signal generator circuit from Ashhar Farhan's (VU2ESE) Web site. He wound his coils on fast-food soda straws, I didn't have any straws lying around, but I did have a bunch of T50-2 powdered-iron toroids, so I decided to use a couple of them. I wasn't entirely sure about the inductance of VU2ESE's coils (he just gave the number of turns), but he did mention that he designed the signal generator circuit to run from 3 to 30 MHz, and given the size of the tuning capacitor, I did some rough calculations concerning what inductance would be required to resonate in that range.
Armed with that information, I went Googling in search of a toroid calculator, and found this calculator site, with exactly the sort of toroid calculator I needed. You just pick the type core you're using (iron powder or ferrite), pick the core size, enter the desired inductance, and the calculator gives you the number of turns to wind on it.
I thought it was pretty cool, anyway, and it saved me some time (and math).
73 de aa0ms