Total posts : 45366
You cited an IEEE paper that isn’t available on the web if one is not an IEEE member, so readers here are not likely to view it.
AM modulation envelope distortion is a known phenomenon and can be a factor in some Class E implementations, particularly where the ratio of the carrier frequency to the modulation bandwidth is low and the Q is very high.
Even with a very low resistance ground connected to the AMT5000 (lower than anyone is likely to achieve in practice), the envelope distortion is negligible. The ground resistance is one component determining Q. There is always a fixed resistance due mainly to coil loss. The toroid has a fairly high Q, roughly equivalent to the Q of a 3.5 in. OD coil wound on PVC pipe using #16 wire. The real advantage is the toroid is inside the TX obviating the need to invest the effort into winding and fabricating an external coil and mounting hardware.
Following is are the results of a Fourier analysis of the AMT5000 with an antenna ground loss resistance of 5 ohms, operating at 1550k Hz and modulated at just under 100% by a 1 kHz sine wave. A simple diode detector was used in the model. The detector was implemented with ideal diodes to minimize detector distortion. The analysis spans one full cycle of the modulation signal. As shown below the THD is 1.25%. With the slight exception of the 3rd harmonic, the harmonic content declines with increasing harmonic number.
I have not done this analysis on a conventional class C or D output, and I have not tried to analyze how much my detector is contributing to the distortion figure, but I think it is reasonable to assume that 1.25% THD distortion on the detected signal is quite acceptable!.
Harmonic Frequency Fourier Normalized
Number [Hz] Component Component
1 1.000e+03 4.840e-03 1.000e+00
2 2.000e+03 3.410e-05 7.046e-03
3 3.000e+03 4.797e-05 9.913e-03
4 4.000e+03 1.093e-05 2.259e-03
5 5.000e+03 6.098e-06 1.260e-03
6 6.000e+03 4.812e-06 9.943e-04
7 7.000e+03 2.888e-06 5.968e-04
8 8.000e+03 1.978e-06 4.088e-04
9 9.000e+03 1.231e-06 2.544e-04
10 1.000e+04 5.893e-07 1.218e-04
11 1.100e+04 5.221e-07 1.079e-04
12 1.200e+04 5.018e-07 1.037e-04
13 1.300e+04 3.996e-07 8.258e-05
14 1.400e+04 3.352e-07 6.926e-05
15 1.500e+04 1.482e-07 3.063e-05
Total Harmonic Distortion: 1.249922%
There is an "aircheck" mp3 on the SSTRAN web site that demonstrates the audio quality of the AMT5000, http://www.sstran.com/pages/AMT5000/AMT5000_aircheck.html. This was done with the transmitter operating indoors with an AC wiring ground which turns out to be have a ground resistance of about 40 ohms so it doesn't substatiate the 5 ohm ground case, but it does at least demonstrate low distortion under ground resistance conditions more likely to be achieved in a typical part 15 installation.