This is the output audio signal from a DC receiver, Soft66LC4. The signal flow is:
Audio SG, WaveGene -> (USB I/F) -> IC-7410 (USB Mod Level 25%, RF output power 2 watts) -> Dummy Load -> (pick up antenna) -> DC receiver -> (analog audio) -> FFT Analyzer, WaveSpectra.
The same configuration, but with USB Mod Level at 35%. Since the RF output power is only 2 watts, I suppose the distortion is due to the baseband stage of the transmitter.