Maybe not as elegant as Gez's effort, but an interesting other way to skin this cat:

http://www.till.com/articles/QuadTrapVCO/discussion.htmlAnd here's another idea: I drew a pair of quad triangles & wondered about how Icould make one into the other, by doing different arithmetic during the four cycles. If you let X be the first triangle wave, and P be half the peak to peak voltage, then you can get a 90 deg delayed triangle Y thus:

1. for 0 to 90 deg, Y = X-P

2 for 90 to 180, Y= -X +P

3. for 180 to 270, Y = X + P

4 for 270 to 360, Y = -X -P

The 4 equations I give above, could be easily done using the classic logic controlled x1 and -x1 output amplifier. And the primary triangle generator can give you the logic levels.