Stefan,
I would use a spectrum analyzer like the TinySA Ultra with the appropriate attenuators.? You need a minimum RBW of 200Hz to see the distinctions in the tones.
An alternative would be to use a PC audio spectrum analyzer sourced from the R56/Q6 junction.? Here, you should see two clear tones around 12kHz.? Most PC-based audio analyzers should be able to go up to 24kHz.? You will need to build a probe to isolate the DC components.? A 1uF or greater capacitor should work.
73
Evan
AC9TU
