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From: Cursitor Doom <cd999666@notformail.com>
Newsgroups: sci.electronics.design
Subject: Re: Distorted Sine Wave
Date: Sun, 2 Jun 2024 19:37:26 -0000 (UTC)
Organization: A noiseless patient Spider
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On Sun, 2 Jun 2024 20:05:43 +0200, Jeroen Belleman wrote:

> On 6/2/24 14:09, Cursitor Doom 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/
> 
> Average power is not the same as average voltage! Average power is
> proportional to the average of the voltage squared. It makes a
> difference!
> 
> Jeroen Belleman

Sorry, but I don't recall anyone claiming average power and average 
voltage were the same thing!