Sujet : Re: Distorted Sine Wave
De : cd999666 (at) *nospam* notformail.com (Cursitor Doom)
Groupes : sci.electronics.designDate : 02. Jun 2024, 15:53:55
Autres entêtes
Organisation : A noiseless patient Spider
Message-ID : <v3hthj$3ccih$2@dont-email.me>
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User-Agent : Pan/0.149 (Bellevue; 4c157ba)
On Sun, 2 Jun 2024 12:59:30 -0000 (UTC), Phil Hobbs wrote:
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
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?