Add the sugar, raspberries and water to a small saucepan set over medium heat. Bring to a gentle simmer and mash the berries with a muddler or a spoon. Simmer for 7 minutes, then strain through a fine mesh sieve, pressing the berries gently to extract the juice. Allow the syrup to cool, then refrigerate until ready to use. The syrup will keep in an airtight container in the refrigerator for 2 to 3 weeks.