1b) how this is applied to explain noise in SC circuits, (2) kT/C noise viewn as a thermal equilibrium state of a system with one degree of freedom, (2b) how this is applied in photo sensorics (c.f. Boyd Fowler), (3) physical cause under debate since Sarpeskar's paper.
Target: not longer than it is now. I also suggest to remove the table with the number of electrons; those who know what this is about really don't need that table; those who would need that table cannot make any use of it.
Dick: I apologize for saying "factual error" in my edit; I promise to not do such a rushed edit again on a Wikipedia article.
Dick, Paul, I'd welcome comments! Hanspi ( talk) 08:42, 9 September 2009 (UTC)
So resistors emit white light? What's the connection to blackbody radiation? 鈥 Omegatron聽( talk) 00:34, 4 May 2008 (UTC)
I quote "The power spectral density, or voltage variance (mean square) per hertz of bandwidth, is given by
\bar v_{n}^2 = 4 k_B T R
where kB is Boltzmann's constant in joules per kelvin, T is the resistor's absolute temperature in kelvins, and R is the resistor value in ohms." Where does 'Hz' come from? [k_B in Joules/Kelvin]x[T in Kelvin]x[R in ohms] = [J][ohms] I can't find anything that leads to 'per Hertz or 1/time. Looking other books in transmission medium noise, they describe another apparently inconsistent result (No = 4*Kb*T) and also verbally express the units as Joules/Hertz!? What happened to [Ohms' in those cases? Loucosta ( talk) 17:16, 15 June 2008 (UTC)loucosta
"The cause of Johnson noise is blackbody radiation within the conductor. This explains
why the noise power is independent of the resistance 鈥 it only depends on the
temperature. However the resistance does affect the observed voltage. The actual charges
in the conductor move so as to try to nullify the electrical fluctuations caused by the
blackbody radiation. But the resistance to their movement limits the nullification, so a net
random time varying potential can be observed across the conductor."
http://www.claysturner.com/dsp/Johnson-Nyquist%20Noise.pdf 鈥擯receding
unsigned comment added by
71.167.60.17 (
talk)
06:13, 3 November 2009 (UTC)
The previous comment is correct - Johnson noise definitely is blackbody emission. To be specific, it is a 1D blackbody in the low frequency limit (hf << kT, also known as the Rayleigh-Jeans limit). To check this, take Planck's law, which is the classical result for a 3D blackbody, multiply by an area of lambda squared (recalling that lambda = the speed of light divided by the frequency), and expand the exponential to first order in the limit hf << kT. This gives the familiar Johnson noise result. Tls60 ( talk) 12:39, 8 September 2011 (UTC)
Do very large resistances have very large noise voltages?
The resistivity of air is about 1013 惟. So for a short distance, a typical resistance is about 10 P惟. Does this then produce 2 V of thermal noise? What about long distances? 鈥擯receding unsigned comment added by 71.167.60.17 ( talk) 05:57, 3 November 2009 (UTC)
What am I missing here? As previously stated the Johnson-Nyquist noise at room temperature in a 6MHz bandwidth is -106dBm. With a receiver noise figure of 3dB the receiver noise power would then be -103dBm. With a receiver noise figure of 1dB the noise power would be -105dBm. Where do you get -112dBm?
"For example a 6 MHz wide channel such as a television channel received signal would compete with the tiny amount of power generated by room temperature in the input stages of the receiver, which, for a TV receiver with a noise figure of 3 dB would be 鈭106 dBm, or one fortieth of a picowatt. For a TV with a noise figure of 1 dB, the noise power would be 鈭112 dBm." -- Golden Eternity ( talk) 21:13, 28 May 2010 (UTC)
From the "Noise voltage and power" section, the following statement is confusing: "For the general case, the above definition applies to". There is not a definition above the sentence (it is the first section). If it's referring to shot noise, it should just say the "For the general case, the above definition of shot noise applies to" to remain clear. 鈥擯receding unsigned comment added by 70.133.83.60 ( talk) 21:06, 16 June 2010 (UTC)
I think that the date (1928) given for Johnson's first thermal noise measurements might be incorrect here - though his detailed Physical Review paper was published (back-to-back with Nyquist's theory paper) in 1928, he published a Nature paper (Nature 119, p. 50, Jan. 8, 1927) in early January 1927 summarizing that work, and an abstract for a paper presented by Johnson at an American Physical Society Annual Meeting (held December 28-30, 1926) is published in a conference proceedings in a February 1927 Physical Review issue (Phys. Rev. 29, p. 367, Feb. 1927).
