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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: Researchers achive real time detection of low gas concentrations
Date: Tue, 14 Jan 2025 15:48:13 +1100
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On 14/01/2025 3:46 am, john larkin wrote:
> On Tue, 14 Jan 2025 01:07:41 +1100, Bill Sloman <bill.sloman@ieee.org>
> wrote:
> 
>> On 13/01/2025 5:15 pm, Jan Panteltje wrote:
>>> Researchers achieve real-time detection of low gas concentrations
>>>    https://www.sciencedaily.com/releases/2025/01/250109125523.htm
>>>     Allowing gas detection and identification in just seconds,
>>>     approach using a coherent control strategy offers promise for
>>>     real-time monitoring in environmental, health and industrial applications
>>> Source:
>>>    Optica
>>> Summary:
>>> Researchers have developed a method for quickly detecting and identifying
>>>    very low concentrations of gases, which, could form the basis for highly
>>>    sensitive real-time sensors for applications such as environmental monitoring,
>>>    breath analysis and chemical process control.
>>>
>>> Interesting method to stop a quartz tuning fork
>>
>> It's a fairly old-fashioned technique - infra-red spectroscopy has been
>> around for a whole,and if you want to detect low gas concentrations you
>> could always use long-path-length absorbtion cells.
>>
>> Ring-down detectors are an interesting variant on that approach.
>>
>> The tuning fork detector may be a novel innovation, but it's less
>> obvious that is a useful one.
>>
>> "Stopping" a high-Q resonator fast is easy enough, but then you have to
>> wait until the resonant movement builds up again - unless you have one
>> of John Larkin's fast-start resonant devices.
> 
> Starting a resonator-based oscillator is trivial,  just get the diff
> equation initial conditions right.

Which turns out to be a less-than-trivial exercise.
> The trick to an instant-start oscillator is really how to stop it,
> totally quench the resonator, so memories of ringing from the previous
> shot doesn't influence the next one.

They aren't "memories" - in electronic oscillators they are circulating 
currents in conductive loops, and charges stored on capacitors - and 
those capacitors can be part of a current loop.

In mechanical resonators, like tuning forks, they are the moving masses.

If you inject exactly the right amount of energy at the right time, you 
stop the movement, but getting it exactly right isn't trivial either.

> HP made one delay generator that used a quartz crystal as the
> triggered resonator. I met the guy who designed that and he was still
> suffering PTSD.
> 
> HP and Tek used triggered delay-line oscillators in many instruments.
> They can be stopped fairly well by terminating both ends.

Except that making a perfect termination isn't trivial either. Even 50R 
coaxial cable isn't exactly 50R. I had a rep try sell me a cable that 
was closer than usual to exactly 50R - it was made by winding the 
insulator onto the centre wire will under closely controlled tension to 
keep outer diameter more nearly precisely correct. We didn't need it.

-- 
Bill Sloman, Sydney