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From: john larkin <JL@gct.com>
Newsgroups: sci.electronics.design
Subject: Re: Researchers achive real time detection of low gas concentrations
Date: Mon, 13 Jan 2025 08:46:40 -0800
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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.

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. 

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.