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From: john larkin <jl@650pot.com>
Newsgroups: sci.electronics.design
Subject: Re: how the laser happened
Date: Thu, 27 Jun 2024 09:26:33 -0700
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On Thu, 27 Jun 2024 10:04:57 -0400, Joe Gwinn <joegwinn@comcast.net>
wrote:

>On Wed, 26 Jun 2024 20:07:12 -0700, john larkin <jl@650pot.com> wrote:
>
>>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.
>
>Yes, this is exactly how all the radar power engineers of my
>acquaintance solve the problem.  LT Spice is their standard tool.
>
>
>>There are cheap tricks to compensate the control loop, once the big
>>power stuff is designed.
>
>Yep.
>
>Joe Gwinn
>>
>>

It certainly helps to know some control theory and classic filter
theory, but that's just a guide to instinctive design and loop tuning.

Current limiting further complicates dynamics. More cases to simulate.

Some topologies get very different at light loads, when they go
discontinuous. I avoid them whenever I can.

Spice-Tweaking filters beyond 3rd order is hard. It's easy to diverge,
to get lost in space. One trick there is to take the AC feedback
early, close to the switcher, before a zillion phase lags pile up.
Starting at the switch node is cool; the transfer function between PWM
demand and voltage there is a dimensionless gain.

https://www.dropbox.com/scl/fi/tid6yeq5owaekv422idyc/P943_Sketch_4.jpg?rlkey=y3wotmyikzs9gum71xy0jqo8b&raw=1