Why do I have to provide an external trigger to my 8000-series oscilloscope?

Why do I have to provide an external trigger to my 8000-series oscilloscope?

Due to the design of sampling oscilloscopes there is no internal trigger pickoff, so there is no signal being routed to the trigger circuitry.

Therefore a signal needs to be applied to the Direct Trigger Input, or the Trigger Prescale Input. Which one you use depends on the frequency of the signal being applied. For signals up to 3Ghz, use the Direct Trigger Input. For signals between 3Ghz and 12.5Ghz, use the Trigger Prescale Input.

There are two exceptions:

Most of the optical sampling modules have clock recovery available as an option. If clock recovery is available, then a separate signal to the trigger inputs is not needed.

For electrical signals there is the 80A05 (Electrical Clock Recovery module). This module can be used for both single and differential signals. This module will provide the signal to the trigger circuitry.