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From: George Herold on 23 Mar 2010 14:26
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.
From: John Larkin on 23 Mar 2010 14:58
On Tue, 23 Mar 2010 10:39:15 -0700 (PDT), George Herold
<ggherold(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-Hide quoted 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

From: Phil Hobbs on 23 Mar 2010 15:20
On 3/23/2010 2:58 PM, John Larkin wrote:
> On Tue, 23 Mar 2010 10:39:15 -0700 (PDT), George Herold
> <ggherold(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-Hide quoted 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
>

The electron arrival times are correlated by electron-electron
scattering, which tends to smooth the flow out. Rolf Landauer (a late
IBM Fellow whom I knew very slightly) showed that the shot noise in a
metal resistor is reduced by a factor of Ls/L, where L is the length of
the element and L_s is the mean free path for electron-electron
scattering. In a typical device, that's 10 nm/1 mm, or 10**-5.

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 15:45
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-Hide quoted 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 net
http://electrooptical.net
From: Paul Hovnanian P.E. on 23 Mar 2010 15:47
Phil Hobbs 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.comandsearch for "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- Hide quoted 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.


So, to paraphrase that: Shot noise is due to the carrier starting or
stopping. The longer the device (greater transit time compared to
recombination time) the more photons (or energy from an accelerating
field as in a PMT) it takes to kick it the length of the soccer field
(sorry about that analogy). But your current depends on the number of
cariers that make it through the goal at the end.

--
Paul Hovnanian mailto:Paul(a)Hovnanian.com
------------------------------------------------------------------
Smoking is one of the leading causes of statistics.
-- Fletcher Knebel
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