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From: Phil Hobbs on 23 Mar 2010 16:23
On 3/23/2010 4:16 PM, John Larkin wrote:
> On Tue, 23 Mar 2010 12:59:12 -0700 (PDT), George Herold
> <ggherold(a)gmail.com> wrote:
>
>> On Mar 23, 2:58 pm, John Larkin
>> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
>>> On Tue, 23 Mar 2010 10:39:15 -0700 (PDT), George Herold
>>>
>>>
>>>
>>>
>>>
>>> <ggher...(a)gmail.com> wrote:
>>>> On Mar 23, 11:33 am, John Larkin
>>>> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
>>>>> On Tue, 23 Mar 2010 06:59:08 -0700 (PDT), George Herold
>>>
>>>>> <ggher...(a)gmail.com> wrote:
>>>>>> On Mar 23, 8:32 am, Wimpie<wimabc...(a)tetech.nl> wrote:
>>>>>>> On 23 mar, 12:57, Wimpie<wimabc...(a)tetech.nl> wrote:
>>>
>>>>>>>> On 22 mar, 23:52, hal-use...(a)ip-64-139-1-69.sjc.megapath.net (Hal
>>>
>>>>>>>> Murray) wrote:
>>>>>>>>> In article<4BA7797C.7020...(a)electrooptical.net>,
>>>>>>>>> Phil Hobbs<pcdhSpamMeSensel...(a)electrooptical.net> writes:
>>>
>>>>>>>>>> Night lights use CdS or CdSSe photoconductors, but they don't have to
>>>>>>>>>> have any kind of stability or repeatability, and they don't. (CdSSe's
>>>>>>>>>> resistance can vary 5X due entirely to previous illumination history.)
>>>
>>>>>>>>> What is the mechanism for that history quirk?
>>>
>>>>>>>>> What is the time scale? How long does it remember the history?
>>>
>>>>>>>>> --
>>>>>>>>> These are my opinions, not necessarily my employer's. I hate spam.
>>>
>>>>>>>> Hello,
>>>
>>>>>>>> Go towww.perkinelmer.comandsearchfor"photocell", or follow the
>>>>>>>> links. When at the photocell page, on the right side you have an
>>>>>>>> application note link. They have a very nice document on the LDRs that
>>>>>>>> includes the memory effect also.
>>>
>>>>>>>> Best regards,
>>>
>>>>>>>> Wim
>>>>>>>> PA3DJSwww.tetech.nl
>>>>>>>> without abc, PM will reach me
>>>
>>>>>>> The document that I have is no longer present on the Perkinelmer
>>>>>>> website. Here:www.tetech.nl/divers/BRO_PhotoconductiveCellsAndAnalogOptoiso.pdf
>>>>>>> you can find the document about LDR. I will leave it there for about
>>>>>>> 10 days.
>>>
>>>>>>> Best regards,
>>>
>>>>>>> Wim
>>>>>>> PA3DJSwww.tetech.nl-Hidequoted text -
>>>
>>>>>>> - Show quoted text -
>>>
>>>>>> Thanks for the document, page 35 talks about shot noise in LDR's. Are
>>>>>> they just throwing equations around or is there shot noise in LDR's?
>>>
>>>>>> George H.
>>>
>>>>> Since photons make current, probably so. But LDRs are so messy it's
>>>>> likely buried in other gunk.
>>>
>>>>> One of my guys is building a test setup to measure shot/excess noise
>>>>> in resistors. We need a ~~ 100M resistor to create a small (50 nA
>>>>> maybe) bias current with below shot-level noise. It's not clear if
>>>>> high-value cermet resistors have shot noise or not, so we plan to
>>>>> measure a bunch.
>>>
>>>>> Metal film resistors don't go to very high values; we can probably get
>>>>> 10M and likely 22M, and maybe even 50M, so we may have to do a series
