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From: john larkin <jl@650pot.com>
Newsgroups: sci.electronics.design
Subject: Re: how the laser happened
Date: Wed, 26 Jun 2024 20:07:12 -0700
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On Thu, 27 Jun 2024 02:49:37 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>john larkin <jl@650pot.com> wrote:
>> On Tue, 25 Jun 2024 17:43:56 -0400, Joe Gwinn <joegwinn@comcast.net>
>> wrote:
>> 
>>> On Tue, 25 Jun 2024 08:19:03 -0700, john larkin <jl@650pot.com> wrote:
>>> 
>>>> On Tue, 25 Jun 2024 11:50:05 +0100, Martin Brown
>>>> <'''newspam'''@nonad.co.uk> wrote:
>>>> 
>>>>> On 21/06/2024 14:05, john larkin wrote:
>>>>>> There was a thread somewhere above about photon wave/particle duality.
>>>>>> 
>>>>>> 
>>>>>> This is worth reading:
>>>>>> 
>>>>>> https://www.amazon.com/How-Laser-Happened-Adventures-Scientist/dp/0195153766
>>>>>> 
>>>>>> Einstein, in one of his fits of genius, predicted in around 1916 that
>>>>>> under the right conditions, a photon could pass by an excited atom and
>>>>>> the atom would kick in another photon, or add to the wave amplitude,
>>>>>> depending on how you feel about these things. He called it stimulated
>>>>>> emission. He also declared that the laws of thermodynamics made this
>>>>>> effect impossible to use in practical situations.
>>>>>> 
>>>>>> In 1951, Charles Townes invented a work-around trick and built the
>>>>>> maser, a gaseous microwave oscillator. His superiors thought he was
>>>>>> crazy to dispute Einstein and almost threw him out of grad school, but
>>>>>> it worked.
>>>>> 
>>>>> More interesting still nature beat him to it.
>>>>> 
>>>>> The natural source W3(OH) dense molecular cloud which has hydroxyl 
>>>>> masers pumped by UV bright young stars embedded in it.
>>>>> 
>>>>> Very bright ultra narrow band point sources on a fuzzy nebulous object.
>>>>> 
>>>>> https://articles.adsabs.harvard.edu/pdf/1981MNRAS.194P..25S
>>>>> 
>>>>>> What's interesting is that any decent neon sign shop could have built
>>>>>> a HeNe laser in 1920.
>>>>> 
>>>>> They would have needed to make the mirror just cavity right though.
>>>> 
>>>> I know a guy who built a HeNe. It wasn't hard.
>>>> 
>>>>> 
>>>>> A nitrogen gas UV pulsed laser is possible just by getting the pressure 
>>>>> right and creating the  population inversion. Self starting - there was 
>>>>> a (dangerous) experiment in SciAm Amateur Scientist column to do it 
>>>>> sometime in the 1970's. June 1974 in fact - cover shows the BZ reaction.
>>>>> 
>>>>> https://www.scientificamerican.com/article/the-amateur-scientist-1974-06/
>>>>> 
>>>>> The failure to discover fullerenes in soot was a lot more surprising 
>>>>> since they were there all the time since the invention of fire just 
>>>>> waiting to be extracted by benzene. For a long time space dust had a 
>>>>> spectrum that could not be reproduced on Earth by any known compound.
>>>>> 
>>>>> Much like Helium was in the sun but more pervasive.
>>>> 
>>>> Too many powerful old farts declare things to be impossible.
>>> 
>>> .<https://en.wikipedia.org/wiki/Planck%27s_principle>
>>> 
>>> This is often paraphrased as "Science progresses one funeral at a
>>> time".
>>> 
>>> Joe Gwinn
>> 
>> I see the same thing in electronic design. People favor accepted
>> practice, validated in textbooks, and apply all their intelligence to
>> showing how new ideas won't work.
>> 
>> A recent case is deciding that the LC's at the output of a switching
>> power supply are "a filter" so must follow  classical filter theory,
>> pole-zeros and Butterworths and such. I tell them "It's just a power
>> supply."
>
>Classical filter theory is very useful for designing a power supply , as
>long as you don’t just wave some canned design over it like a dead chicken.
>
>
>Controlling rolloff and ringing over a wide range of conditions is easier
>with a bit of theory—you can estimate the overshoot via the Q of the
>network, for instance. 
>
>Canned designs such as Butterworth, Chebyshev, and so on assume constant,
>resistive source and load. While that’s a useful fiction in lots of
>signal-level applications, it’s not remotely true in a power supply. 
>
>Cheers 
>
>Phil Hobbs 

My switching power supply filters are usually dominated by the first
inductor. It has to let some tolerable ripple current into the
downstream caps, has to not saturate, and must not get too hot in the
minimum expected air stream, from core loss and copper loss. And fit
available space and not cost too much and be available for purchase.

I'll often have a secondary high-current ferrite bead to reduce EMI
spikes, typically maybe a per cent of the main inductance.

None of that is classic filter theory.

Only Spice can predict the power supply load response. It's too
nonlinear for classic filter theory.

There are cheap tricks to compensate the control loop, once the big
power stuff is designed.