Technical Itch

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I have a technical question which I thought I had researched and answered myself, but I find myself doubting my interpretation.

I am measuring THD using FFT, rather I am measuring Harmonic content.

I have two DSO I can use at work, which can do FFT.

One measures in dBm, the other dBV.

I have used Microsemi AN30 for guidance by the way, but I still doubt myself (when trying to work quickly)

If I havent misunderstood something, both of these measures are referenced to absolutes, 0 dBm to 1 Watt ( 0 dBmW to 1mW); and 0 dBV to 1V RMS.

Since they are absolutes, the ratio between them, when examining a FFT spectrum, should be the same?

Is this correct? This is what I am seeing when comparing the two devices and outputs, Distortion Factor etc, even if the Fundamental value, In dBm or dBV is not identical.

The screen outputs and ratio between harmonic levels seem to tie up closely, and seems to validate the approach - this is the intention, to check one device against another, and give me further confidence that I'm not making some silly mistake somewhere, and then posting all about here with results that mean nothing (I'd hope the members here would point that out, rather than let me look a fool, but...ya never know!

So, in order to use a Excel sheet to calculate THD % I have been using dBm for some time and have had some confidence in the method.

Now when switching to using the device which outputs dBV, I am a little stumped- is my method really valid at all?

Do I need to take the dBV and convert to dBm using my load resistance?

Am I missing something?

Any help appreciated here. After considering a Picoscope 2204 for some time, I'm not sure it will have any better noise floors or offer anything above what two DSO can, and perhaps I'll be disappointed with it
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Joined 2014
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If I havent misunderstood something, both of these measures are referenced to absolutes, 0 dBm to 1 Watt ( 0 dBmW to 1mW); and 0 dBV to 1V RMS.

I think you have some units mixed there. Do you mean 0dB ref 1W and 0dBm ref 1mW? That is normal convention.

I think where you need to check your working is impedance. dB/dBm is usually referenced to 50Ohms. From that you can off course calculate dBV. and if your dBV setup is 50 Ohms you should be able to go the other way remembering that dBV is 10log not 20log.

I've not done it in over a decade so I'm getting brain twisted on something I once could do in my head.


Joined 2003
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Almost certainly both instruments measure Voltage and not Power.

The dBV is obviously voltage.

dBm is power; but voltage can only be known if you know the Impedance.

AUDIO gear normally takes "dBm" as 600 Ohms. 0dBm @ 600r is 0.7746V

RF gear usually takes "dBm" as 50 Ohms. 0.224V

Yes, the ratios should be consistent. However large differences in bandwidth may skew the results (a GHz analyser will read a lot of hypersonic garbage which *may* not matter to audio, especially if you are measuring individual spikes instead of a total).

> remembering that dBV is 10log not 20log.

I'm not clear on that. One of us may be twisted.
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Joined 2017
Yes this is mindboggling (spellcheck alarms) stuff. I'm not sure either. #2 an #3 points accordingly. Get the exact specs of the measurements devices alongside and compare.
I would like to invite you at my table to sort this out (with a good bottle an well filled glass), but we're separated by salt seas and tectonic plates.
I should be asleep, but again I have woken.

It's the mix if convention that I believe has me in a muddle, as well as lack of mental rest (and sleep!) Ultimately, I thought it's not the unit that matters, but the ratio - or is it? Am I using the correct unit? Does it matter?

Annoying as 5 years ago, I would have rattled off the definitions, by heart.

Citing Analog devices here:

Utilities:VRMS/dBm/dBu/dBV calculators | Design Center | Analog Devices

dBm, dBu, dBV, VPEAK and VRMS (ANSI T1.523-2001 definitions). dBm is a power ratio relative to 1mW, dBu and dBV are voltage ratios, relative to 0.775V and 1V, respectively.

Oddly at 3am its clearer.

20log is for amplitude, Voltage.
10log is for power

I believe that all of you have helped here.

PRR thank you again, confirming that the ratios are indeed equivalent - it's what I am seeing anyways and what I believe(d) to be correct, so I'm not entirely sure why I am doubting that.

I was befuddled that I could not get my spreadsheet calculation to marry up and give the same results, when switching from dBm to dBV.

I'm willing to bet I've missed something in the formulas (garbage in, garbage out)- after all, it's not rocket science!


Thanks, that made me laugh. I've not enjoyed the hospitality of Nederlanders in some years now. I'm sure that, armed with a full glass, my appetite for talking dB would quickly vanish!

Thank goodness it just my tired brain, refusing to switch off, and causing doubts - and I am not going crazy!
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As for me I don't use those relative values (power dBm, dbW, voltage dBV, etc). Because it causes more questions than answers. I only use dB (and put 0 dB level in Volts, but sometimets it isn't impotant) - it is easy and clear (so I recommend it).
And I dont know why here many people use that different types of dBV ets (to make questions), and especially power-related! dBw.
Yeah, first I heard of tech itch was in 2002, the trance'n'bass mix by John B for mixmag.
Pretty good tunes all around. I still prefer the '94-'96 drum'n'bass sound, but it was fun hearing a more trance kind of spin to it. Funny that some of the tunes from '92 sound fresher and more innovative than much of the new stuff, despite the fact that computer based music production at the time was almost a strictly underground (IE happ'nin in some loners bedroom) phenomenon.
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