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From: Jeroen Belleman <jeroen@nospam.please>
Newsgroups: sci.electronics.design
Subject: Re: noise question
Date: Mon, 15 Jul 2024 19:33:52 +0200
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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