Peter Hall, VK6HP
Thanks for the continuing interest. I've consolidated a few responses and comments below. First, it might be useful to point out that my Class D PA is built around the G0MRF module, with details of my implementation described in a short document at http://users.tpg.com.au/adslif89/ph/vk6hp/MF_amp/VK6HP_630m_Amplifier.pdf
Rik, many thanks for the correction and update - very useful. If I extrapolate the rise times to the FST4-15 mode, we might expect a rise time of about 20 ms, which seems excessive for spectral containment. Perhaps we can verify that number but, in any case, it's clear that the envelope rise on the longer transmissions is very slow indeed. Most of my measurements were done with FST4W-120, for a direct comparison with WSPR. With that mode set, I see rise time numbers very similar to the ones mentioned by you and Chris.
Paul, the tail of the transmission is also extended to a similar extent. I suspect that, in fact, my T/R sequencing (which effectively removes drive to the PA) may be activating prior to the trailing envelope causing any issues. Regarding the hardware "noise-gate" circuit, I am using a JFET as a level activated switch. Actually, I have a second (similar) circuit just about to be tested now. It's based on a guitar noise gate (!) PCB available on eBay and, if you're interested, I can post details and results when I've fired it up and done some tests.
Chris, I suspect the fault condition in your friend's G0MRF PA is "over-current" rather than "reverse power". The two conditions get "OR-d" to produce a timed shutdown of the PA drive. In practice, and depending on the load, one can also trigger the other.
Reino, it'd certainly be interesting to have a look at the tone-spaced carrier. My experience of those modes is in a digital exciter where, for 475 kHz operation (for example), the carrier is at 950 kHz and the tones spaced x2, producing the correct 475 kHz WSPR drive for a push-pull PA after dividing by 2. However, the transceiver systems most commonly used (Kenwood, Flex, ....) produce a 475 kHz signal at low level, normally requiring an external doubler prior to the PA divider. Such transceivers don't generally allow the generation of a 950 kHz carrier. In fact, I am testing a PA input circuit based on an LT1016 fast comparator which produces very symmetrical square waves, over a range of levels, from a 475 kHz signal. This removes the need for the doubler in the push-pull PA but it's largely irrelevant in the present discussion.
On balance, I think Rik's take on the fix is correct: allow the WSJT-X user to select whether envelope shaping is used for the FST4 modes. It's probably true that the majority of LF/MF power amplifiers are non-linear types and the proper place to control the envelope of such systems is in the PA, not the exciter. For example, see the CW conditioning arrangements in the G0MRF amplifier shown in my earlier URL.