Part 2: Oscilloscopes – A silly scope?
During the previous article we discussed in outline the advantages of an oscilloscope over a digital voltmeter and how these advantages can be exploited to good effect during some diagnostic procedures. Let us now discuss how to set up the ‘scope correctly to maximise its effectiveness.
On an oscilloscope, the voltage value is displayed on the vertical scale (axis) and the time value is on the horizontal scale (axis). The voltage scale is selectable as it would be on a normal voltmeter and it can be adjusted to display as much or as little of the pattern as you wish. If the scale selected is too high the pattern will appear smaller in height and a low scale will only display part of the pattern with some of it overflowing the top of the screen. It does not normally matter if you select a voltage scale that is too low, as long as the signal voltage is not higher than that which the unit can safely handle.
On some oscilloscopes the voltage scale represents the total voltage that can be fully displayed, i.e. 20 Volts. Alternatively, the screen will be marked in grids or divisions and the scale will be measured in volts per division, which means that each vertical division represents a proportion of the total voltage. In the case quoted above where the total scale is 20 Volts, when 4 divisions are marked on the display, the scale selected should be 5 Volts per division, giving a total of 20 Volts as before. Some oscilloscopes display both scales as well as digital values for the peak and average voltages in addition to the trace itself.
It is also possible to adjust the time scale to suit specific needs. The time scale chosen e.g. 100ms on the diagram determines how much of the pattern you wish to see across the display, from left to right. The ranges available may be displayed in time per division or total time, similar to the voltage ranges. Typical time scales can range from microseconds (millionths of a second) to a number of complete seconds per division or total scale.
When no connections are made to a circuit, a trace should be displayed as a straight line at zero volts. Sometimes the line will not be straight, usually because of interference from room lighting etc. Simply joining the signal and earth test leads together will normally ensure a straight line at zero volts. The interference is not usually a problem when actually testing a circuit as the leads are effectively joined together by the circuit itself.
Some scopes can display more than one trace at the same time, each of which can be positioned on the screen where you wish. A different voltage range can be selected for each trace although the timescale will be the same for all traces displayed. More than one trace allows signals to be compared, not only for their relative timing but also for their relative voltage and pattern shape.
Triggering the trace
Whilst on many occasions simply determining that a pattern actually exists is sufficient for a diagnosis, it is sometimes necessary to examine the pattern more closely. For this it would, of course be preferable if the pattern remains in the same place on the screen and a “Trigger” function is provided for this purpose. Without a trigger the pattern will scroll across the screen in a random fashion, making close examination impossible. To set a trigger it is necessary to tell the oscilloscope the voltage at which you wish to trigger, or start the display, when the pattern passes through this voltage, the trace will remain “still”. The trigger voltage level must be set somewhere between the minimum and maximum operating voltage of the trace; otherwise nothing at all will be displayed. Bear in mind that some traces will never reach the maximum or minimum voltage available and if you set a trigger level outside the normal operating range you may see nothing and assume a fault exists, even if the signal is ok. If you are unsure of the operating voltage of the circuit, it is usually preferable to switch off the trigger when first checking for a trace and select the trigger after you are confident that the trace exists.
Some oscilloscopes provide a function for the trigger point to be moved left and right across the screen – the pattern will follow the trigger allowing you to position the display exactly where you wish on the screen. It is also possible to select whether you wish the trigger point to be active when the voltage is rising or falling. This helps you to measure the on/off times or duty cycles of some signals accurately.
Connecting to a circuit
In order to check most signals accurately it is best to connect to a complete, live circuit by backprobing a plug or using a special breakout harness in the circuit under test. Only when confirming the actual power supply terminals to sensors and actuators does it sometimes become necessary to disconnect a plug. Note: Avoid testing a disconnected live circuit whenever possible as this may cause fault codes to be stored and/or warning lamps to be illuminated. In some cases default values may be set by the ECM, confusing your diagnosis. In addition to this, disconnection will prevent current from flowing in the relative circuits and any high resistance faults that may be present will not be seen.
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Measuring the signal on a complete, operating circuit is effectively connecting your oscilloscope in parallel with the signal source. The signal probe (various colours) should be connected to the circuit and the earth probe (normally black) to a good earth point on the vehicle chassis or body.
There is no need to be nervous about connecting to a circuit in this way; it is normally very safe to do so with modern digital oscilloscopes, which present an extremely large impedance (resistance) to the circuit under test. The process can be compared to reading a book; you take the information in but the words themselves are left on the page for future readers. Any problems usually only arise through physical damage to the connectors caused by rough handling.
If you wish to display more than one trace it is normal for the earth connection to be “common” to all the signal probes.