Ham Radio Blog
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.
The above figure is from the book, “An Introduction to Satellite Communications”. Some part, including this figure, is available at Google Books.
http://www.teledyne-mec.com/appnotes/appnotes.pdf
Because you will get 2dB improvement of the 3rd IMD for every 1dB of input backoff, I thought a linear amplifier stage of the transmitter gives you no problem as far as you are operating in digital modes such as PSK31 since the required Eb/No and hence the RF output power is much less than in the SSB mode.
This is the 3rd order IMD of TS-990 at its full power of 200 watts. (See QST Product Review, for IC-7410.) If you are operating at 20 watts (-10dB to the full power), the 3rd IMD should be 20dB better than in this figure.
So the question is: how much power do you need for your satisfaction when you operate in digital modes? Of course, the answer depends on you.