>>>>> string. Even then I want to measure them to make sure they behave. I'm
>>>>> guessing that axials are better than surface mount, because of the
>>>>> bigger available surface for depositing metal.
>>>
>>>>> It's an interesting problem, trying to generate a nA-range DC current
>>>>> with low noise. Low voltage across a low-value metal-film resistor
>>>>> doesn't work because of Johnson noise. High value resistors are noisy
>>>>> in their own right.
>>>
>>>>> John- Hide quoted text -
>>>
>>>>> - Show quoted text -
>>>
>>>> "> Since photons make current, probably so. But LDRs are so messy it's
>>>>> likely buried in other gunk."
>>>
>>>> Hmm, Yeah this is a bit confusing. The photons make the charge
>>>> carriers and reduce the resistance... and as Phil H. says there is
>>>> generation and recombination noise. But this should only depend on
>>>> the light level and not on how the device is biased. For a given
>>>> light level if I double the bias voltage (and thus double the current)
>>>> does the noise go up?
>>>
>>> What's amazing to me is that there are resistors that *don't* have
>>> shot noise.
>>>
>>> Any time you have a current in which the electrons arrive randomly,
>>> there's shot noise. It's pure raindrops-on-the-roof statistics. That
>>> happens when the electrons are knocked loose by photons or when some
>>> thin barrier, like a P-N junction, doles out electrons across a
>>> surface. Metal wire is unique in having electron interactions that
>>> smooth out the flow. As far as I know, all semiconductor junction
>>> currents and leakages have shot noise. Tubes have shot noise.
>>>
>>> The shot noise current depends only on the average current; it goes up
>>> as the square root of I. Of course, some devices have more noise than
>>> pure shot noise.
>>>
>>>
>>>
>>>> I'm using 100M and 1 G resistors from Ohmite to make a 10nA current
>>>> source. I'll look at the noise next chance I get.
>>>
>>> I'd appreciate that. We'll share whatever we learn. This is not very
>>> easy to measure.
>>>
>>> John- Hide quoted text -
>>>
>>> - Show quoted text -
>>
>> "> I'd appreciate that. We'll share whatever we learn. This is not
>> very
>>> easy to measure."
>>
>> Hmm I hadn't thought it would be that hard... but then I'm probably
>> missing something. I was going to take the the 10nA current source
>> and fed it into a TIA opamp circuit with 100Meg as feed back resistor,
>> (giving me a volt of DC across the resistor) And then see how noisy it
>> is.
>
> Fine, as long as that 100M resistor has no excess noise of its own,
> and you account for its Johnson noise, and the TIA has very little
> input current noise. And all the supplies are very quiet. And
> everything is really well shielded.
>
> We're going to make a voltage divider from two identical RUTs, and AC
> couple that into an opamp with a gain of +1000. The AC coupling RC
> will add shunt Johnson noise but no excess noise of its own. We're
> figuring on a polystyrene cap and a 1G resistor maybe. Opamp =
> ADA4817. I figure we'd measure from 1 KHz to 100K maybe.
>
> John
>

