When expressing the noise voltage across, say a 50 ohm resistor, it will be called 1nV/sqrt(1Hz) I am trying to come up with an explanation as to why this notation is used. For example, if you want to determine the noise voltage in a 100 Hz bandwidth on the same 50 ohm resistor I think you would say that the noise voltage is 10nV. Or would you say 10nV/sqrt(100Hz)? I am thinking that this notation is used to remind the user to take the noise voltage up by the square root of the Bandwidth , and that it does not really have any other significance than that. Agree?
Question about noise voltage notation
Started by ●September 7, 2011
Reply by ●September 7, 20112011-09-07
"brent"> > When expressing the noise voltage across, say a 50 ohm resistor, it > will be called > > 1nV/sqrt(1Hz) > > I am trying to come up with an explanation as to why this notation is > used.** It expresses the noise characteristic of the part independent of the bandwidth in use. Makes it the most efficient way.> For example, if you want to determine the noise voltage in a 100 Hz > bandwidth on the same 50 ohm resistor I think you would say that the > noise voltage is 10nV. Or would you say 10nV/sqrt(100Hz)?** No. That is stupid.> I am thinking that this notation is used to remind the user to take > the noise voltage up by the square root of the Bandwidth , and that it > does not really have any other significance than that.** See above. .... Phil
Reply by ●September 7, 20112011-09-07
On 09/07/2011 09:10 PM, brent wrote:> When expressing the noise voltage across, say a 50 ohm resistor, it > will be called > > 1nV/sqrt(1Hz) > > I am trying to come up with an explanation as to why this notation is > used. > > For example, if you want to determine the noise voltage in a 100 Hz > bandwidth on the same 50 ohm resistor I think you would say that the > noise voltage is 10nV. Or would you say 10nV/sqrt(100Hz)? > > I am thinking that this notation is used to remind the user to take > the noise voltage up by the square root of the Bandwidth , and that it > does not really have any other significance than that. > > Agree?No, it's a different unit, as in feet vs square feet. If you change the bandwidth, the rms noise voltage goes as the square root of the bandwidth. If you have a narrow pulse signal, the rms signal voltage goes linearly with the bandwidth. If you want the noise voltage to come out in volts when you're done, you need the square root. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●September 8, 20112011-09-08
"Phil Hobbs" brent wrote:>> When expressing the noise voltage across, say a 50 ohm resistor, it >> will be called >> >> 1nV/sqrt(1Hz) >> >> I am trying to come up with an explanation as to why this notation is >> used. >> >> For example, if you want to determine the noise voltage in a 100 Hz >> bandwidth on the same 50 ohm resistor I think you would say that the >> noise voltage is 10nV. Or would you say 10nV/sqrt(100Hz)? >> >> I am thinking that this notation is used to remind the user to take >> the noise voltage up by the square root of the Bandwidth , and that it >> does not really have any other significance than that. >> >> Agree? > > No, it's a different unit, as in feet vs square feet. If you change the > bandwidth, the rms noise voltage goes as the square root of the bandwidth. > If you have a narrow pulse signal, the rms signal voltage goes linearly > with the bandwidth. > > If you want the noise voltage to come out in volts when you're done, you > need the square root.** Try reading the OP's question again. Your reply is completely redundant. .... Phil
Reply by ●September 8, 20112011-09-08
On 09/07/2011 11:07 PM, Phil Allison wrote:> "Phil Hobbs" > brent wrote: >>> When expressing the noise voltage across, say a 50 ohm resistor, it >>> will be called >>> >>> 1nV/sqrt(1Hz) >>> >>> I am trying to come up with an explanation as to why this notation is >>> used. >>> >>> For example, if you want to determine the noise voltage in a 100 Hz >>> bandwidth on the same 50 ohm resistor I think you would say that the >>> noise voltage is 10nV. Or would you say 10nV/sqrt(100Hz)? >>> >>> I am thinking that this notation is used to remind the user to take >>> the noise voltage up by the square root of the Bandwidth , and that it >>> does not really have any other significance than that. >>> >>> Agree? >> >> No, it's a different unit, as in feet vs square feet. If you change the >> bandwidth, the rms noise voltage goes as the square root of the bandwidth. >> If you have a narrow pulse signal, the rms signal voltage goes linearly >> with the bandwidth. >> >> If you want the noise voltage to come out in volts when you're done, you >> need the square root. > > > ** Try reading the OP's question again. > > Your reply is completely redundant. > > > > ..... PhilI suppose it would seem so, if you think that the only difference between a length and an area is to remind you to take the square. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●September 8, 20112011-09-08
"Phil Hobbs" Phil Allison wrote:>> "Phil Hobbs" >> brent wrote: >>>> When expressing the noise voltage across, say a 50 ohm resistor, it >>>> will be called >>>> >>>> 1nV/sqrt(1Hz) >>>> >>>> I am trying to come up with an explanation as to why this notation is >>>> used. >>>> >>>> For example, if you want to determine the noise voltage in a 100 Hz >>>> bandwidth on the same 50 ohm resistor I think you would say that the >>>> noise voltage is 10nV. Or would you say 10nV/sqrt(100Hz)? >>>> >>>> I am thinking that this notation is used to remind the user to take >>>> the noise voltage up by the square root of the Bandwidth , and that it >>>> does not really have any other significance than that. >>>> >>>> Agree? >>> >>> No, it's a different unit, as in feet vs square feet. If you change the >>> bandwidth, the rms noise voltage goes as the square root of the >>> bandwidth. >>> If you have a narrow pulse signal, the rms signal voltage goes linearly >>> with the bandwidth. >>> >>> If you want the noise voltage to come out in volts when you're done, you >>> need the square root. >> >> >> ** Try reading the OP's question again. >> >> Your reply is completely redundant. >> > > I suppose it would seem so,** FFS read it again - you pompous as. YOU MISREAD IT !!!!!!
