Cathode Ray Oscilloscope [CRO] ( Basics )

What does an oscilloscope do?

An oscilloscope is easily the most useful instrument available for testing circuits because it allows you to see the signals at different points in the circuit. The best way of investigating an electronic system is to monitor signals at the input and output of each system block, checking that each block is operating as expected and is correctly linked to the next. With a little practice, you will be able to find and correct faults quickly and accurately.

An oscilloscope is an impressive piece of kit:

Faced with an instrument like this, students typically respond either by twiddling every knob and pressing every button in sight, or by adopting a glazed expression. Neither approach is specially helpful. Following the systematic description below will give you a clear idea of what an oscilloscope is and what it can do.

The function of an oscilloscope is extremely simple: it draws a V/t graph, a graph of voltage against time, voltage on the vertical or Y-axis, and time on the horizontal or X-axis.

As you can see, the screen of this oscilloscope has 8 squares or divisions on the vertical axis, and

10 squares or divisions on the horizontal axis. Usually, these squares are 1 cm in each direction:

Many of the controls of the oscilloscope allow you to change the vertical or horizontal scales of the V/t graph, so that you can display a clear picture of the signal you want to investigate. 'Dual trace' oscilloscopes display two V/t graphs at the same time, so that simultaneous signals from different parts of an electronic system can be compared.

Setting up

1. Someone else may have been twiddling knobs and pressing buttons before you. Before you switch the oscilloscope on, check that all the controls are in their 'normal' positions. For the CRO shown in above figure, this means that:

·     all push button switches are in the OUT position

·     all slide switches are in the UP position

·     all rotating controls are CENTERED

·       the central TIME/DIV and VOLTS/DIV and the HOLD OFF controls are in the calibrated, or CAL position

Check through all the controls and put them in these positions:

2. Set both VOLTS/DIV controls to 1 V/DIV and the TIME/DIV control to 0.2 s/DIV, its slowest setting:

3. Switch ON, red button, top centre:

The green LED illuminates and, after a few moments, you should see a small bright spot, or

trace, moving fairly slowly across the screen.

4. Find the Y-POS 1 control:

What happens when you twiddle this?

The Y-POS 1 allows you to move the spot up and down the screen. For the present, adjust the trace so that it runs horizontally across the centre of the screen.

5. Now investigate the INTENSITY and FOCUS controls:

When these are correctly set, the spot will be reasonably bright but not glaring, and as sharply focused as possible. (The TR control is screwdriver adjusted. It is only needed if the spot moves at an angle rather than horizontally across the screen with no signal connected.)

6. The TIME/DIV control determines the horizontal scale of the graph which appears on the oscilloscope screen.

With 10 squares across the screen and the spot moving at 0.2 s/DIV, how long does it take for the spot to cross the screen? The answer is 0.2 x 10 = 2 s. Count seconds. Does the spot take 2 seconds to cross the screen?

Now rotate the TIME/DIV control clockwise:

With the spot moving at 0.1 s/DIV, it will take 1 second to cross the screen.

Continue to rotate TIME/DIV clockwise. With each new setting, the spot moves faster. At around 10 ms/DIV, the spot is no longer separately visible. Instead, there is a bright line across the screen. This happens because the screen remains bright for a short time after the spot has passed, an effect which is known as the persistence of the screen. It is useful to think of the spot as still there, just moving too fast to be seen.

Keep rotating TIME/DIV. At faster settings, the line becomes fainter because the spot is moving very quickly indeed. At a setting of 10 µ s/DIV how long does it take for the spot to cross the screen?

7. The VOLTS/DIV controls determine the vertical scale of the graph drawn on the oscilloscope screen.

Check that VOLTS/DIV 1 is set at 1 V/DIV and that the adjacent controls are set correctly:

The Hameg HM 203-6 has a built in source of signals which allow you to check that the oscilloscope is working properly. A connection to the input of channel 1, CH 1, of the oscilloscope can be made using a special connector called a BNC plug, as shown below:

The diagram shows a lead with a BNC plug at one end and crocodile clips at the other. When the crocodile clip from the red wire is clipped to the lower metal terminal, a 2 V square wave is connected to the input of CH 1.

Adjust VOLTS/DIV and TIME/DIV until you obtain a clear picture of the 2 V signal, which should look like this:

Check on the effect of Y-POS 1 and X-POS:

What do these controls do?

Y-POS 1 moves the whole trace vertically up and down on the screen, while X-POS moves the whole trace from side to side on the screen. These control are useful because the trace can be moved so that more of the picture appears on the screen, or to make measurements easier using the grid which covers the screen.

You have now learned about and used the most important controls on the oscilloscope.

You know that the function of an oscilloscope is to draw a V/t graph. You know how to put all the controls into their 'normal' positions, so that a trace should appear when the oscilloscope is switched on. You know how the change the horizontal scale of the V/t graph, how to change the vertical scale, and how to connect and display a signal.

What is needed now is practice so that all of these controls become familiar.

Connecting a function generator

The diagram shows the appearance of a Thandar TG101 function generator, one of many types used in UK schools:

Again, your function generator, or signal generator, may look different but is likely to have similar controls.

The Thandar TG101 has push button controls for On/Off switching and for selecting either sine, square, or triangular wave shapes. Most often the 600 ohmoutput is used. This can be connected to the CH 1 input of the oscilloscope using a BNC-BNC lead, as follows:

Switch on the function generator and adjust the output level to produce a visible signal on the oscilloscope screen. Adjust TIME/DIV and VOLTS/DIV to obtain a clear display ond investigate the effects of pressing the waveform shape buttons.

The rotating FREQUENCY control and the RANGE switch are used together to determine the frequency of the output signal. With the settings shown in the diagram above, the output frequency will be 1 kHz. How would you change these setting to obtain an output frequency of

50 Hz? This is done by moving the RANGE switch to '100' and the FREQUENCY control to '.5':

Experiment with these controls to produce other frequencies of output signal, such as 10 Hz, or

15 kHz. Whatever frequency and amplitude of signal you select, you should be able to change the oscilloscope settings to give a clear V/t graph of the signal on the oscilloscope screen.

The remaining features of the function generator are less often used. For example, it is possible to change the output frequency by connecting suitable signals to the 'Sweep in' input. The DC Offset switch and the Offset control allow you to add a DC voltage component to the output signal producing a complex waveform.

The output level switch is normally set to 0 dB:

This gives an output signal with a peak amplitude which can be easily adjusted up to several volts. In the -40 dB position, the amplitude of the output signal is reduced to a few millivolts. Such small signals are used for testing amplifier circuits.

The TTL output produces pulses between 0 V and 5 V at the selected frequency and is used for testing logic circuits.

The arrangement outlined below is a very convenient way of setting up an oscilloscope to make measurements from the prototype circuit:

Once the crocodile clip corresponding to the black lead has been connected to 0 V, it can be ignored. This leaves the test probe which can be connected to any point in the circuit to monitor the signals present.

Connect the test probe to the prototpye circuit. Increase the sensitivity of the VOLTS/DIV control by rotating it clockwise until you can see changes on the oscilloscope screen when you talk into the microphone. Adjust TIME/DIV until the shape of the signals is clear.

How does an oscilloscope work?

An outline explanation of how an oscilloscope works can be given using the block diagram shown below:

Like a televison screen, the screen of an oscilloscope consists of a cathode ray tube. Although the size and shape are different, the operating principle is the same. Inside the tube is a vacuum. The electron beam emitted by the heated cathode at the rear end of the tube is accelerated and focused by one or more anodes, and strikes the front of the tube, producing a bright spot on the phosphorescent screen.

The electron beam is bent, or deflected, by voltages applied to two sets of plates fixed in the tube. The horizontal deflection plates, or X-plates produce side to side movement. As you can see, they are linked to a system block called the  time base. This produces a sawtooth waveform. During the rising phase of the sawtooth, the spot is driven at a uniform rate from left to right across the front of the screen. During the falling phase, the electron beam returns rapidly from right ot left, but the spot is 'blanked out' so that nothing appears on the screen.

In this way, the time base generates the X-axis of the V/t graph.

The slope of the rising phase varies with the frequency of the sawtooth and can be adjusted, using the TIME/DIV control, to change the scale of the X-axis. Dividing the oscilloscope screen into squares allows the horizontal scale to be expressed in seconds, milliseconds or microseconds per division (s/DIV, ms/DIV, µ s/DIV). Alternatively, if the squares are 1 cm apart, the scale may be given as s/cm, ms/cm or µ s/cm.

The signal to be displayed is connected to the input. The AC/DC switch is usually kept in the DC position (switch closed) so that there is a direct connection to the  Y-amplifier. In the AC position (switch open) a capacitor is placed in the signal path. As will be explained in Chapter 5, the capacitor blocks DC signals but allows AC signals to pass.

The Y-amplifier is linked in turn to a pair of Y-plates so that it provides the Y-axis of the the V/t graph. The overall gain of the Y-amplifier can be adjusted, using the  VOLTS/DIV control, so that the resulting display is neither too small or too large, but fits the screen and can be seen clearly. The vertical scale is usually given in V/DIV or mV/DIV.

The trigger circuit is used to delay the time base waveform so that the same section of the input signal is displayed on the screen each time the spot moves across. The effect of this is to give a stable picture on the oscilloscope screen, making it easier to measure and interpret the signal.

Changing the scales of the X-axis and Y-axis allows many different signals to be displayed. Sometimes, it is also useful to be able to change the positions of the axes. This is possible using the X-POS and Y-POS controls. For example, with no signal applied, the normal trace is a straight line across the centre of the screen. Adjusting Y-POS allows the zero level on the Y-axis to be changed, moving the whole trace up or down on the screen to give an effective display of signals like pulse waveforms which do not alternate between positive and negative values.