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?

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?

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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