The Tektronix Type 5T3 is a timing plug-in for the Type 661 sampling oscilloscope. It uses TD253B tunnel diodes for triggering. It has a high-impedance external trigger input that is buffered by an 8056 Nuvistor cathode follower. The 5T3 also has a 50-ohm external trigger input with no buffering stage. This low-impedance trigger input is capable of operation up to 5GHz.