Combine first 3 ingredients in a large bowl; stir until sugar dissolves.
Place thawed raspberries in a blender; process until smooth. Strain puree through a fine sieve over a bowl. Discard seeds.
Add puree to yogurt mixture.
Pour raspberry mixture into the freezer can of an ice-cream freezer; freeze according to manufacturer's instructions. Spoon into a freezer-safe container; cover and freeze 1 hour or until firm.