If you use two FET op amps going into two scope channels, and use the
math functions to multiply the two inputs, FFT the results, and average
over N traces, you'll get just the noise of the resistors, because
everything else cancels out. How big N is depends on how far below the
amplifier noise you want to go.

Quiet power supplies are of course vital.

Sounds like a really interesting measurement.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
From: George Herold on 23 Mar 2010 16:27
On Mar 23, 3:45 pm, Phil Hobbs
<pcdhSpamMeSensel...(a)electrooptical.net> wrote:
> On 3/23/2010 1:59 PM, George Herold wrote:
>
>
>
>
>
> > On Mar 23, 11:43 am, Phil Hobbs
> > <pcdhSpamMeSensel...(a)electrooptical.net>  wrote:
> >> On 3/23/2010 9:59 AM, George Herold wrote:
>
> >>> On Mar 23, 8:32 am, Wimpie<wimabc...(a)tetech.nl>    wrote:
> >>>> On 23 mar, 12:57, Wimpie<wimabc...(a)tetech.nl>    wrote:
>
> >>>>> On 22 mar, 23:52, hal-use...(a)ip-64-139-1-69.sjc.megapath.net (Hal
>
> >>>>> Murray) wrote:
> >>>>>> In article<4BA7797C.7020...(a)electrooptical.net>,
> >>>>>>    Phil Hobbs<pcdhSpamMeSensel...(a)electrooptical.net>    writes:
>
> >>>>>>>    Night lights use CdS or CdSSe photoconductors, but they don't have to
> >>>>>>> have any kind of stability or repeatability, and they don't.  (CdSSe's
> >>>>>>> resistance can vary 5X due entirely to previous illumination history..)
>
> >>>>>> What is the mechanism for that history quirk?
>
> >>>>>> What is the time scale?  How long does it remember the history?
>
> >>>>>> --
> >>>>>> These are my opinions, not necessarily my employer's.  I hate spam..
>
> >>>>> Hello,
>
> >>>>> Go towww.perkinelmer.comandsearchfor"photocell", or follow the
> >>>>> links. When at the photocell page, on the right side you have an
> >>>>> application note link. They have a very nice document on the LDRs that
> >>>>> includes the memory effect also.
>
> >>>>> Best regards,
>
> >>>>> Wim
> >>>>> PA3DJSwww.tetech.nl
> >>>>> without abc, PM will reach me
>
> >>>> The document that I have is no longer present on the Perkinelmer
> >>>> website. Here:www.tetech.nl/divers/BRO_PhotoconductiveCellsAndAnalogOptoiso.pdf
> >>>> you can find the document about LDR. I will leave it there for about
> >>>> 10 days.
>
> >>>> Best regards,
>
> >>>> Wim
> >>>> PA3DJSwww.tetech.nl-Hidequoted text -
>
> >>>> - Show quoted text -
>
> >>> Thanks for the document, page 35 talks about shot noise in LDR's.  Are
> >>> they just throwing equations around or is there shot noise in LDR's?
>
> >>> George H.
>
> >> The conductivity comes from photogenerated carriers, so there's one copy
> >> of the shot noise there.  Another copy comes from the stochastic
> >> recombination, so a photoconductor with unity gain (i.e. where the
> >> transit time equals the carrier lifetime) has exactly twice the shot
> >> noise.
>
> >> If there's a lot of gain, i.e. you get to reuse the same carriers lots
> >> of times before they recombine, the shot noise gets amplified too--just
> >> like an APD or PMT.
>
> >> Cheers
>
> >> Phil Hobbs
>
> >> --
> >> Dr Philip C D Hobbs
> >> Principal
> >> ElectroOptical Innovations
> >> 55 Orchard Rd
> >> Briarcliff Manor NY 10510
> >> 845-480-2058
> >> hobbs at electrooptical dot nethttp://electrooptical.net-Hide quoted text -
>
> >> - Show quoted text -
>
> > Thanks Phil,  I must admit I don't quite 'see' how that works out.  If
> > I 'reuse' a photo generated charge carrier say ten times I get ten
> > times the shot noise?  (OK twenty times since I get twice the shot
> > noise to begin with.)  I'll have to think about that.   The bias
> > current divided by the photo current is something like the gain of the
> > LDR.
>
> > Say (speaking of noise) I think I remember you mumbling sometime ago
> > that you had a nice derivation of the Johnson noise formula.
> > Something that didn't involve the counting of modes in an infinite
> > transmission line as Nyquist first derived it.  Have you written this
> > up anywhere?
>
> > George H.
>
> It isn't original--it's just classical equipartition of energy plus the
> linearity of resistors and capacitors.
>
>    Consider a parallel RC circuit, isolated from everything else, and at
> a temperature T.  Because it is a single classical degree of freedom,
> the energy stored in a capacitor has an RMS value of kT/2, which leaks
> away through the resistor with a time constant of t=RC.
>
> In order for this to be statistically stationary (which thermal
> equilibrium always is), the rms power supplied by the resistor to the
> capacitor must be the same as the rms power dissipated in the resistor
> due to the voltage that's already on there.
>
> Thus (1/2)*CV_n**2 = kT/2, so V_n**2 = kT/C.
>
> Because the resistor is linear, we can consider the dissipation current
> (draining off the kT/C voltage) and the fluctuation current separately.
>   (Key step.)  The current in the resistor that is dissipating the
> capacitor's energy is
>
> I_diss**2 = V_n**2/R**2 = (kT/C)/R**2
>
> The bandwidth of this current is the noise bandwidth of the RC, which is
> 1/(4RC) (one-sided BW, i.e. analytic signal basis), and we need to
> divide by the BW to get the spectral density in A**2/Hz.
>
> Since this is in thermal equilibrium, I_n**2 == I_diss**2, so the 1-Hz
> noise is
>
> i_n = sqrt(kT/(R**2*C)*4RC) = sqrt(4kT/R), which is the classical
> Johnson noise formula.
>
> Cheers
>
> Phil Hobbs
>
> --
> Dr Philip C D Hobbs
> Principal
> ElectroOptical Innovations
> 55 Orchard Rd
> Briarcliff Manor NY 10510
> 845-480-2058
> hobbs at electrooptical dot nethttp://electrooptical.net- Hide quoted text -
>
> - Show quoted text -

Excellent, Thanks Phil!!! All I need to convince myself of, is that
the bandwidth of the RC is 1/4RC, but that should be easy.

