SMPTE IMD (mental problem)

The SMPTE IMD approach is described in the QA401 manual as well as in the QA40x and IMD Measurements dedicated report. I am having mental trouble comparing the two examples and then addressing the method to my test platform. The user manual has the 60Hz signal level set to -8dBV and the report has it set to zero. Both docs differentiate the two measurements: 101dB for the User Manual example and 94.7 for the report doc (graph) dBr but the text in the report doc says it is 100.7 dB. Where does the extra 6dB come from? Maybe the “below the carrier” text is confusing me? Nobody here at work gets it either so I have to ask…

Hi @bklein, I think the manual is more correct. Let me check on the blog post and it will be corrected if needed.

Hi @bklein,

OK, here’s how the math works out on this. In the blog post below (QA40x and IMD Measurements), the 60 Hz is at 0 dB, and the 7 kHz is at -12 dB (that is ratio prescribed by SMTPE). The amplitude of the 7060 Hz harmonic is -94.7 dB relative to the 7 kHz. Which means, relative to the 60 Hz it is -94.7-12 = -106.7. And there are two 2nd-order products at about the same level. So, when those are RMS added, that would take you to -103.7. The blog post wrongly assumed it would be a 6 dB adder. I’ve updated the blog post. Thanks!