I'm looking at used function generators and most have at least one output that says 50 ohms, The manuals say the specs apply to a 50 ohm impedance (or load). Some have an additional output that provides TTL levels. What's the difference between the two (50 ohm and TTL)? I presume I can plug the TTL output directly into a TTL circuit, but not the 50 ohm. How am I supposed to use the latter? Thanks.