Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Phil Hobbs Newsgroups: sci.electronics.design Subject: Re: Distorted Sine Wave Date: Sun, 2 Jun 2024 12:59:30 -0000 (UTC) Organization: A noiseless patient Spider Lines: 106 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 14:59:30 +0200 (CEST) Injection-Info: dont-email.me; posting-host="25d46efa73679e903c678283a3f37d14"; logging-data="3535801"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1+3ZV/kk8U8iAZzppEAmIat" User-Agent: NewsTap/5.5 (iPhone/iPod Touch) Cancel-Lock: sha1:sxDpeSuzLhloZL5ODsVyPF3QjZ4= sha1:mIrH8odrqG/wPu7LEaNKIPsP0n8= Bytes: 6047 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 -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics