One of the fundamental issues when using an oscilloscope is how to stabilize the displayed waveform. This article explains what norm and auto mean on the mode selection switch of an oscilloscope.
This involves the triggering operation, which is a skill that is relatively difficult to master when using an oscilloscope because it relates to the triggering principles of the oscilloscope.
The oscilloscope displays the measured signal through scanning, showing a portion of the measured signal with each scan. To ensure that the displayed waveform is stable and unchanging, it is necessary for each displayed waveform to be exactly the same, that is, overlapping. For periodic signals, as long as the starting phase of the waveform displayed in each scan is the same, the displayed waveform will also be the same, thus making the displayed waveform stable. To achieve this, in addition to sending the measured signal to the cathode ray tube, the oscilloscope also splits off a portion to form a trigger pulse using a voltage comparator, which controls the horizontal scanning direction to ensure that the starting point of each horizontal scan aligns with the same phase point of the measured signal.
The trigger pulse is formed as follows: a portion of the measured signal is sent to a voltage comparator, while the other end of the comparator is connected to a DC voltage adjusted by the trigger level knob (Trigger LEVEL). When the instantaneous voltage of the measured signal exceeds the trigger level, the voltage comparator outputs a high level, and when the instantaneous voltage is below the trigger level, it outputs a low level. Thus, the voltage comparator outputs a rectangular waveform trigger pulse.
The scanning sawtooth waveform is formed as follows: when the leading edge of the trigger pulse arrives, the positive cycle of the sawtooth begins, but the duration of the positive cycle is determined by the scanning switch (TIME/DIV), while the negative cycle duration is fixed. If the negative cycle ends without the arrival of the leading edge of a trigger pulse, the scanning sawtooth remains at a low level until the leading edge of a trigger pulse arrives, at which point the second scanning sawtooth’s positive cycle begins.
When the trigger mode switch (Trigger MODE) is set to the NORM position, the oscilloscope scans according to the above method. Clearly, if there is no measured signal, or if there is a measured signal but it cannot form a trigger pulse, there will be no scanning sawtooth, and thus no scanning line will appear on the screen. When the trigger mode switch is set to the AUTO position, the oscilloscope will automatically form a scan, so scanning lines will always appear regardless of whether there is a measured signal. However, when there is a measured signal, the oscilloscope will immediately switch to the aforementioned operating mode.
Under normal circumstances, the oscilloscope should be set to automatic triggering mode. However, when the frequency of the measured signal is below 25Hz, the oscilloscope should be switched to normal triggering mode.
