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From: john larkin <jlarkin_highland_tech>
Newsgroups: sci.electronics.design
Subject: Re: noise question
Date: Mon, 15 Jul 2024 09:09:01 -0700
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On Mon, 15 Jul 2024 17:35:08 +0200, Jeroen Belleman
<jeroen@nospam.please> wrote:

>On 7/15/24 16:30, john larkin wrote:
>> Does a negative 50-ohm resistor make as much noise as a regular 50 ohm
>> resistor?
>> 
>> I'd sorta guess the current noise to be the same, and maybe the
>> open-circuit voltage noise is infinite.
>> 
>> I could Spice that, at least the current noise, if Spice handles it
>> right. LT Spice noise analysis is kind of weird.
>> 
>
>I just tried it: In LTspice the sign doesn't matter,
>only the absolute value. Also, if you put a positive
>resistor in series with negative one, the noise
>voltages add RMS-wise, like you'd expect of independent
>sources.

Cool. Thanks.

>
>In real life, a negative resistor may have more or
>less noise than an actual resistor, depending on the
>low-noise design skills of the designer.
>
>I think you knew that...
>
>Jeroen Belleman

Sure, I was considering an ideal neg resistor, without added noise
from active parts.

As a college project, I built a 2-terminal negative resistor and
plugged the negative value into a bunch of equations (voltage
dividers, RCs, LRCs, things like that) and demonstrated that they
worked that way in real life. That was fun.

What I was thinking lately was about making an LC oscillator with very
low phase noise, namely low jitter in my world. The finite Q of the
parallel LC is equivalent to a shunt resistor so I'd expect it to have
the Johnson noise of that equivalent resistance. Then the active stuff
must look like a negative resistor, which is noisy too.

LT Spice noise analysis is very limited. I have sometimes added some
random-noise BV blocks in series with resistors and such, so I can do
genuine nonlinear sims with noise. It's actually easier to breadboard.