Oscilloscope Basics – by David Wagstaff AAE MIMI Master Technician

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There will come a time when we get a car in with a fault and despite following our diagnostic procedure – with visual inspections, code reads, live data checks, voltage checks and volt drop testing – we still won’t have enough information to give us a clear answer to what the underlying cause of the problem is. This situation may occur on something as simple as a crank sensor fault or a more complex issue such as a communication network fault. This is often the point when the parts cannon gets fired, guessing the cause of the issue. Take for instance our crank sensor fault, you’ve done all the checks, so it must be the sensor that’s faulty. Crank sensors are cheap and easy to fit, so worth a guess, right? What if you fit the sensor and it still doesn’t cure the issue? What if it had been a £1,500 ECU, would you be prepared to gamble your own money on it? Or would you want to be more certain it was faulty before committing? What next?

The answer is to use an oscilloscope to test the input or output signal to prove a fault before we commit to a new part. An oscilloscope is a very useful and powerful tool but seems to
be the tool that holds most fear in the hearts of technicians.

I guess the reason for that is that the machine doesn’t give you a definitive answer in writing to a question, it requires the technician to interpret what is displayed on the screen to come to their own conclusion. To an inexperienced user this can be confusing to say the least.

Let’s start with what an oscilloscope is and what it is capable of. If I take you back to the days of your school science classes, you may remember doing an experiment where you measured the temperature of water as it was being heated. You would have measured the temperature, perhaps once each minute and plotted the results onto a graph, with time along the bottom and temperature up the side. Each of your measurements would have been plotted as a dot on the graph and when you finished you drew a line through the dots to give you a curve. That is exactly what a modern digital oscilloscope does, except where you may have been taking a sample once a minute, the oscilloscope will be taking thousands, if not millions, of voltage samples per second, plotting them all as dots and joining them with lines to display a waveform on the screen.

GETTING SET UP

There are three basic settings that as a beginner you need to worry about. The first is the voltage scale or maximum voltage that can be displayed on the screen. It should be appropriate for what you are expecting to measure. If you are trying to measure something that has a small voltage and have the scope set to show high voltage, the image will be too small to be seen clearly. Conversely, with the scope set on too low a voltage, the image will be off the top of the screen. Many automotive systems work on 12-14 Volts, so probably the most common scale to use would be the one with a maximum 20V. That is 0V at the bottom of the screen and 20V at the top. If you don’t know what you are expecting to see, start with a high voltage setting, then drop the voltage down until the image is displayed somewhere around the middle of the screen without going off the top.

The next is the time base, or how much time is shown across the screen. Different scopes may show this as either the total time across the screen, or as the time of each division across the screen. Around 50ms (milliseconds) per division or 500ms across the screen is a good place to start. Normally a good setting is within a couple of clicks of this, but if in doubt, start at a long time base (1 to 2 seconds across the screen) to check that you are actually picking up a signal, then zoom in by reducing the time to show more detail in the waveform.

The final step and one that seems to give a lot of trouble is triggers. If you run the scope without a trigger the image is drawn on the screen randomly and tends to jump around a lot from side to side making it difficult to view. A trigger is simply an event that the scope needs to see happen before it starts drawing the image. This is often a rising or failing edge, meaning that the scope needs to see the voltage rise or fall past a set voltage point. You can normally set the trigger at different points across the screen, so you see what happened leading up to the trigger point or after it. The most important part about the trigger is that it is used to stabilise the image on the screen.

So, once you have a stable image on the screen, what next? This is where it gets tricky – you must interpret what is on the screen. The easiest way to do this is to compare what is presented with a known good waveform. This could mean having to get another car that you know is good to measure, easy enough if you work for a dealer, but more difficult as an independent garage to have a similar car available. This is where I believe that PicoScope has an advantage over most of the other scopes on the market. Within the menu will be a pre-set to get you close to a good image on the screen and a waveform library of known good or bad images for you to compare with your own, to help you make a decision. Waveforms can be emailed to others with more experience to look at or posted on forums for support.

If you are going to take the next step towards fixing those tricky faults and decide to invest in an oscilloscope, then some training to get to grips with the basics is a must. Then the most important thing is not to hide it away under the bench and only get it out when the going gets tough, but to use it all the time. If you spend time looking at waveforms that are good, you’ll spot the problematic ones more easily.

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