[time-nuts] "Q for dummies"

[Tim Shoppa]

Just two sentences is quite a challenge! Maybe one of the two sentences
should explain the downsides of a system design that uses filters or
networks with high Q.

"High-Q filters and matching networks will have circulating currents
resulting in poorer efficiency, and higher variation in group and phase
delay across the passband and with environmental conditions, when compared
to low-Q networks."

On Tue, Jan 26, 2021 at 11:52 AM Dana Whitlow <k8yumdoober at gmail.com> wrote:

> If anybody can even approach doing justice to the Q concept, including why
> it
> matters, in just two sentences, that person will have definitely earned the
> "Qulitzer prize" in technical journalism.
>
> Here's my entry:
>
> "A circuit's Q is closely related to its internal energy losses compared to
> external
> energy exchanges.  A high Q can mean better efficiency, better conformance
> with
> expected performance (especially in filter applications), longer
> ringdown times
> (wineglass compared to milk glass) and (unfortunately) higher price."
>
> Dana
>
>
> On Tue, Jan 26, 2021 at 8:14 AM Lux, Jim <jim at luxfamily.com> wrote:
>
> > On 1/26/21 5:29 AM, Dave Daniel wrote:
> > > That doesn’t really illustrate the fundamental concept of Q. I’ll try
> > and think of something that is still non-mathematical but nonetheless
> > illustrates the concept of Q.
> > >
> > > DaveD
> >
> > Ultimately, Q is about internal (or external) losses (leaving aside
> > antennas) - how much energy is stored, vs (how much is radiated) or lost
> > to heat
> >
> > A big heavy pendulum will swing for a lot longer than a light one,
> > because more energy is stored in the system, relative to the amount lost
> > to air and mechanical friction.
> >
> > And that, like the wine glass (which is a nice mechanical analogy - the
> > fine crystal has less internal damping) is a "impulse response" system,
> > not a "driven system"
> >
> >
> >
> > But how that relates to bandwidth is a bit tricky, because then you need
> > to get into a discussion "driven systems" and then into resonance - and
> > that is where the train will come off the rails, particularly with
> > respect to antennas.
> >
> >
> >
> >
> > >> On Jan 26, 2021, at 07:28, Ole Petter Ronningen <
> opronningen at gmail.com>
> > wrote:
> > >>
> > >> Hi, All
> > >>
> > >> I am going to give a presentation to non-nuts, and in one of the
> slides
> > I
> > >> touch on Q - not wanting to spend more than a sentence or two on the
> > >> subject, I wonder if the following analogy works:
> > >>
> > >> "A quality long-stemmed, thin-walled wine glass will ring for a long
> > time
> > >> after we give it a little tap - this is high Q. A thick-walled, stubby
> > milk
> > >> glass will barely ring at all, just a dull "plink" - this is low Q.
> The
> > >> energy we put in dies out very quickly."
> > >>
> > >> As I am sure is embarrassingly evident, I have a rather tentative grip
> > on
> > >> the subject myself..
> > >>
> > >> Ole
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