The Oscilloscope

The oscilloscope allows one to measure voltage signals that are changing in time. The voltage of the signal one wants to measure is read on the vertical scale of the screen of the cathode ray tube, and the time variation of the signal is read on the horizontal scale of the screen.

There are four main groups of controls on the oscilloscope. The first group controls the appearance and position of the electron beam which causes the fluorescent bright line to cross the screen. This fluorescent line is called the trace. The second controls the vertical deflection of the electron beam, or trace, and the third controls the horizontal deflection by means of and internal voltage ramp generator or by an external horizontal input. If you have trouble finding the beam, preset the controls as described in the scope instruction manual. The fourth set controls what signals are used to cause the electron beam to sweep across the screen. They are called triggering controls. See the figure below for a look at a generic oscilloscope. The oscilloscope that you use may appear different, but have patience, with a little looking you will find the same groups of controls.

1. The Intensity knob adjusts the brightness of the trace by adjusting the potential of a grid controlling number of electrons reaching the screen. The Focus adjusts the sharpness of the trace. On some scopes one sees a trace rotation control. This knob allows the operator to adjust the trace to be parallel with the horizontal grid lines on the screen.

2. The Volts/cm and Variable knobs provide coarse and fine adjustment of the pattern height by varying the amplification of signals fed into the terminals marked INPUT. The vertical input terminals will be referred to as the Y input. The Volts/cm and Variable knobs also set vertical scale voltage sensitivity, i.e., the vertical scale grid lines are set to specific voltage differences which are read on the Volts/cm (or Voltage difference between horizontal grid lines) knob. To read the correct volts/cm shown on the black volts/cm scale, the Variable knob must be in the calibrate position. For dual trace scopes, there are two sets of Vertical input controls. There is a position knob nearby which adjusts the vertical position of the trace.

   Near the Input terminals, there is a switch for grounding the input (GND), coupling through a capacitor (AC) or a direct connection (DC). Ordinarily use just the (AC) input.

3. The Time/cm and Variable knobs control the sweep generator which causes the electron beam to sweep across the screen of the cathode ray tube. Just as the vertical Volts/cm knob sets the units of the vertical scale in specific voltage units, so the Time/cm knob set the horizontal scale in specific time units. One easily makes measurements of the period of a signal and inverts to calculate the frequency of a signal. For the Time/cm scale to be accurate the Variable knob must be at the calibrate position.

4. The Trig and Level knobs control the triggering of the horizontal sweep. These adjustments allow the vertical signal to be synchronized with respect to the horizontal sweep so that a stationary trace on the screen is produced. To trigger and sweep automatically set Level to Auto. To trigger only on large signals, use the normal mode and adjust the Level knob.

Try playing with a virtual oscilloscope . This group has a rather extensive java applet library. Check it out!