When I was a kid, I never believed the Sprite commercials when they said that it was supposed to be a lemon-lime flavor. They did almost have me fooled into thinking that there was such thing as a "lymon", though.
I always thought that Sprite, 7-up, and other lemon-lime clones were sort of flavorless. They were sweet, they were clean, they were refreshing, but they weren't distinctly lemon or lime. Then one day as a child I tasted club soda and it brought everything into context.
Snap to 25 years later, and here's my take on the clear citrus beverage. Sweetened, carbonated, lemon-water. I do enjoy a lemon in my water from time to time, and I thought it would be a good match to other green-bottled clear citrus sodas. It is perhaps the simplest soda next to plain club soda (carbonated water).
1 lb sugar
1 tsp Citric Acid
, or additional 2 tbsp lemon juice
1 gallon (4 L) carbonated water
The great thing about this is that you can add as much syrup to your carbonated water or as little as you want and it will always taste great (well, within reason). What I've outlined here is a pretty decent match to the commercial versions, but without the lime.
First, zest the lemon and steep the zest in about 1 cup of water. You should get a pretty good yellow color, with it, too. Unfortunately, this doesn't necessarily show up in the finished product.
Next, Strain out your lemon zest/peel, and add your sugar directly to the water that you've used to steep the lemon in. I normally invert my sugar to get more sweetness out of it, but this comes out nicely without the inversion. Using pure cane sugar comes out a little clearer, and a little less syrupy, but you do end up with the same amount of calories for less sweetness for those making their own soda for health reasons. You can obviously always use less sugar for a less sweet drink, also.
To top it off, you'll need to add some tartness. Again, you can add as much or as little as you'd like depending on your tastes. For a crisp, clean drink, Citric Acid can be found online or at some health food or specialty food stores. If you prefer, you can use Lemon or Lime juice, as they are great sources for citric acid, but you will need to add more, and they will add more lemon/lime flavor and some cloudiness.
Add this syrup to your carbonated water and there you have it. This makes syrup for about 1 gallon of finished beverage depending on how sweet or lemony you want to end up with.