  We examine the phase and the period of the radiation-induced
oscillatory-magnetoresistance in GaAs/AlGaAs devices utilizing in-situ magnetic
field calibration by Electron Spin Resonance of DiPhenyl-Picryl-Hydrazal. The
results confirm a $f$-independent 1/4 cycle phase shift with respect to the $hf
= j\hbar\omega_{c}$ condition for $j \geq 1$, and they also suggest a small
($\approx$ 2%) reduction in the effective mass ratio, $m^{*}/m$, with respect
to the standard value for GaAs/AlGaAs devices.