George H.
From: Phil Hobbs on 23 Mar 2010 16:33
On 3/23/2010 4:20 PM, Robert Macy wrote:
> On Mar 23, 11:26 am, George Herold<ggher...(a)gmail.com> wrote:
>> On Mar 23, 1:42 pm, Robert Macy<m...(a)california.com> wrote:
>>
>>
>>
>>
>>
>>> On Mar 23, 10:39 am, George Herold<ggher...(a)gmail.com> wrote:
>>
>>>> On Mar 23, 11:33 am, John Larkin
>>
>>>> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
>>>>> On Tue, 23 Mar 2010 06:59:08 -0700 (PDT), George Herold
>>
>>>>> <ggher...(a)gmail.com> wrote:
>>>>>> On Mar 23, 8:32 am, Wimpie<wimabc...(a)tetech.nl> wrote:
>>>>>>> On 23 mar, 12:57, Wimpie<wimabc...(a)tetech.nl> wrote:
>>
>>>>>>>> On 22 mar, 23:52, hal-use...(a)ip-64-139-1-69.sjc.megapath.net (Hal
>>
>>>>>>>> Murray) wrote:
>>>>>>>>> In article<4BA7797C.7020...(a)electrooptical.net>,
>>>>>>>>> Phil Hobbs<pcdhSpamMeSensel...(a)electrooptical.net> writes:
>>
>>>>>>>>>> Night lights use CdS or CdSSe photoconductors, but they don't have to
>>>>>>>>>> have any kind of stability or repeatability, and they don't.. (CdSSe's
>>>>>>>>>> resistance can vary 5X due entirely to previous illumination history.)
>>
>>>>>>>>> What is the mechanism for that history quirk?
>>
>>>>>>>>> What is the time scale? How long does it remember the history?
>>
>>>>>>>>> --
>>>>>>>>> These are my opinions, not necessarily my employer's. I hate spam.
>>
>>>>>>>> Hello,
>>
>>>>>>>> Go towww.perkinelmer.comandsearchfor"photocell", or follow the
>>>>>>>> links. When at the photocell page, on the right side you have an
>>>>>>>> application note link. They have a very nice document on the LDRs that
>>>>>>>> includes the memory effect also.
>>
>>>>>>>> Best regards,
>>
>>>>>>>> Wim
>>>>>>>> PA3DJSwww.tetech.nl
>>>>>>>> without abc, PM will reach me
>>
>>>>>>> The document that I have is no longer present on the Perkinelmer
>>>>>>> website. Here:www.tetech.nl/divers/BRO_PhotoconductiveCellsAndAnalogOptoiso.pdf
>>>>>>> you can find the document about LDR. I will leave it there for about
>>>>>>> 10 days.
>>
>>>>>>> Best regards,
>>
>>>>>>> Wim
>>>>>>> PA3DJSwww.tetech.nl-Hidequotedtext-
>>
>>>>>>> - Show quoted text -
>>
>>>>>> Thanks for the document, page 35 talks about shot noise in LDR's. Are
>>>>>> they just throwing equations around or is there shot noise in LDR's?
>>
>>>>>> George H.
>>
>>>>> Since photons make current, probably so. But LDRs are so messy it's
>>>>> likely buried in other gunk.
>>
>>>>> One of my guys is building a test setup to measure shot/excess noise
>>>>> in resistors. We need a ~~ 100M resistor to create a small (50 nA
>>>>> maybe) bias current with below shot-level noise. It's not clear if
>>>>> high-value cermet resistors have shot noise or not, so we plan to
>>>>> measure a bunch.
>>
>>>>> Metal film resistors don't go to very high values; we can probably get
>>>>> 10M and likely 22M, and maybe even 50M, so we may have to do a series
>>>>> string. Even then I want to measure them to make sure they behave. I'm
>>>>> guessing that axials are better than surface mount, because of the
>>>>> bigger available surface for depositing metal.
>>
>>>>> It's an interesting problem, trying to generate a nA-range DC current
>>>>> with low noise. Low voltage across a low-value metal-film resistor
>>>>> doesn't work because of Johnson noise. High value resistors are noisy
>>>>> in their own right.
>>
>>>>> John- Hide quoted text -
>>
>>>>> - Show quoted text -
>>
>>>> "> Since photons make current, probably so. But LDRs are so messy it's
>>
>>>>> likely buried in other gunk."
>>
>>>> Hmm, Yeah this is a bit confusing. The photons make the charge
>>>> carriers and reduce the resistance... and as Phil H. says there is
>>>> generation and recombination noise. But this should only depend on
>>>> the light level and not on how the device is biased. For a given
>>>> light level if I double the bias voltage (and thus double the current)
>>>> does the noise go up?
>>
>>>> I'm using 100M and 1 G resistors from Ohmite to make a 10nA current
>>>> source. I'll look at the noise next chance I get.
>>
>>>> George H.
>>
>>> sorry for the VERY naive question, but would some form of junction
>>> leakage have lower noise? Or are junction leakages at and above
>>> johnson?- Hide quoted text -
>>
>>> - Show quoted text -
>>
>> Not naive at all! Phil is the expert here, but I'll answer anyway...
>> (put my foot in mouth once again.) So, as I understand it, the
>> thermal (Johnson) noise from a biased junction is 1/2 of the thermal
>> noise from a resistor that has the same resistance as the junction. (R
>> = kT/(eV * I). I beleive that Phil has actully used this 'trick' to
>> make lower noise photodiode front ends... but it's a trick that is
>> beyond my ability.
>>
>> George H.
>
> Thank you for your reply.
>
> Next naive question is Can the leakage be provided by a DC/DC
> converter circuit where the supply's noise is caused by caps and
> inductors, oh wait, no noise there. But I'm talking a physically
> realizable circuit.

