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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: Instead scopes
Date: Sat, 31 Aug 2024 17:59:04 -0000 (UTC)
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john larkin <jlarkin_highland_tech> wrote:
> On Thu, 29 Aug 2024 21:50:19 +0100, liz@poppyrecords.invalid.invalid
> (Liz Tuddenham) wrote:
> 
>> john larkin <jl@650pot.com> wrote:
>> 
>>> On Thu, 29 Aug 2024 16:15:59 GMT, Jan Panteltje <alien@comet.invalid>
>>> wrote:
>>> [...]
>>>> Calculators yes.
>> 
>>> We weren't alowed to use a calculator on exams because it would give
>>> an unfair advantage to the students that could afford one.
>> 
>> We were allowed mechanical calculators (Facits etc.) but there were only
>> enough for half the candidates.  Half of us were locked in a room with a
>> spare envigilator acting as a guard while the other half sat the exam
>> then we swapped places and the ones who had taken the exam were locked
>> in while the second group sat the exam.
>> 
>> Electronic calculators did not exist, but we did learn to program an
>> analogue computer where 100v = 1 Machine Unit.
> 
> A lot of current engineering practise is left over from the days when
> computing was expensive or non-existant. Things like s-parameters and
> Smith charts.

I’ve done a lot of lowish-power RF stuff, and mostly agree with you about
the practicality of using S parameters in hand calculations. 

However, I cordially disagree with your sentiments regarding Smith charts. 

For one thing, they’re super useful for designing optical coatings, but
that’s a minority interest on SED. 

In RF work one runs into a lot of matching jobs involving modulated sine
waves. 

One typical example from my work is coupling sine modulation into a diode
laser, for modulation-generated carrier interferometry.  (*)
 
A Smith chart makes it super easy to try out different schemes, such as
series/shunt stubs, lumped elements, or any combination thereof. 

Useless for bandwidths of an octave or more, and so apt to be undervalued
by crass time-domain types. ;)

Cheers 

Phil Hobbs 

(*)MGC is dear to my heart—it’s about the only thing I’ve ever invented
that was published by someone else first. (There are an amazing number of
things to work on.) 

You put sinusoidal FM on the laser, and adjust the amplitude to the first
carrier null (M=2.405 radians) at the interferometer output. At that point
the first (Q phase) and second (I phase) signals are the same size, and
account for about 85% of the total signal power. 

That lets you measure the complex signal just by looking at the amplitudes
of the first two harmonics.  Great for fiber sensors and other situations
where there’s a lot of low frequency instability. 

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