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From: Cursitor Doom <cd999666@notformail.com>
Newsgroups: sci.electronics.design
Subject: Re: Distorted Sine Wave
Date: Sun, 2 Jun 2024 12:09:48 -0000 (UTC)
Organization: A noiseless patient Spider
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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/