Linkage for Tracing Inverse Figures

Lengths of pink and blue bars: b/2 + b/2 = b
Lengths of green bars: a
The linkage maintains always:
Points A, P and Q are in line.
AP.AQ = (a.a – b.b) = constant
P and Q trace figures (in orange and green) that are inverse to each other.
If Q moves following a circle, P traces a circle.