Reply by ●September 8, 20112011-09-08
On 09/07/2011 11:21 PM, Phil Allison wrote:> "Phil Hobbs" > Phil Allison wrote: >>> "Phil Hobbs" >>> brent wrote: >>>>> When expressing the noise voltage across, say a 50 ohm resistor, it >>>>> will be called >>>>> >>>>> 1nV/sqrt(1Hz) >>>>> >>>>> I am trying to come up with an explanation as to why this notation is >>>>> used. >>>>> >>>>> For example, if you want to determine the noise voltage in a 100 Hz >>>>> bandwidth on the same 50 ohm resistor I think you would say that the >>>>> noise voltage is 10nV. Or would you say 10nV/sqrt(100Hz)? >>>>> >>>>> I am thinking that this notation is used to remind the user to take >>>>> the noise voltage up by the square root of the Bandwidth , and that it >>>>> does not really have any other significance than that. >>>>> >>>>> Agree? >>>> >>>> No, it's a different unit, as in feet vs square feet. If you change the >>>> bandwidth, the rms noise voltage goes as the square root of the >>>> bandwidth. >>>> If you have a narrow pulse signal, the rms signal voltage goes linearly >>>> with the bandwidth. >>>> >>>> If you want the noise voltage to come out in volts when you're done, you >>>> need the square root. >>> >>> >>> ** Try reading the OP's question again. >>> >>> Your reply is completely redundant. >>> >> >> I suppose it would seem so,I suppose it would seem so,>> if you think that the only difference between a length and an area >> is to remind you to take the square. (snippage restored)> > > ** FFS read it again - you pompous as. > > YOU MISREAD IT !!!!!!How so? My reply quotes the relevant part of the OP's post nearly verbatim. The physics is that for fundamental noise sources. e.g. shot noise and Johnson noise, there are no correlations between noise components at different frequencies, so the noise power goes as the bandwidth. Other wideband signals such as pulses may have quite different behaviour, because the correlations between the different frequency components are nonzero. For instance, if you have a narrow pulse at t=0, all the frequency components have phase 0 at t=0, i.e. only the cosine components contribute. It's a different unit, not just a mnemonic for behaviour vs. bandwidth. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●September 8, 20112011-09-08
On Wed, 7 Sep 2011 18:10:29 -0700 (PDT), the renowned brent <bulegoge@columbus.rr.com> wrote:>When expressing the noise voltage across, say a 50 ohm resistor, it >will be called > >1nV/sqrt(1Hz)Think about what is meant by "it" here..>I am trying to come up with an explanation as to why this notation is >used.It's used when describing Johnson noise in a resistor, which is assumed to be "white", iow to have a constant power spectral density in the region of interest. Since the _power_ spectral density is constant, the voltage squared per Hz is constant, and thus the voltage per sqrt(Hz) is constant. This is assumed to be true EVERYWHERE, not just from 0 to xx Hz. IOW, if the noise voltage from 0 to 100 Hz is 10nV, it's also the noise voltage over the bandwidth 100.0kHz to 100.1kHz>For example, if you want to determine the noise voltage in a 100 Hz >bandwidth on the same 50 ohm resistor I think you would say that the >noise voltage is 10nV.Yes. 10nV (RMS)> Or would you say 10nV/sqrt(100Hz)?No. That's not a voltage. Voltage is measured in volts ... or (kg*m^2)/(A*S^3).. this is a voltage spectral density.>I am thinking that this notation is used to remind the user to take >the noise voltage up by the square root of the Bandwidth , and that it >does not really have any other significance than that. > >Agree?No. Those are the proper units for this kind of noise. Dimensional analysis? 10nV/sqrt(Hz) * sqrt(100Hz) = 10 * 10 * nV * sqrt(Hz)/sqrt(Hz) = 100nV. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Reply by ●September 8, 20112011-09-08
"Phil Hobbs = Wanker ">>>> ** Try reading the OP's question again. >>>> >>>> Your reply is completely redundant. >>>> >>> >>> I suppose it would seem so,I suppose it would seem so, > >> if you think that the only difference between a length and an area > >> is to remind you to take the square. >> >> >> ** FFS read it again - you pompous as. >> >> YOU MISREAD IT !!!!!! > > How so? My reply quotes the relevant part of the OP's post nearly > verbatim.** GIANT Huh ???????????? Got SFA to do with you misreading it. Imbecile. ... Phil
Reply by ●September 8, 20112011-09-08
On 09/07/2011 11:34 PM, Phil Allison wrote:> "Phil Hobbs = Wanker" > >>>>> ** Try reading the OP's question again. >>>>> >>>>> Your reply is completely redundant. >>>>> >>>> >>>> I suppose it would seem so,I suppose it would seem so, >>>> if you think that the only difference between a length and an area >>>> is to remind you to take the square. >>> >>> >>> ** FFS read it again - you pompous as. >>> >>> YOU MISREAD IT !!!!!! >> >> How so? My reply quotes the relevant part of the OP's post nearly >> verbatim. > > ** GIANT Huh ???????????? > > Got SFA to do with you misreading it. > > Imbecile. > > > .... PhilSo you say, but you explain nothing. Don't just tell me, show me. Cheers Phil Hobbs