I suspect that since I had to dig this all up, this could be running afoul of WP:NOR, but it's clear to me that Johnson must have discovered thermal noise no later than December 1926, earlier than the 1928 date given. Does anyone disagree that the February 1927 abstract publication - for a December 1926 conference - is sufficient evidence? Should I change it to 1926 (with the reference given)? 00:54, 8 September 2011 (UTC) 鈥 Preceding unsigned comment added by Lambda(T) ( talk 鈥 contribs)
This does not follow from "the equation above" (which equation?). It is for the case of a matched load. Very misleading. 鈥 Preceding unsigned comment added by Peter1c ( talk 鈥 contribs) 12:19, 8 May 2013 (UTC)
The discution I start here reffers to the following statemant found on the page:
"A resistor in a short circuit dissipates a noise power of" (this line follows a formula of power).
Now I want to put the question -> into what type of power/energy is transformed that power quantity (4*k*T*deltaF).
The word 'dissipation' reffer generaly at transforming work (in various forms, including electrical work) into heat (a type of energy that rises the temperature of things (or changes thier phase -> melding, vaporising)).
The noise power exist all the time and it cannot "dissipate" into heat because otherwise a resistor will heat spontaneosely.
So I suggest that the words: "a circuit disipates a noise power" is not suitable because the noise power does not transform into anything (it does not dissipates, it stays into the device in the same form all the time).
A more intuitive phrase would be: "the circuit present at the terminals a noise power of ... ". 鈥 Preceding unsigned comment added by 84.247.79.142 ( talk) 15:51, 4 September 2014 (UTC)
I think it would be nice to have a separate page called " reset noise" for capacitor reset noise, any objections?
Reasons: One way to derive the thermal reset noise on a capacitor (mean square voltage of kTC) is using Johnson noise, but that's not the only way to derive it. Even basic statistical mechanical arguments work so there is no need for justifying kTC noise with fluctuation-dissipation theorems. Another motivation for the split is that Johnson noise is a dynamically changing noise whereas reset noise is a static fluctuation.
One possible problem is that presently " thermal noise" redirects to this article.-- Nanite ( talk) 11:13, 15 June 2015 (UTC)
I have changed the rather common and derogatory, off-handed remark that Johnson was the "first to measure" to "first to discover and measure."
Johnson did discover it in 1926 (it had clearly not been "predicted" by Nyquist in 1928, as is frequently claimed) it was formulated by Nyquist as a direct result of its discovery/observation by Johnson, who had isolated it sufficiently to understand that it was an intrinsic thermal noise (by comparison, one cannot make the same eponymous claim about Brownian motion, which was completely misunderstood by it's discoverer). Wikibearwithme ( talk) 03:19, 16 September 2016 (UTC)
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this is wrong
corrected
values in the table are 6.2 dB to low 26.85掳C is a funny room temperature.
If no one objects I will correct the formula.
Someone did not know to handle dBm 聽?驴 But at dBm are also problems, there is a value stated 0.4 dB different to this here (and also wrong).
See https://www.sengpielaudio.com and many other sources.
-- AK45500 ( talk) 21:44, 8 December 2020 (UTC)
in the best case (a matched load)
Googled "thermal noise power formula"
[3]I found the sengpielaudio calculator. They calculate dBu and dBv, neither of which is dBm.
I also found
[4] Thermal noise in a 50 惟 system at room temperature is -174 dBm / Hz.
[5] calculator produces -173.8277942 dBm
[6] Eq. 2.121 verifies formula for power P = kTB
[7] Verifies Thermal noise power equation
[8] -173.9 dBm鈦凥z
Here is an old but good one from the IEEE: "the available thermal noise power, from a resistor, a lossy network, a lossy dielectric, or an antenna is always kTB" [9]
Constant314 ( talk) 21:02, 10 December 2020 (UTC)
and some printed sources
"Noise power at 300 K = -173.83 dBm/Hz" [1]:鈥260鈥
"Noise power = kTB" [2]:鈥621鈥
"P = kTB" [3]:鈥203鈥 Constant314 ( talk) 22:03, 10 December 2020 (UTC)
I and many engineers have memorized the following helpful fact when doing noise analysis. I would like to add the following to the article. Before I do that I would like to hear any comments on the wording and format.
Memory aid
The noise voltage of a 1k惟 resister at room temperature is 4 nV per root hertz.
Constant314 ( talk) 14:19, 27 June 2024 (UTC)