3 - TRIGGER CONTROL

This section is by far the most important and the most difficult to grasp and understand.

The electron beam presents a single dot on the screen. The resemblance of a trace appears on the screen because the dot moves across the screen rapidly as a result of an oscillator ramp circuit in the horizontal function of the oscilloscope. A line is observed because of the persistence of the phosphor on the screen and the persistence of vision in the human eye.

If the horizontal oscillator is allowed to run in "free-run" mode, signals would appear at random on the screen. This is fine for random waveforms such as audio, voice and music.

In order for a repetitive waveform to remain stationary on the screen the internal horizontal oscillator must be synchronized with the incoming signal. This is the function of the trigger section. The horizontal oscillator is held off until a trigger event is detected. The oscillator generates a single ramp and then goes back to stand-by mode.

MODE
AUTO
NORM
TV - V - ignore this for now
TV - H - ignore this for now

NORM
Connect the CH1 probe to CAL 0.5V square wave test output.
Set MODE to NORM
Set SOURCE to INT

The scope will trigger on the next available signal when the trigger criteria are met.
Experiment with LEVEL control and PULL (-) SLOPE.

PULL (-) SLOPE selects a falling transition of the signal. Default is rising transition.
LEVEL allows you to select the voltage point on the signal that triggering begins.

If you remove the source of signal, (e.g. disconnect the probe or change SOURCE to EXT), the trace should vanish.

AUTO
Set the MODE to AUTO
In AUTO mode the scope will trigger even when there is no detectable signal.
Remove the signal source and you should still see a horizontal line.

LINE
Set SOURCE to LINE
This means that the oscilloscope will trigger on 60Hz AC line frequency (50Hz for some countries). This is useful for viewing AC line frequency induced noise. With the 1kHz CAL signal as your input signal, the trace will not be synchronized. It will appear randomly across the screen.

 

Probe Frequency Compensation Adjustment

To wrap up this tutorial on using your oscilloscope, it is important that you adjust your x10 probe correctly. Failure to do so may result in incorrect voltage amplitude measurements at high frequencies.

Set the probe to x10 attenuation.
Connect CH1 probe to CAL .5V reference output.
Locate the trimmer capacitor on the body of the probe. Use a small insulated screwdriver to make this adjustment.

Adjust the trimmer capacitor to achieve the flattest square wave (top and bottom flat portions) as shown in the diagrams below.

Do the same with the other probe.

You only need to do this once. You need to do this again if you change the probe or use the same probes on a different oscilloscope.

 

Locate the trimmer capacitor on the body of the probe.

Sometimes it's at the connector end.

With a x1, x10, gnd probe, a lot of times the window gets covered by the switch in the gnd and x1 position when it makes no sense to do the adjustment.