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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: Distorted Sine Wave
Date: Sun, 2 Jun 2024 12:59:30 -0000 (UTC)
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Cursitor Doom <cd999666@notformail.com> wrote:
> On Sun, 2 Jun 2024 13:49:16 +0200, Jeroen Belleman wrote:
> 
>> On 6/2/24 00:24, piglet wrote:
>>> piglet <erichpwagner@hotmail.com> wrote:
>>>> Cursitor Doom <cd999666@notformail.com> 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