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Question about noise voltage

Started by brent September 7, 2011
I have been pondering the notation for noise voltage.  The noise
voltage across a 50 ohm resistor is expressed as 1nV/sqrt(1Hz)

As I ponder this it seems to me that one is concerned about the noise
voltage period.  For instance, if you have a 1Hz BW signal you would
say the noise voltage on a 50 ohm resistor is 1nV.

If you have a 100 Hz signal you would say that the noise voltage is 10
nV (on a 50 ohm resistor with a 100 Hz BW)

Would you say 10 nV/sqrt(100Hz) ? (I don't think so)

It seems to me that the notation of 1nV/sqrt(1Hz) is really just a
reminder that when calculating the noise voltage that the noise
voltage goes up by the square root of the Bandwidth, and is not really
relevant other than for the reminder.

Agree?
On Sep 7, 9:43&#2013266080;pm, brent <buleg...@columbus.rr.com> wrote:
> I have been pondering the notation for noise voltage. &#2013266080;The noise > voltage across a 50 ohm resistor is expressed as 1nV/sqrt(1Hz) > > As I ponder this it seems to me that one is concerned about the noise > voltage period. &#2013266080;For instance, if you have a 1Hz BW signal you would > say the noise voltage on a 50 ohm resistor is 1nV. > > If you have a 100 Hz signal you would say that the noise voltage is 10 > nV (on a 50 ohm resistor with a 100 Hz BW) > > Would you say 10 nV/sqrt(100Hz) ? (I don't think so) > > It seems to me that the notation of 1nV/sqrt(1Hz) is really just a > reminder that when calculating the noise voltage that the noise > voltage goes up by the square root of the Bandwidth, and is not really > relevant other than for the reminder. > > Agree?
No.
On Sep 7, 9:43&#2013266080;pm, brent <buleg...@columbus.rr.com> wrote:
> I have been pondering the notation for noise voltage. &#2013266080;The noise > voltage across a 50 ohm resistor is expressed as 1nV/sqrt(1Hz) > > As I ponder this it seems to me that one is concerned about the noise > voltage period. &#2013266080;For instance, if you have a 1Hz BW signal you would > say the noise voltage on a 50 ohm resistor is 1nV. > > If you have a 100 Hz signal you would say that the noise voltage is 10 > nV (on a 50 ohm resistor with a 100 Hz BW) > > Would you say 10 nV/sqrt(100Hz) ? (I don't think so) > > It seems to me that the notation of 1nV/sqrt(1Hz) is really just a > reminder that when calculating the noise voltage that the noise > voltage goes up by the square root of the Bandwidth, and is not really > relevant other than for the reminder. > > Agree?
Please ignore this thread and reply to other one. Google took over an hour to post first message so I reposted thinking it was lost -- bah.
On 9/7/2011 9:43 PM, brent wrote:
> I have been pondering the notation for noise voltage. The noise > voltage across a 50 ohm resistor is expressed as 1nV/sqrt(1Hz)
Did you just make that up, or did you read it somewhere?
> As I ponder this it seems to me that one is concerned about the noise > voltage period.
What do you call a voltage period?
> For instance, if you have a 1Hz BW signal you would > say the noise voltage on a 50 ohm resistor is 1nV. > > If you have a 100 Hz signal you would say that the noise voltage is 10 > nV (on a 50 ohm resistor with a 100 Hz BW) > > Would you say 10 nV/sqrt(100Hz) ? (I don't think so) > > It seems to me that the notation of 1nV/sqrt(1Hz) is really just a > reminder that when calculating the noise voltage that the noise > voltage goes up by the square root of the Bandwidth, and is not really > relevant other than for the reminder. > > Agree?
You seem to have misinterpreted something you read. Instead, read http://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noise It will dispel some of the half truths that plague you. Jerry -- Engineering is the art of making what you want from things you can get.
On Sep 7, 10:29&#2013266080;pm, dvsarwate <dvsarw...@yahoo.com> wrote:
> On Sep 7, 9:43&#2013266080;pm, brent <buleg...@columbus.rr.com> wrote: > > > > > > > > > > > I have been pondering the notation for noise voltage. &#2013266080;The noise > > voltage across a 50 ohm resistor is expressed as 1nV/sqrt(1Hz) > > > As I ponder this it seems to me that one is concerned about the noise > > voltage period. &#2013266080;For instance, if you have a 1Hz BW signal you would > > say the noise voltage on a 50 ohm resistor is 1nV. > > > If you have a 100 Hz signal you would say that the noise voltage is 10 > > nV (on a 50 ohm resistor with a 100 Hz BW) > > > Would you say 10 nV/sqrt(100Hz) ? (I don't think so) > > > It seems to me that the notation of 1nV/sqrt(1Hz) is really just a > > reminder that when calculating the noise voltage that the noise > > voltage goes up by the square root of the Bandwidth, and is not really > > relevant other than for the reminder. > > > Agree? > > No.
I love Usenet!
On Sep 7, 10:52&#2013266080;pm, Jerry Avins <j...@ieee.org> wrote:
> On 9/7/2011 9:43 PM, brent wrote: > > > I have been pondering the notation for noise voltage. &#2013266080;The noise > > voltage across a 50 ohm resistor is expressed as 1nV/sqrt(1Hz) > > Did you just make that up, or did you read it somewhere? > > > As I ponder this it seems to me that one is concerned about the noise > > voltage period. > > What do you call a voltage period? > > > &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; For instance, if you have a 1Hz BW signal you would > > say the noise voltage on a 50 ohm resistor is 1nV. > > > If you have a 100 Hz signal you would say that the noise voltage is 10 > > nV (on a 50 ohm resistor with a 100 Hz BW) > > > Would you say 10 nV/sqrt(100Hz) ? (I don't think so) > > > It seems to me that the notation of 1nV/sqrt(1Hz) is really just a > > reminder that when calculating the noise voltage that the noise > > voltage goes up by the square root of the Bandwidth, and is not really > > relevant other than for the reminder. > > > Agree? > > You seem to have misinterpreted something you read. Instead, readhttp://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noiseIt will > dispel some of the half truths that plague you. > > Jerry > -- > Engineering is the art of making what you want from things you can get.
Jerry, It is the notation in this very article (and in op amp data sheets) that is prompting my questions. For example, In the article they state that the noise voltage for a 1K resistor is 4.07nV/sqrt(1Hz).
On 9/7/2011 10:58 PM, brent wrote:
> On Sep 7, 10:52 pm, Jerry Avins<j...@ieee.org> wrote: >> On 9/7/2011 9:43 PM, brent wrote: >> >>> I have been pondering the notation for noise voltage. The noise >>> voltage across a 50 ohm resistor is expressed as 1nV/sqrt(1Hz) >> >> Did you just make that up, or did you read it somewhere? >> >>> As I ponder this it seems to me that one is concerned about the noise >>> voltage period. >> >> What do you call a voltage period? >> >>> For instance, if you have a 1Hz BW signal you would >>> say the noise voltage on a 50 ohm resistor is 1nV. >> >>> If you have a 100 Hz signal you would say that the noise voltage is 10 >>> nV (on a 50 ohm resistor with a 100 Hz BW) >> >>> Would you say 10 nV/sqrt(100Hz) ? (I don't think so) >> >>> It seems to me that the notation of 1nV/sqrt(1Hz) is really just a >>> reminder that when calculating the noise voltage that the noise >>> voltage goes up by the square root of the Bandwidth, and is not really >>> relevant other than for the reminder. >> >>> Agree? >> >> You seem to have misinterpreted something you read. Instead, readhttp://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noiseIt will >> dispel some of the half truths that plague you. >> >> Jerry >> -- >> Engineering is the art of making what you want from things you can get. > > Jerry, > > It is the notation in this very article (and in op amp data sheets) > that is prompting my questions. > > For example, In the article they state that the noise voltage for a 1K > resistor is 4.07nV/sqrt(1Hz).
No they don't. They specifically state that that the noise voltage depends on temperature. The noise is statistical. The variance -- noise power -- depends on resistance and temperature. The measured voltage is determined by standard deviation, hence the square root. Jerry -- Engineering is the art of making what you want from things you can get.
On Sep 7, 10:55&#2013266080;pm, brent <buleg...@columbus.rr.com>
sarcastically commented:

Brent2> I love Usenet!

when I responded "No" to his query

Brent1>  Agree?

following a series of mis-statements and half-truths.

What answer other than Yes or No is there to the question
asked?  If you don't like the answer, and would have
preferred the response Yes, too bad.  Other responses
such as Jerry Avins's more loquacious

Jerry>  You seem to have misinterpreted something you
Jerry>  read. Instead, read http://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noise
Jerry>  It will dispel some of the half truths that plague you.

indicate that nobody agrees with what Brent said in
his original post.

Dilip Sarwate


On Sep 8, 7:52&#2013266080;am, dvsarwate <dvsarw...@yahoo.com> wrote:
> On Sep 7, 10:55&#2013266080;pm, brent <buleg...@columbus.rr.com> > sarcastically commented: > > Brent2> I love Usenet! > > when I responded "No" to his query > > Brent1> &#2013266080;Agree? > > following a series of mis-statements and half-truths. > > What answer other than Yes or No is there to the question > asked? &#2013266080;If you don't like the answer, and would have > preferred the response Yes, too bad. &#2013266080;Other responses > such as Jerry Avins's more loquacious > > Jerry> &#2013266080;You seem to have misinterpreted something you > Jerry> &#2013266080;read. Instead, readhttp://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noise > Jerry> &#2013266080;It will dispel some of the half truths that plague you. > > indicate that nobody agrees with what Brent said in > his original post. > > Dilip Sarwate
I don't know why you are explaining so much here. Your one word answer was exactly what I needed, which is why I said I love usenet , because your two letter answer provided tremendous insight into my question about this. Please chime in with as few (but insightful) letters as possible in the future. The fewer the letters the more helpful! Thanks!!!!
if you'd write "volts per Hertz", it would imply that volts are integrated
over frequency. That's not correct.
I guess one could try some substitution of variables in the integration,
getting via "watts per Hertz" and "sqrt(Watts) per Hertz" to "volts per
sqrt(Hertz)".