Probably because with a DC level your scope isn't triggering. You need to change the trigger type so that it's basically in "free run", "continuous", "normal", or whatever it's called for that model of the scope.
Do you mean I have to change the second item into Normal trigger. But it is Normal. I have tried in my device. It displays well in the AUTO(Untriggered Roll) Mode. I would like to know how to modify the circuit to meet this requirement.
