I should elaborate a bit on my previous post.

I don't want to give the impression that the Plan B is the only
thing out there since the 258/9 with expo and linear FM. Blacet
has it, MOTM has it as did the Wiard 300. There are probably
more.

But...I went to great lengths to duplicate the 258's FM response
in the Model 15 through unique means - meaning I didn't just lift
the 258's expediential converte (It actually wouldn't have worked
for 1V/OCT tracking). But I will say that the architecture of the
expo converter. the sine waveshaper and incorporation of a
triangle core are key ingredients here and the Model 15 was
designed around those three requirements. All of the other
functions were designed AFTER I got the FM indexing nailed.

- P