Function Generator, BNC T and Termination
The generator has a source impedance of 50 ohms.  For best results it should be connected to the device under test with a 50 ohm coax cable which is then terminated with a 50 ohm load. Without the 50 ohm load, the generator is essentially unterminated and the signal will reflect back since it sees an impedance mismatch. The best termination is a 50 ohm feedthru but a T connector will suffice if the feedthru is not available.