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Failed to connect to MySQL: (1203) User howardkn already has more than 'max_user_connections' active connectionsPath: ...!eternal-september.org!feeder2.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: john larkin Newsgroups: sci.electronics.design Subject: Re: 80dB now but still needs improvement at 1KHz Date: Tue, 19 Nov 2024 12:16:05 -0800 Organization: A noiseless patient Spider Lines: 50 Message-ID: References: <1r37pyc.1bodve2fz5t4wN%liz@poppyrecords.invalid.invalid> <1r3a1m9.1lg1mngftnegwN%liz@poppyrecords.invalid.invalid> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Injection-Date: Tue, 19 Nov 2024 21:13:42 +0100 (CET) Injection-Info: dont-email.me; posting-host="02fde32b45fa01503e0808e8ceb9b45a"; logging-data="2120719"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX19UMEMi8LlEHXYv7YpwTRSH" User-Agent: ForteAgent/8.00.32.1272 Cancel-Lock: sha1:dgMqm1VJtOMDNRn4HFqhgutp3Oo= Bytes: 3540 On Tue, 19 Nov 2024 13:50:46 -0500, Joe Gwinn wrote: >On Tue, 19 Nov 2024 19:12:18 +0100, Jeroen Belleman > wrote: > >>On 11/19/24 17:47, Edward Rawde wrote: >>> "Liz Tuddenham" wrote in message news:1r3a1m9.1lg1mngftnegwN%liz@poppyrecords.invalid.invalid... >>[...] >>>> >>>> The startup transient of the FFT would have to be included, so that may >>>> set a limit to the accuracy of the simulation measurements. Even if you >>>> start on a zero-axis crossing, you will get spurious results because of >>>> the finite length of the sample. >>> >>> Yes I understand that. It's actually a sinewave multiplied by a rectangular function. >>> That's why I try to use a sample length of many tens of seconds. >>> A window function might help but then I'd have to decide which window to use. >>> >>>> >>>> This can be demonstrated by doing FFT on samples of fewer and fewer >>>> cycles from the middle of an apparently pure sinewave. As the sample >>>> length decreases, spurious harmonics begin to appear, even though they >>>> don't exist in reality. >> >>An FFT implicitly connects the end of the recording back to the >>beginning. It's sufficient that the number of periods is integer. > >If testing with sine waves that all fit with an integral number of >cycles per window, the FFT noise floor will be determined by the >precision of the arithmetic used. > >But there is often a splice error at that join because the fit into >the window is approximate, which is a good reason to use a window >function. The default choice in the radar world is Taylor. > >Joe Gwinn I wouldn't trust LT Spice for simulating a low-distortion oscillator. Run a sim and zoom up on the top of some sine wave. It looks all segmented and ratty. I was just simulating something and had a KHz range loop oscillation that refused to be compensated. Turns out I had a BI behavioral current source driving an inductor, and that oscillated all by itself. A G current source does the same thing. At really low distortion, the opamp models can getcha too.