There was a big foofaraw 15 years or so back where some guy claimed to
have figured out how to make an active circuit that functioned as a
noiseless resistor. It was all over IEEE Spectrum and places like that.

There are various means to take quiet active things, e.g. the beta of a
BJT or the g_m of a good JFET, and apply feedback so as to make the
equivalent of a very quiet resistor. I recently designed a TIA that's
shot noise limited down to the low tens of nanoamps in a 1-MHz
bandwidth, using techniques like that. It's about 20 dB better than I
thought I could do, which was a very pleasant surprise. You just have
to get rid of the 300 kelvin resistors.

(John L. and I collaborated on it, along with one of his guys, Jonathan
Dufour--you'll be able to buy them soon, if all goes well. Buy
lots--I'll have two kids in college this fall, and John's ski place
needs a new laboratory.) ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
From: Phil Hobbs on 23 Mar 2010 16:50
On 3/23/2010 4:27 PM, George Herold wrote:
> On Mar 23, 3:45 pm, Phil Hobbs
> <pcdhSpamMeSensel...(a)electrooptical.net> wrote:
>> On 3/23/2010 1:59 PM, George Herold wrote:
>>
>>
>>
>>
>>
>>> On Mar 23, 11:43 am, Phil Hobbs
>>> <pcdhSpamMeSensel...(a)electrooptical.net> wrote:
>>>> On 3/23/2010 9:59 AM, George Herold wrote:
>>
>>>>> On Mar 23, 8:32 am, Wimpie<wimabc...(a)tetech.nl> wrote:
>>>>>> On 23 mar, 12:57, Wimpie<wimabc...(a)tetech.nl> wrote:
>>
>>>>>>> On 22 mar, 23:52, hal-use...(a)ip-64-139-1-69.sjc.megapath.net (Hal
>>
>>>>>>> Murray) wrote:
>>>>>>>> In article<4BA7797C.7020...(a)electrooptical.net>,
>>>>>>>> Phil Hobbs<pcdhSpamMeSensel...(a)electrooptical.net> writes:
>>
>>>>>>>>> Night lights use CdS or CdSSe photoconductors, but they don't have to
>>>>>>>>> have any kind of stability or repeatability, and they don't. (CdSSe's
>>>>>>>>> resistance can vary 5X due entirely to previous illumination history..)
>>
>>>>>>>> What is the mechanism for that history quirk?
>>
>>>>>>>> What is the time scale? How long does it remember the history?
>>
>>>>>>>> --
>>>>>>>> These are my opinions, not necessarily my employer's. I hate spam..
>>
>>>>>>> Hello,
>>
>>>>>>> Go towww.perkinelmer.comandsearchfor"photocell", or follow the
>>>>>>> links. When at the photocell page, on the right side you have an
>>>>>>> application note link. They have a very nice document on the LDRs that
>>>>>>> includes the memory effect also.
>>
>>>>>>> Best regards,
>>
>>>>>>> Wim
>>>>>>> PA3DJSwww.tetech.nl
>>>>>>> without abc, PM will reach me
>>
>>>>>> The document that I have is no longer present on the Perkinelmer
>>>>>> website. Here:www.tetech.nl/divers/BRO_PhotoconductiveCellsAndAnalogOptoiso.pdf
>>>>>> you can find the document about LDR. I will leave it there for about
>>>>>> 10 days.
>>
>>>>>> Best regards,
>>
>>>>>> Wim
>>>>>> PA3DJSwww.tetech.nl-Hidequoted text -
>>
>>>>>> - Show quoted text -
>>
>>>>> Thanks for the document, page 35 talks about shot noise in LDR's. Are
>>>>> they just throwing equations around or is there shot noise in LDR's?
>>
>>>>> George H.
>>
>>>> The conductivity comes from photogenerated carriers, so there's one copy
