| Pulse-Width Triggering with Fluke’s ScopeMeter® 190C Series Test Tools
Glitches. Timing jitters. System crashes. They’re the bane of a field service engineer’s existence, devilish problems that can hide in a network just out of reach of a conventional test tool, no matter how powerful. Sometimes you need a tool with a little something extra. You need pulse-width triggering. Pulse-width triggering, sometimes called time-qualified triggering or glitch triggering, moves beyond measuring the edge of a pulse. It’s a powerful way to capture a specific positive or negative pulse by triggering on the pulse duration rather than on its edge. That’s important. In a logic circuit, for example, a glitch – say, a pulse much faster than the clock pulse – can be a serious source of problems. Measuring the edge simply isn’t enough. The ability to trigger uniquely on the glitch, investigate what generated it and determine its effect on the rest of the system – all possible with pulse-width triggering - provides important diagnostic information. Whether it’s an error of synchronous logic, problems with a rotary encoder or a serial data transmission error, oscilloscopes with pulse-width triggering capability such as the powerful, handheld ScopeMeter® 190C Series help service technicians ferret out even the most troublesome buried problems. Handheld scopes offering pulse-width triggering are still a rarity, but because field service engineers are just as likely to need this capability as laboratory-based engineers, Fluke included pulsewidth triggering in its advanced ScopeMeter 190C oscilloscope series. The ability to trigger uniquely on the glitch, investigate what generated it and determine its effect on the rest of the system provides the service engineer with an important diagnostic tool. In addition to glitches, many timing problems in circuits are caused by pulses that appear too long (which can, for example, indicate a missing pulse). To capture these, you can set a scope with pulse-width triggering capabilities to trigger on pulses longer than a given duration. Triggering on a long pulse also is useful in many bus protocols where a long pulse often occurs at the beginning of a data stream. To contend with all likely eventualities, the pulse-width triggering function on the ScopeMeter 190C Series scopes offers four time qualifiers: ‘less than’ (< t), ‘greater than’ (> t), ‘equal to’ (= t) and ‘not equal to’ (≠ t), where the time interval is selectable in minimum steps of 0.01 divisions or 50 ns. The scopes also offer a time delay of nine div pre-triggering and 1000 div post triggering. To be able to set the correct triggering conditions, however, it’s necessary to know something about the signal you are looking for, such as the likely pulse duration, or whether the condition you’re investigating is likely to lead to a glitch or a pulse longer than the normal signal (Figures 1 and 2). Tracing errors in synchronous logicA typical problem with synchronous logic systems is an unexpected timing delay caused by slow peripheral components in the signal path. On a microprocessor board, for example, a single clock controls all timing function. Two clock-derived pulses passing simultaneously... Download the full PDF:Pulse-width triggering (.pdf) » | |