Operating instructions for a simple oscilloscope
Teaching Guidance for 14-16
A CRO (cathode ray oscilloscope) is a good voltmeter. The electron stream from the electron gun obeys instructions to move up or down almost instantaneously. The input resistance of the device is very high so that very little current is taken from the circuit under investigation. Voltmeters take a trickle of current.
The spot moves sideways when the time-base is switched on, and the trace can be looked on as the x-axis or time axis of a graph. The signal is applied, normally, across the y-terminals, and so the variation in the potential difference (p.d.) is plotted along the y-axis. The screen plots out the 'graph' of p.d. against time.
- To prepare the oscilloscope for use, plug into the mains supply and set the controls as follows:
- Brightness to OFF
- Focus to the mid position
- X-gain fully anticlockwise
- X-shift to the mid position
- Trig control to +
- Time-base: time/cm (or time/div) control to 1 ms
- Time-base: variable control fully clockwise
- Stability control fully clockwise
- Trig level control fully clockwise
- Amplifier: volts/cm control to 0.5 V/cm
- Y-shift to the mid-position
- Input switch to DC
- Switch on by means of the 'Brightness' control. After warming up for about one minute, turn 'Brightness' clockwise until a trace appears. Set the control so that the trace is clearly visible, but not excessively bright.
- If no trace appears, leave the 'Brightness' in the fully clockwise position, and adjust 'X-shift' and 'Y-shift' until the trace appears. This is best done by rotating 'X-shift' backwards and forwards, whilst slowly advancing 'Y-shift' from the fully anticlockwise position. Immediately the trace is found, reduce 'Brightness' to a convenient level.
- Now centre the trace with the 'X-shift' and 'Y-shift' controls, and adjust' Focus' to give a sharp trace.
- Slowly turn 'Stability' anticlockwise until the trace just disappears. Finally, rotate 'Trig Level' anticlockwise and switch it to the auto position. The trace (which reappears when 'Trig Level' is rotated) may dim when this is done, but will brighten again when an input is applied.
- The oscilloscope is now ready for use, but it is important to be familiar with the function of the various controls. This experience is best gained by a 50 Hz wave-form from an AC low-voltage power supply, and then exploring the action of the various controls (excepting 'Stability' and 'Trig Level' controls which are set by procedure d above). A possible routine for those unfamiliar with such instruments is to:
- put 2 to 4 volts, 50 Hz AC on the input and change volts/cm back to 5
- turn the variable time-base control fully anticlockwise and then back to the calibrated position (fully clockwise)
- change time-base to 100 μs, and then return it to 1 ms
- change Trig + to - (if the sine curve trace is not inverted by this, turn the Stability control very slightly anticlockwise until it is).
Further work should bring increasing confidence.
For most experiments, the 'Trig Level' can be left at AUTO. To give a steady trace, the 'Stability' should be turned as far as possible counter-clockwise without losing the trace. This setting may vary a little with different time-base speeds.
Further details on the oscilloscope
AC – DC switch The switch should normally be set to DC, even when the oscilloscope is used for AC. In the AC position there is a capacitor in series with the input and this will separate the AC component from a wave-form such as the one in the sketch. The AC position of the switch should be used only for this purpose.
When the oscilloscope is used for the pure AC, setting the switch to AC will cause a smaller deflection at very small frequencies because C and R modify the signal fed to the tube. This is another reason for not using it except for the purpose indicated above.
Time base When time-base is switched off the spot is automatically centred. There is no X-shift control.
When the time-base is switched on, the speed of the spot is determined by the setting of the ‘Range’ and ‘Variable’ controls. However, the frequency of repetition of the time-base is not much increased at the higher speeds and the time-base is often interrupted by slow changes of the input voltages. For these reasons it is better to have the time-base off when the oscilloscope is being used as a DC voltmeter.
When an alternating voltage is connected to the input, it automatically triggers the time-base and makes it a very steady trace.
X-input The time-base should first be switched off by turning the 'Variable' control fully anti-clockwise to the OFF position. AC inputs may then be connected to the X-input and sockets on the back of the oscilloscope. (Note. The socket on the back and the E terminal on the front are connected internally.)
The 'X-gain' control will give a variation of 2:1 in the amplification. The spot will be deflected horizontally to the full screen width by AC voltages between 3 V r.m.s. and 6 V r.m.s. The sensitivity varies from 2 V/cm to 1 V/cm.
(There is no direct coupling between the X-input socket and the cathode ray tube. DC inputs will give only momentary deflections.)
Z-input If a sine wave or square wave input is connected between the Z-input and Earth sockets on the back of the oscilloscope, the brightness of the trace may be varied by these inputs.
With sine wave inputs, at least 20 V r.m.s. is needed at a frequency of 50 Hz. This reduces to 1 V r.m.s. at 20 kHz. It is necessary to dim the trace, so that the variation in brightness may be easily noticed.
With square-wave inputs, 30 V peak to peak is necessary at 50 Hz. This reduces to 2 V peak to peak at 20 kHz. The variation in brightness is much clearer with square waves. With low frequencies, quite sudden increases or decreases in brilliance can be seen.
Sweep output When the time-base is switched on, a p.d. corresponding to the X deflection may be taken from the 'sweep output' and Earth terminals. The potential of the 'sweep output' terminal varies from about +40 V, when the trace is on the left of the tube, to about +20 V when the trace is on the right. Too much current should not be taken, unless distortion of the time-base is permissible.
As a rough guide, the time-base will not be affected if a 0.1 μF capacitor (to block the DC component) is connected in series with the sweep output, and the load resistance is not less than 100 kΩ. At some sweep speeds, much more current may be taken.
It is easiest to see if the load circuit is distorting the time-base by unplugging it momentarily.
The sweep output may be used for triggering any transient effect repeatedly so that a steady pattern occurs on the tube.
It is also interesting, and helps to understand the operation of the time-base, to connect the sweep output of one oscilloscope to the Y-input of a second oscilloscope.
A video showing how to use an oscilloscope: