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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: A collection of monographs on high accuracy electronics
Date: Mon, 17 Jun 2024 14:36:07 -0000 (UTC)
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john larkin <jl@650pot.com> wrote:
> On Sun, 16 Jun 2024 22:20:43 +0100, JM <sunaecoNoSpam@gmail.com>
> wrote:
> 
>> On Tue, 11 Jun 2024 19:11:51 -0700, john larkin <jl@650pot.com> wrote:
>> 
>>> On Wed, 12 Jun 2024 02:50:19 +0100, JM <sunaecoNoSpam@gmail.com>
>>> wrote:
>>> 
>>>> On Mon, 10 Jun 2024 15:14:40 -0400, Phil Hobbs
>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>>> 
>>>>> On 2024-06-09 21:43, Phil Hobbs wrote:
>>>>>> On 2024-06-09 20:55, JM wrote:
>>>>>>> On Mon, 10 Jun 2024 00:29:17 -0000 (UTC), Phil Hobbs 
>>>>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>>>>>> 
>>>>>>>> JM <sunaecoNoSpam@gmail.com> wrote:
>>>>>>>>> On Sun, 9 Jun 2024 18:09:24 -0000 (UTC), Phil Hobbs
>>>>>>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>>>>>>>> 
>>>>>>>>>> Jeroen Belleman <jeroen@nospam.please> wrote:
>>>>>>>>>>> On 6/9/24 19:02, ehsjr wrote:
>>>>>>>>>>>> On 6/7/2024 9:14 PM, JM wrote:
>>>>>>>>>>>>> A collection of monographs on high accuracy electronics written 
>>>>>>>>>>>>> by Mr.
>>>>>>>>>>>>> Chris Daykin, following his career predominantly in metrology.
>>>>>>>>>>>>> 
>>>>>>>>>>>>> Unfortunately Chris will be unable to complete the unfinished
>>>>>>>>>>>>> monographs (having started end of life care) but there is plenty of
>>>>>>>>>>>>> interest to any analogue engineer.
>>>>>>>>>>>>> 
>>>>>>>>>>>>> https://1drv.ms/b/c/1af24d72a509cd48/EZhO_rP5-glDmxtc4ZHycvYBhrsqmyC5tuZjt2NFFsS0gQ?e=Wq2Yj0
>>>>>>>>>>>>> 
>>>>>>>>>>>>> 
>>>>>>>>>>>> 
>>>>>>>>>>>> 
>>>>>>>>>>>> Thanks!
>>>>>>>>>>>> Ed
>>>>>>>>>>> 
>>>>>>>>>>> I have an issue with his definition of resistor noise power
>>>>>>>>>>> as the product of open-circuit noise voltage and short-circuit
>>>>>>>>>>> current. That makes no sense.
>>>>>>>>>>> 
>>>>>>>>>>> There's more than that, probably, but that just jumped out at
>>>>>>>>>>> me.
>>>>>>>>>>> 
>>>>>>>>>>> Jeroen Belleman
>>>>>>>>>>> 
>>>>>>>>>> 
>>>>>>>>>> It?s four times too high, for a start.
>>>>>>>>>> 
>>>>>>>>>> Cheers
>>>>>>>>>> 
>>>>>>>>>> Phil Hobbs
>>>>>>>>> 
>>>>>>>>> "It is shown elsewhere [1] that the noise power is four times the heat
>>>>>>>>> energy which would flow down the conductors
>>>>>>>>> from a warm source resistor to a matching cold resistor."
>>>>>>>>> 
>>>>>>>> 
>>>>>>>> Which, if true, would solve all our energy problems, except that
>>>>>>>> thermodynamic systems would all be unstable.
>>>>>>>> 
>>>>>>>> The thermal noise power produced by a resistor into a matched load is kT
>>>>>>>> per hertz.
>>>>>> 
>>>>>>> 
>>>>>>> Sure, which is what he states.  By mentioning a hot and cold resistor 
>>>>>>> he makes it clear that net energy flow is from hot to cold, and that 
>>>>>>> the T refers to the hot source.
>>>>>>> 
>>>>>> But apparently he says that it's four times larger than that.
>>>>>> 
>>>>>> I'm not making a microsoft account just to download the PDF, so if you 
>>>>>> want to discuss it further, you could email it to me.
>>>>>> 
>>>>>> Cheers
>>>>>> 
>>>>>> Phil Hobbs
>>>>>> 
>>>>>> 
>>>>>> 
>>>>> Bill was kind enough to send me a copy (thanks again, Bill), and right 
>>>>> there on P. 374, the author says,
>>>>> 
>>>>> Pn = 4kTB
>>>>> 
>>>>> which is a factor of four too high.
>>>>> 
>>>> 
> 
> 
>>>> No it isn't. He is calculating the thermal noise power dissipated in
>>>> an unloaded resistor - something (or at least the related noise
>>>> voltage) which is actually required in the design process of a
>>>> transducer/amplifier low S/N system. 
>  ===============================================
> 
> 
>>> 
>>> What does that mean? Do unconnected resistors get hot?
>>> 
>>> A box of resistors could start a fire!
>> 
>> And why would that occur.  In thermal equilibrium there is no net
>> transfer of energy either from or to the resistor (when averaged over
>> any time interval of interest appropriate to the bandwidth of current
>> electronic circuits).
>> 
>> The so called resistor thermal "available noise power" KTB implies there
>> is a net power delivery from a source to a load.  In the case of maximum
>> transfer the source must dissipate within itself exactly the same as it
>> delivers to the load (due to having the same resistance).  However if
>> the so called load is at the same temperature as the source it also
>> delivers KTB to the source and dissipates KTB within its own resistance.
>>  Thus there is no net transfer of energy between the two resistors in
>> thermal equilibrium.  If one is at a lower temperature than the other
>> there will be a net transfer of energy, but this will be completely
>> dwarfed in any practical system by the energy transferred due to thermal
>> conductivity between the two resistors.  
>> 
>> So the power dissipated in a system of two equal value resistors is
>> 4KTB.  But this also holds if the two resistors have different values,
>> including the situation where one of the resistors is a short or open
>> circuit (i.e. leaving a single open or short circuit resistor).  So it
>> is entirely reasonable to state (as many engineers do) that the thermal
>> noise power of a resistor is 4kTB.
> 
> If as stated "the thermal noise power dissipated in an unloaded
> resistor" is nonzero, a reel of 0805s is a fire hazard. What would
> noise power mean if it can't be extracted or dissipated or even
> measured?
> 
> I'm now designing a signal generator that would be more sellable if it
> had lower voltage noise density. Given that I've got the noise as low
> as I can without cryogenics, around 8 nV/rtHz, the only thing left is
> to average multiple channels. Noise declines as the square root of
> applied dollars.
> 
> 

If there are enough people willing to read Chris’s doc carefully, maybe we
could put together an errata list so that John M. could issue a corrected
version. 

That would probably make it more useful, and so a better memorial. 

Cheers 

Phil Hobbs 

-- 
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics