Prepare popcorn by putting it into a grocery sized paper bag, set aside. In a large glass mixing bowl (2 quart) put syrup, butter, brown sugar and salt. Cook in microwave 2 minutes or until it boils. Remove from microwave and mix ingredients together well. Add and mix in baking soda. Cook another 2 minutes. Poor mixture over popcorn in bag, tossing popcorn to coat all kernels. Roll up end of bag to enclose popcorn and put it into the microwave. Cook for 1 1/2 minutes. Take out bag and shake, turn bag over and place in microwave again. Cook 1 minute. Shake out caramel popcorn onto two cookie sheets to cool. Once completely cooled it can be stored in an airtight container.