If you look at the specs of the wavetek 145, rise time is under 20ns rise time, also the signal is 50 ohms terminated and it looks like you are just plugging it directly to your scope without a 50 ohm terminator.
Edit, and it seems you are also using probes to measure the signal, the ground lead is going to do a lot of ringing that way.
You need a BNC T connector and a 50 ohm terminator, not that it's going to make the rise time faster on the wavetek.
Edit: green terminators in this setting are 50 ohms, purple one is 75 ohms (for VGA signal)