Oh, another thing.
You might notice that the 3IMD column don't begin until the audio generator has
dropped to -6 dB.
That is because the two tones for IMD testing need to each be 6 dB lower than
full scale (i.e., -6dBFS), since summing them gets the same peak voltage of a
single tone.
Following common audio measurements practice, I have used dBc (a single IMD
component relative to a single tone) instead of dB PEP which the ARRL uses to
characterize IMD for amateur transmitters. The dB PEP vs dBc difference
precisely explains the missing first 6 dB -- i.e., when measured relative to
PEP, the devices appear 6.02 dB better than when measured with dBc.
The audio world tends to use dBc.
In the professional RF world, both dBc and dB PEP as acceptable (as long as you
mention which one). But once the ARRL chose dB PEP for transmitters, all the
ham manufacturers followed, since it makes their transmitters appear 6 dB
better than some competition who uses the dBc number :-). Pretty much
everybody today uses dB PEP in their transmitter specs.
Anyhow, be aware of the dBc vs dB PEP difference. They are only directly
comparable when you lop off 6 dB from one of them.
The other thing in the audio world is that they tend to spec their SNR in terms
of A Weighted spectrum. This is a little different from the 2125/2295 tone
pair :-).
Interestingly, though, my Presonus measurement came in within 0.5 dB or
Presonus' A-Weighted dynamic range spec (they claimed 118 dB of A-weighted
dynamic range, I measured 117.5 dB for unweighed dynamic range).
73
Chen, W7AY
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