Wagner Lipnharski wrote:

> Integrate the square wave in two capacitors, using two resistors, one
> for each capacitor in a traditional R+C configuration.

  ... et al.

  In short, use a current driver to convert it to a precision triangle
wave, low-pass filter it (or use a "slicer") to obtain the mid-point and
compare the two to derive your 90¡ shift.  Very elegant Wagner!
-- 
  Cheers,
        Paul B.