Post by Cursitor DoomPost by Jeroen BellemanPost by pigletPost by pigletPost by Cursitor DoomPost by Jeroen BellemanPost by Cursitor DoomI'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,
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.
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.
Post by Jeroen BellemanWhat'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
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
