Phase-Variation Ramsey Spectroscopy of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msup> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> <mml:msub> <mm

The 2 ^{3}S_{1}→2 ^{3}P_{2} interval (ν_{2}) in positronium has been measured using a phase-variation separated oscillatory fields technique. A beam of positronium atoms in the 2 ^{3}S_{1} state was passed through two spatially separated coherent microwave fields tuned close to the ν_{2} resonance frequency (8626.71±0.08 MHz), and the surviving beam fraction was recorded as a function of the relative phase between the fields. The resonance frequency was obtained from the frequency dependence of the phase of the interference signal without scanning over the full spectral line shape of the transition. We obtain ν_{2}=8626.39±1.17_{stat}±0.80_{sys} MHz, which is in agreement with QED theory.