From: Ryan O'Connor <rocifier@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Mon, 8 Feb 2021 19:15:46 -0800
Subject: Re: [EE] Low noise attenuator
Thread-Topic: [EE] Low noise attenuator
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Well, they have low resistance. But they have reactance?

On Tue, 9 Feb 2021 at 16:12, David VanHorn <microbrix@gmail.com> wrote:

> Resistors have thermal noise, capacitors have essentially no resistance
>
> On Mon, Feb 8, 2021, 6:14 PM Ryan O'Connor <rocifier@gmail.com> wrote:
>
> > David do you have any theory or practice which can explain this? I woul=
d
> be
> > interested to read more about why this helps. I'm trying to read up on
> > Sean's topics and I'm finding it pretty complex. I have more questions
> than
> > before, such as what is the relationship between impedence matching to
> > reduce noise and impedence matching to gain maximum power? Do they happ=
en
> > to co-inside, or how much out of phase are they (literally can't find
> > anything on the net about this)
> >
> > Cheers
> > Ryan
> >
> > On Tue, 9 Feb 2021 at 13:57, David VanHorn <microbrix@gmail.com> wrote:
> >
> > > Use capacitive dividers.
> > >
> > > On Mon, Feb 8, 2021, 4:20 PM Sean Breheny <shb7@cornell.edu> wrote:
> > >
> > > > A fundamental property of absorbative attenuators (i.e. those which
> > > > dissipate the power removed from the signal) is that they introduce
> > > > (1-a)kTB thermal noise in watts, where a is the attenuation factor,=
 k
> > is
> > > > Boltzman's constant, T is kelvin temperature of the attenuator, and=
 B
> > is
> > > > the downstream circuit bandwidth in Hz. So, for example, a 10dB
> > > attenuator
> > > > (a=3D0.1), introduces 90% of a full equivalent of thermal noise.
> > > >
> > > > It is possible to attenuate reflectively (i.e., produce an impedanc=
e
> > > > mismatch) and that can often circumvent this limitation although it
> > > depends
> > > > on the source characteristics. There is a common practice in low
> noise
> > > > amplifiers call "noise matching" where instead of matching the
> > impedance
> > > of
> > > > the source to that of the amplifier input, a purposeful mismatch is
> > > > introduced which reduces the contribution of noise due to the real
> part
> > > of
> > > > the amplifier's input impedance. I think the fundamental principle =
is
> > the
> > > > same.
> > > >
> > > > Sean
> > > >
> > > >
> > > >
> > > > On Mon, Feb 8, 2021 at 5:00 PM Ryan O'Connor <rocifier@gmail.com>
> > wrote:
> > > >
> > > > > Hi, I am curious about a general topic which mostly interests me =
in
> > the
> > > > > area of sensitive audio signals.
> > > > >
> > > > > What options do I have for attenuating a copper line signal while
> > > > > introducing a very low amount of noise? For example, if I grounde=
d
> > L+R
> > > > > channels using independent resistors would that work? OR do I nee=
d
> a
> > > > proper
> > > > > high-pass or low-pass filter design? But then what about
> > > > > capacitors/inductors or series resistors introducing thermal nois=
e?
> > > > >
> > > > > Open to discussion on types of noise as well :)
> > > > >
> > > > > Ryan
> > > > > --
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