Warren Allgyer <allgyer@...>
I am not sure if you are soliciting my opinion specifically but I will respond as if that is the case.
First, I suspect the four filters will suffice in this case. Because you are using the push-pull amplifiers the pesky second harmonic issue is effectively dealt with. The first harmonic that we have to really worry about is the third and, as can be seen in my photos, it is really the 5th 7th and 9th that are the issues on the lower bands. These are so widely space from the carrier frequency that the suppression can be derived from just 4. I think! :-)
In order to maintain backward compatibility I would design a daughter card that could press fit onto a pattern of posts on the existing main card. The posts should be existing holes that allow picking up input, several grounds, and output. Retrofit would involve removal of two or three strategic components on the existing motherboard, installation of the pins, and plugging the daughter card onto them. I would then supply a “y” jumper cable to pick up power and filter selection signals from the existing connector.
In designing the daughter card it is critical that the input to output isolation be measured carefully with the board unpopulated. If it is not sufficient in this condition it is only going to get worse. Then populate the prototype and measure it again. Finally, install the working model and check it again.
Finally, I think we can likely rely on the software community for this radio to come up with a means of keying and injecting a low level tone that is processed through the filters as SSB in order to remove the discrepancy in harmonic content between SSB and CW.
The modifications should be optional. You have created an incredible product with a crucial price point and its popularity is proof of just that. Many amateurs will turn a pragmatic eye away from the currently non-compliant harmonics and operate the radio as it is. At these levels it is very likely that no one will ever be the wiser and I highly doubt anyone is going to get cited for it.
But, in providing the option, you have given hams who wish to operated according to the letter of the law a pathway to doing exactly that in a fashion that maintains software compatibility across either version.
That is my opinion. Thank you for asking!