Total posts : 45366
Yes, bench testing should reveal envelope distortion. With a 30-ohm dummy load, however, loaded Q would not be particularly high–perhaps 75, or so, taking into account something like ten or more ohms loading coil loss resistance. To detect the distortion, higher modulation frequencies would have to be used. I believe that the AMT-5000 is rated for about 16 kHz maximun modulation frequency. Using the five ohms load resistance that was mentioned can increase the loaded Q to the vicinity of 200, but the maximum modulation frequency would have to be reduced compared to 16 kHz. Neil: Can you get access to a distortion meter to use for testing?
Until 1580 posted his compaint, I, too thought that, although envelope distortion definitely exists in Class E AM transmitters, it is minimal. In fact, the class E modulation waveforms I have personally observed looked pretty good; but 1580 caused me to look more closely into the matter. The symtoms he described do have the earmarks of envelope distortion.
The loaded Q of the tuning circuit of a normal AM transmitter is rarely greater than about 15, but Part 15 AM, because of the short antenna, and the high-inductance loading coil that must be used to tune it, can have considerably higher loaded tuning circuit Q, which increases the possibility of envelope distortion.