To indicate a degree of approach of a test object signal to a trigger condition.

The test object signal is digitalized by an ADC 315, and a digital sample is supplied to a trigger event detector 305. In response to the trigger signal, the digital sample is stored in an acquisition memory 325 and is displayed on a display 335. A processor 345 and an instruction unit 350 display a first marker indicating a real time trigger function factor calculated on the basis of the digital sample showing the test object signal, and second marker means indicating a level to be overcome by the performance factor in order to allow the trigger event detector to detect a trigger event and to generate a trigger signal.