Path: ...!feeds.phibee-telecom.net!3.eu.feeder.erje.net!feeder.erje.net!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Cursitor Doom Newsgroups: sci.electronics.design Subject: Re: Distorted Sine Wave Date: Sun, 2 Jun 2024 13:53:55 -0000 (UTC) Organization: A noiseless patient Spider Lines: 109 Message-ID: References: <3lcf5jd7li0a3c0fgddt7o8lnfocvls2pr@4ax.com> <48bd78e1-7da8-3bba-2879-d22962203fa3@electrooptical.net> <9olh5j9al34fhrebr4grqq8h6c8javjpp1@4ax.com> <1n0i5jh257hiinlj2dhaatlo11s33m5n0e@4ax.com> <9k2i5jpfhu3ncfpm28ukusrok4hugal80s@4ax.com> <9tok5j9p388ookujrtbsofskjlbekfuhjb@4ax.com> <60rk5jti9l5154hqaqicohmj3u1lfd16g3@4ax.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Injection-Date: Sun, 02 Jun 2024 15:53:56 +0200 (CEST) Injection-Info: dont-email.me; posting-host="40a1d383888994c45f6bff6cd4b36ba9"; logging-data="3551825"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1+s+VJlASuZXa5ntPtX0+Ci65kZEyTPRtU=" User-Agent: Pan/0.149 (Bellevue; 4c157ba) Cancel-Lock: sha1:MRpJgK/AOsvyAfv6J/9/kNl4qeI= Bytes: 6274 On Sun, 2 Jun 2024 12:59:30 -0000 (UTC), Phil Hobbs wrote: > Cursitor Doom wrote: >> On Sun, 2 Jun 2024 13:49:16 +0200, Jeroen Belleman wrote: >> >>> On 6/2/24 00:24, piglet wrote: >>>> piglet wrote: >>>>> Cursitor Doom wrote: >>>>>> On Sat, 1 Jun 2024 15:44:17 +0200, Jeroen Belleman wrote: >>>>>> >>>>>>> On 6/1/24 14:07, Cursitor Doom wrote: >>>>>> >>>>>>>> I've taken a shot of the waveform into the 50 ohm input. It's >>>>>>>> around 850mV peak-peak. Hopefully the slight distortion I spoke >>>>>>>> about is visible; the slightly more leisurely negative-going >>>>>>>> excursions WRT their positive-going counterparts. So it's not a >>>>>>>> pure sine wave as one would expect. Does it matter? I don't know! >>>>>>>> >>>>>>>> https://disk.yandex.com/i/7cuuBimDbOIBZw >>>>>>> >>>>>>> The shape looks perfectly acceptable to me. This is +3dBm into 50 >>>>>>> Ohms. >>>>>>> Is that what it's supposed to be? Canned reference oscillators >>>>>>> most often deliver +13dBm, sometimes +10dBm. >>>>>> >>>>>> Is it? I only make it about half your figure: +1.65dBm. >>>>>> I admit I'm frequently prone to careless errors, so stand to be >>>>>> corrected, >>>>>> but here's my method: >>>>>> 850mV peak to peak is 425mV peak voltage. Average of that is >>>>>> 0.425x0.636 = >>>>>> 0.27V. Average power is average volts squared divided by the load >>>>>> impedance of 50 ohms = 1.46mW = +1.65dBm. >>>>>> >>>>>> I shall consult the manual to see what it ought to be - if I can >>>>>> find it, that is, as PDF manuals are a nightmare to navigate IME. >>>>>> >>>>>> >>>>>> >>>>> Use 0.71 for RMS instead of 0.636 ! I make that about 1.8mW or >>>>> +2.6dBm ? >>>>> >>>>> >>>>> >>>>> >>>> Or +2.9dBm if using the 0.88v pk-pk I think is shown in the scope pic >>>> rather than the 0.85v figure of your message. >>>> >>>> >>> To CD: >>> >>> The above is what I did. 30 + 10*log( (0.88/(2*sqrt(2)))^2 / 50) = >>> 2.869 dBm. Rounded to 3dBm. >> >> OK, thanks for that clarification. Anyway, I finally measured the power >> of that oscillator with my HP RF power meter and it comes out at 1.74mW >> (or about +2.5dBm off the top of my head). Seems a tad on the low side, >> but I can't find what it's supposed to be in the manual. >> >> >>> What's the issue with RMS vs. average? >> >> When you dig into it, you find that what people really mean when they >> talk about "RMS Watts" is actually *average* power. I found this on the >> web which attempts to explain it: >> >> https://agcsystems.tv/rms-power-fallacy/ >> >> > It’s really not this hard. > > “RMS” stands for “root mean square”, which is a shorthand description of > how you calculate the power delivered by an arbitrary voltage waveform > (or equivalently current) in a resistive circuit. > > You square the instantaneous voltage, compute the mean (I. e. time > average), and then take the square root. > > All those fudge factors like 0.5, 0.636, 0.707, and so forth, can be > useful for quick calculations, but they just summarize the results of > the above procedure _for_specific_situations_. Without first doing the > math, and understanding the situation, they’re worse than useless. > > The ‘rms power’ thing came as a response to lying advertisements for > stereo systems, starting in the 1970s iirc. Crappy stereos were > advertised as producing “250 watts PMP”, for “peak music power”, as > though that were a thing. That led to very optimistic numbers, even > before actual lies were added, which they usually were. > > People started pushing back by insisting on knowing what sine wave power > the amp could put out continuously without distorting or overheating. > > That’s a very conservative spec, since music waveforms have a high > peak/rms ratio and the ear is most sensitive to transient distortion on > the peaks. > It does have some basis in reality, though, and is easy to measure > unambiguously, which cuts through the Audio BS” (tm). > > While saying “rms watts“ is indeed redundant, strictly speaking, > nevertheless it’s a useful shorthand for describing audio amps, Chinese > switchers, and (I suppose) power FETs. > > Cheers > > Phil Hobbs Phil, I believe you also have an 8566B. Do you know what the 10Mhz reference oscillator output level should be? Is yours anything close to +2.5dBm?