Hi Chris

Thank you for the information.  It's very useful to know of a similar problem, especially since you're using an entirely different exciter.

I condensed the story a bit for a concise description of the issue but I actually use a doubler after the transceiver output, followed by a divide-by-two flip-flop to give very symmetrical drive waveforms to the MCP1404 FET drivers and push-pull HEXFETs.  With the FST4 modes, the (Schmitt) FF clocking becomes intermittent as the RF envelope spends a long time rising through the switching point, activating the protection circuit until things have settled down. However, using the segmented memory triggering options on my scope to account for delays, I can quite clearly see the problem starts with the envelope shaping of the waveform emerging from the TS-890S.

I did wonder if the rise time issue may be a product of an over-zealous attempt to contain "key clicks" with fast FST4 but I come back to the demonstration by Rick, ON7YD, with his bare bones sound-card system:

https://groups.io/g/rsgb-lf-group/topic/77212765#11958

However, perhaps if there's a capable transceiver selected, there's an attempt made to slow things down.  Just my speculation, of course.  Anyway, with the FST4 modes aimed at LF/MF users, it'd obviously be good to have those modes compatible with typical hardware.

73, Peter.