>>>> of the shot noise there. Another copy comes from the stochastic
>>>> recombination, so a photoconductor with unity gain (i.e. where the
>>>> transit time equals the carrier lifetime) has exactly twice the shot
>>>> noise.
>>
>>>> If there's a lot of gain, i.e. you get to reuse the same carriers lots
>>>> of times before they recombine, the shot noise gets amplified too--just
>>>> like an APD or PMT.
>>
>>>> Cheers
>>
>>>> Phil Hobbs
>>
>>>> --
>>>> Dr Philip C D Hobbs
>>>> Principal
>>>> ElectroOptical Innovations
>>>> 55 Orchard Rd
>>>> Briarcliff Manor NY 10510
>>>> 845-480-2058
>>>> hobbs at electrooptical dot nethttp://electrooptical.net-Hide quoted text -
>>
>>>> - Show quoted text -
>>
>>> Thanks Phil, I must admit I don't quite 'see' how that works out. If
>>> I 'reuse' a photo generated charge carrier say ten times I get ten
>>> times the shot noise? (OK twenty times since I get twice the shot
>>> noise to begin with.) I'll have to think about that. The bias
>>> current divided by the photo current is something like the gain of the
>>> LDR.
>>
>>> Say (speaking of noise) I think I remember you mumbling sometime ago
>>> that you had a nice derivation of the Johnson noise formula.
>>> Something that didn't involve the counting of modes in an infinite
>>> transmission line as Nyquist first derived it. Have you written this
>>> up anywhere?
>>
>>> George H.
>>
>> It isn't original--it's just classical equipartition of energy plus the
>> linearity of resistors and capacitors.
>>
>> Consider a parallel RC circuit, isolated from everything else, and at
>> a temperature T. Because it is a single classical degree of freedom,
>> the energy stored in a capacitor has an RMS value of kT/2, which leaks
>> away through the resistor with a time constant of t=RC.
>>
>> In order for this to be statistically stationary (which thermal
>> equilibrium always is), the rms power supplied by the resistor to the
>> capacitor must be the same as the rms power dissipated in the resistor
>> due to the voltage that's already on there.
>>
>> Thus (1/2)*CV_n**2 = kT/2, so V_n**2 = kT/C.
>>
>> Because the resistor is linear, we can consider the dissipation current
>> (draining off the kT/C voltage) and the fluctuation current separately.
>> (Key step.) The current in the resistor that is dissipating the
>> capacitor's energy is
>>
>> I_diss**2 = V_n**2/R**2 = (kT/C)/R**2
>>
>> The bandwidth of this current is the noise bandwidth of the RC, which is
>> 1/(4RC) (one-sided BW, i.e. analytic signal basis), and we need to
>> divide by the BW to get the spectral density in A**2/Hz.
>>
>> Since this is in thermal equilibrium, I_n**2 == I_diss**2, so the 1-Hz
>> noise is
>>
>> i_n = sqrt(kT/(R**2*C)*4RC) = sqrt(4kT/R), which is the classical
>> Johnson noise formula.
>>
>> Cheers
>>
>> Phil Hobbs
>>
>> --
>> Dr Philip C D Hobbs
>> Principal
>> ElectroOptical Innovations
>> 55 Orchard Rd
>> Briarcliff Manor NY 10510
>> 845-480-2058
>> hobbs at electrooptical dot nethttp://electrooptical.net- Hide quoted text -
>>
>> - Show quoted text -
>
> Excellent, Thanks Phil!!! All I need to convince myself of, is that
> the bandwidth of the RC is 1/4RC, but that should be easy.
>
> George H.


