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1.3 Triggering ControlThis is the tricky bit that no-one understands, so engage your brain…
Imagine that the input signal to an oscilloscope is a sinusoidal voltage. The scope repeatedly1
draws a trace across the screen that represents the time-varying voltage. The trace is drawn
hundreds or thousands of times each second. Now imagine that each time the trace is drawn
across the screen, the drawing begins on a different part of the sine wave. The trace of the
sine wave will flicker horizontally, backwards and forwards across the screen. I am sure that
you have seen something like this. To stop the flickering and “freeze” the trace on the screen,
the scope must start to draw the sine wave on exactly the same part of the wave every time the
wave is drawn. This is what the TRIGGERING control lets you do.
Triggering works by setting a reference (DC) voltage level, which the scope compares with
the input signal. When the input signal voltage reaches the trigger voltage, the scope begins
drawing the trace on the screen. The trace remains rock-solid on the screen, as if by magic!
Actually, what is happening is that every time the trace is drawn (thousands of times per
second) it is drawn in exactly the same place.
Of course, there are a couple of circumstances where this will not give a stable screen trace:
? If the input signal voltage never reaches the trigger voltage, the scope never draws the
trace. I am sure that you have (or have not?) seen this, too.
? If the input signal is non-repetitive. For this, you need a storage scope – see later.
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