Path: ...!Xl.tags.giganews.com!local-1.nntp.ord.giganews.com!nntp.supernews.com!news.supernews.com.POSTED!not-for-mail NNTP-Posting-Date: Mon, 15 Jul 2024 16:09:01 +0000 From: john larkin Newsgroups: sci.electronics.design Subject: Re: noise question Date: Mon, 15 Jul 2024 09:09:01 -0700 Message-ID: <4aha9j9es0d8bhvforetas77fiatfr8f5q@4ax.com> References: <82ca9jl6d27llb1m5gagmlteop59p3cc0a@4ax.com> User-Agent: ForteAgent/8.00.32.1272 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Lines: 49 X-Trace: sv3-MScggGN7yzMV4OnjDoJQ6ADJzdO4re8/55rHuFCpVvYTv4k24pqF9SpfFLOXlfZeXMvtmpzH7XRPKJ/!pqk0EOAR+2M8Mm7ovjV6T57dJwETUtyfCPuUFX/yhGvtvhQYvael6asxCLolRdGd0FEdFfi97Ooc!cyBvQw== X-Complaints-To: www.supernews.com/docs/abuse.html X-DMCA-Complaints-To: www.supernews.com/docs/dmca.html X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly X-Postfilter: 1.3.40 Bytes: 2869 On Mon, 15 Jul 2024 17:35:08 +0200, Jeroen Belleman 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.