It's pretty simple: the squared modulus of the transfer function is


|H(f)|^2 = 1/(1+(2*pi*f*R*C)**2), and the noise power bandwidth is

infinity
BW = integral 2*|H(f))|**2 df
0

(The factor of 2 is for the analytic signal--otherwise you have to use
the two-sided integral.)

Substituting tan u = 2 pi f R C, and using the identity 1+tan**2 u =
sec**2 u, the integral becomes

pi/2
BW = 1/(pi*RC) integral cos**2(u)
0

Now cos**2(u) = 1/2 + cos(2u)/2, and the integral of cos(2u) from 0 to
pi/2 is 0, so

BW = 1/(pi*RC) (pi/4) = 1/(4RC), i.e. pi/2 times the 3 dB bandwidth.


Cheers


Phil Hobbs


--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
From: George Herold on 23 Mar 2010 17:04
On Mar 23, 4:16 pm, John Larkin
<jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> On Tue, 23 Mar 2010 12:59:12 -0700 (PDT), George Herold
>
>
>
>
>
> <ggher...(a)gmail.com> wrote:
> >On Mar 23, 2:58 pm, John Larkin
> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> >> On Tue, 23 Mar 2010 10:39:15 -0700 (PDT), George Herold
>
> >> <ggher...(a)gmail.com> wrote:
> >> >On Mar 23, 11:33 am, John Larkin
> >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> >> >> On Tue, 23 Mar 2010 06:59:08 -0700 (PDT), George Herold
>
> >> >> <ggher...(a)gmail.com> wrote:
> >> >> >On Mar 23, 8:32 am, Wimpie <wimabc...(a)tetech.nl> wrote:
> >> >> >> On 23 mar, 12:57, Wimpie <wimabc...(a)tetech.nl> wrote:
>
> >> >> >> > On 22 mar, 23:52, hal-use...(a)ip-64-139-1-69.sjc.megapath.net (Hal
>
> >> >> >> > Murray) wrote:
> >> >> >> > > In article <4BA7797C.7020...(a)electrooptical.net>,
> >> >> >> > >  Phil Hobbs <pcdhSpamMeSensel...(a)electrooptical.net> writes:
>
> >> >> >> > > >  Night lights use CdS or CdSSe photoconductors, but they don't have to
> >> >> >> > > >have any kind of stability or repeatability, and they don't..  (CdSSe's
> >> >> >> > > >resistance can vary 5X due entirely to previous illumination history.)
>
> >> >> >> > > What is the mechanism for that history quirk?
>
> >> >> >> > > What is the time scale?  How long does it remember the history?
>
> >> >> >> > > --
> >> >> >> > > These are my opinions, not necessarily my employer's.  I hate spam.
>
> >> >> >> > Hello,
>
> >> >> >> > Go towww.perkinelmer.comandsearchfor"photocell", or follow the
> >> >> >> > links. When at the photocell page, on the right side you have an
> >> >> >> > application note link. They have a very nice document on the LDRs that
> >> >> >> > includes the memory effect also.
>
> >> >> >> > Best regards,
>
> >> >> >> > Wim
> >> >> >> > PA3DJSwww.tetech.nl
> >> >> >> > without abc, PM will reach me
>
> >> >> >> The document that I have is no longer present on the Perkinelmer
> >> >> >> website. Here:www.tetech.nl/divers/BRO_PhotoconductiveCellsAndAnalogOptoiso.pdf
> >> >> >> you can find the document about LDR. I will leave it there for about
> >> >> >> 10 days.
>
> >> >> >> Best regards,
>
> >> >> >> Wim
> >> >> >> PA3DJSwww.tetech.nl-Hidequotedtext -
>
> >> >> >> - Show quoted text -
>
> >> >> >Thanks for the document, page 35 talks about shot noise in LDR's.  Are
> >> >> >they just throwing equations around or is there shot noise in LDR's?
>
> >> >> >George H.
>
> >> >> Since photons make current, probably so. But LDRs are so messy it's
> >> >> likely buried in other gunk.
>
> >> >> One of my guys is building a test setup to measure shot/excess noise
> >> >> in resistors. We need a ~~ 100M resistor to create a small (50 nA
> >> >> maybe) bias current with below shot-level noise. It's not clear if
> >> >> high-value cermet resistors have shot noise or not, so we plan to
> >> >> measure a bunch.
>
> >> >> Metal film resistors don't go to very high values; we can probably get
