Re: noise question

On 7/15/24 18:09, john larkin wrote:

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.

Yes, that's what I'd expect 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.
 

BTDT. What's with the nonlinear bit? LTspice noise analysis is
basically an AC analysis, no?
Jeroen Belleman