A test and measurement apparatus, system, and method for synchronizing an acquisition or triggering system to a specific burst of interest. The subject apparatus and method triggers on varying energy content of a signal qualified by time in the presence of high-frequency input signal bursts, by using an adjustable pulse width envelope detector, disposed in the signal path of the trigger circuitry, as a digital rectifier or to otherwise process and extract an envelope signal. An RF envelope probe having an analog envelope detector among other suitable components is disclosed. A method is implemented for isolating an interval of interest in a signal under test. An envelope detector circuit produces an envelope signal from the signal. Trigger circuitry receives the envelope signal from the envelope detector, and isolates the interval of interesting in the signal under test using the envelope signal.