> >> >> 10M and likely 22M, and maybe even 50M, so we may have to do a series
> >> >> string. Even then I want to measure them to make sure they behave. I'm
> >> >> guessing that axials are better than surface mount, because of the
> >> >> bigger available surface for depositing metal.
>
> >> >> It's an interesting problem, trying to generate a nA-range DC current
> >> >> with low noise. Low voltage across a low-value metal-film resistor
> >> >> doesn't work because of Johnson noise. High value resistors are noisy
> >> >> in their own right.
>
> >> >> John- Hide quoted text -
>
> >> >> - Show quoted text -
>
> >> >"> Since photons make current, probably so. But LDRs are so messy it's
> >> >> likely buried in other gunk."
>
> >> >Hmm, Yeah this is a bit confusing.  The photons make the charge
> >> >carriers and reduce the resistance... and as Phil H. says there is
> >> >generation and recombination noise.  But this should only depend on
> >> >the light level and not on how the device is biased.  For a given
> >> >light level if I double the bias voltage (and thus double the current)
> >> >does the noise go up?
>
> >> What's amazing to me is that there are resistors that *don't* have
> >> shot noise.
>
> >> Any time you have a current in which the electrons arrive randomly,
> >> there's shot noise. It's pure raindrops-on-the-roof statistics. That
> >> happens when the electrons are knocked loose by photons or when some
> >> thin barrier, like a P-N junction, doles out electrons across a
> >> surface. Metal wire is unique in having electron interactions that
> >> smooth out the flow. As far as I know, all semiconductor junction
> >> currents and leakages have shot noise. Tubes have shot noise.
>
> >> The shot noise current depends only on the average current; it goes up
> >> as the square root of I. Of course, some devices have more noise than
> >> pure shot noise.
>
> >> >I'm using 100M and 1 G resistors from Ohmite to make a 10nA current
> >> >source.  I'll look at the noise next chance I get.
>
> >> I'd appreciate that. We'll share whatever we learn. This is not very
> >> easy to measure.
>
> >> John- Hide quoted text -
>
> >> - Show quoted text -
>
> >"> I'd appreciate that. We'll share whatever we learn. This is not
> >very
> >> easy to measure."
>
> >Hmm I hadn't thought it would be that hard... but then I'm probably
> >missing something.  I was going to take the the 10nA current source
> >and fed it into a TIA opamp circuit with 100Meg as feed back resistor,
> >(giving me a volt of DC across the resistor) And then see how noisy it
> >is.
>
> Fine, as long as that 100M resistor has no excess noise of its own,
> and you account for its Johnson noise, and the TIA has very little
> input current noise. And all the supplies are very quiet. And
> everything is really well shielded.
>
> We're going to make a voltage divider from two identical RUTs, and AC
> couple that into an opamp with a gain of +1000. The AC coupling RC
> will add shunt Johnson noise but no excess noise of its own. We're
> figuring on a polystyrene cap and a 1G resistor maybe. Opamp =
> ADA4817. I figure we'd measure from 1 KHz to 100K maybe.
>
> John- Hide quoted text -
>
> - Show quoted text -

Hi John, The current source is just a 10 Volt reference (REF102)
feeding the big resistor with an opamp on the bottom driving the
'ground' pin of the reference. The references has a bunch of noise so
I'm not sure how quite the current source will be. We'll find out
though.

The ADA4817 is that the screaming 1GHz FET? I was going to use the
much slower opa134.

George H.
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