  In scale-invariant models of fundamental physics all mass scales are
generated via spontaneous symmetry breaking. In this work, we study inflation
in scale-invariant quadratic gravity, in which the Planck mass is generated
classically by a scalar field, which evolves from an unstable fixed point to a
stable one thus breaking scale-invariance. We investigate the dynamics by means
of dynamical system standard techniques. By computing the spectral indices and
comparing them with data, we put some constraints on the three dimensionless
parameters of the theory. We show that certain regions of the parameter space
will be within the range of future CMB missions like CMB-S4, LiteBIRD and
STPol. The second half of the paper is dedicated to the analysis of
inflationary first-order tensor perturbations and the calculation of the power
spectrum of the gravitational waves. We comment on our results and compare them
with the ones of mixed Starobinsky-Higgs inflation.
