Using Oscilloscopes on Cars, Part 2

This article was first published in 2004.

Last week (Using Oscilloscopes on Cars, Part 1) we looked at how the changing technology of car systems has meant that in many situations a multimeter is useless for monitoring and displaying signals. Basically, if the signal involves rapid changes (ie a waveform or pulsetrain), you need an oscilloscope to see it.

Just as a digital multimeter replaced the test light, in many instances the scope is now replacing the multimeter. And there's another parallel too. When they were first introduced, digital multimeters were quite expensive, but now they're available for next to nothing. At this stage in their adoption, many digital scopes are also expensive. However, the prices of digital scopes are dropping rapidly and some models are now within reach of the private modifier.

But how do you go about sourcing a scope? They're available as complete standalone units, as adaptors that turn a laptop PC into a scope, or as integrated multimeter/scopes. And what about all those specifications? Which are important?

In this story we'll take a look at the type of scopes useful for looking at the signals racing around in car systems. The sky's the limit in digital scope pricing and functionality, but here we'll stay towards the cheaper end of the range.

Digital Scope Specifications

If you're new to the area, the specifications for different scopes are baffling. So then, what do all those things mean?

• Sampling Speed

As briefly indicated last week, an analog scope effectively draws the waveform as it occurs. However, a digital scope samples the voltages coming into the scope - it isn't continuously measuring the input signal but instead is only measuring bits of it. The waveform is then reconstructed from these separate samples and displayed on the screen. It's a join-the-dots process.

How often the scope samples the signal is known as sampling speed, expressed in samples/second.

All else being equal, the higher the sampling speed, the higher the frequency of the signal that can be accurately displayed. Or to put it another way, the higher the sampling speed, the shorter the event that can be captured.

Let's take a look at an example of what happens if the digital scope has an insufficient sampling rate. Here the input signal waveform has a 4 volt peak-to-peak range with some complex waveform changes. But displayed on the scope is a signal that looks nothing like the original - it's lost the high frequency changes and now has a peak to peak voltage of only 1 volt!

If the sampling speed of the scope is inappropriate for the signal measurement being made, the reconstructed signal shown on the scope may not bear any resemblance to the true signal....

Sampling.... and Sampling There are actually two types of digital scope sampling: Real Time Sampling and Equivalent Sampling. Real Time is the maximum rate that the samples can be acquired for a one-off event, while Equivalent Sampling takes advantage of the fact that most signals are continuously repeated (eg a sine wave) and so better definition can be gained by sampling the signal many times before displaying it. To capture one-off events (eg a very short signal drop-out), it's the Real Time Sampling that needs to be fast. As an example, the Fluke 123 Scopemeter has an Equivalent Sampling speed of up to 1.25 giga samples/second but a Real Time Sampling rate of 5-25 mega samples/second.

• Bandwidth

In addition to sampling speed, the maximum frequency that the scope can accurately measure is also influenced by the scope's input amplifiers and filters. This factor is called 'bandwidth'.

• Memory

The amount of memory that the scope has is also important - especially in automotive applications. Memory (sometimes referred to as Record Length or Buffer Size) is relevant for two reasons:

• The more closely spaced the samples are, the more memory that's required to hold them before a complete waveform can be displayed. In other words, high sampling rates require more memory.

• The longer the length of time over which the waveform needs to be displayed (called the time-base), the more samples that need to be kept if the sampling resolution is to be retained.

In automotive use, where most often quite slow time bases are used, the second point is the more important of the two. For example, when the full width of the screen shows 90 nano-seconds, a scope may be able to sample at 10 mega-samples/second, but if you lengthen the period that you want to display to 9 milliseconds, the effective sampling rate (dictated by how many samples can be memorised) may drop to only 10 kilo-samples/second. In some scopes you can go even further, setting the timebase to hours! In this case you want lots and lots of memory if you're to be able to store what's basically become a data-log record of the signal.

The amount of memory available is also relevant if the scope has the ability to zoom in on waveforms after they have been frozen. In order to gain that extra waveform detail, more memory will be required, especially if you have a long timebase.

• Analog to Digital Converter

In addition to sampling speed and bandwidth, the analog to digital converter (ADC) resolution of the scope is important. Most have 8 bit vertical resolution which limits the voltage variation that can be measured to just under 0.4 per cent. On the other hand, 12-bit scopes can resolve changes in voltage levels of only 0.024 per cent. The following table shows the significance of the analog-to-digital converter that is used.

Oscilloscope resolution Number of steps Smallest change that can be detected 6 bit oscilloscopes 64 1.6% (16000ppm) 8 bit oscilloscopes 256 0.39% (4000ppm) 12 bit oscilloscopes 4096 0.024% (244ppm) 16 bit oscilloscopes 65536 0.0015% (15ppm)

A graphic demonstration of the difference in 8-bit and 12-bit digital oscilloscopes can be seen here:

In the upper section the waveform looks well-shaped but when the waveform is zoomed in on (lower picture), the discrete steps making up the trace can be clearly seen.

In contrast, this 12-bit scope has a much smoother waveform with better resolution. For example, note the 937mV measurement possible here, while the above 8-bit design shows a peak of 923mV.

In addition, it's important to realise that some small digital scopes have less resolution than indicated by the ADC bit number. An 8-bit scope might have an LCD screen capable of displaying only 6-bit data, for example.

• Software

Especially in designs where add-on modules are used to turn laptop PCs into digital scopes, the functionality of the software is important. In addition to scope functions, many of these designs can also act as spectrum analysers (that is, showing on a vertical bar graph the magnitude of all the different frequencies), multimeters (although often with quite limited ranges) and as data-loggers.

All manufacturers of this type of scope allow web downloads of demo, trial or fully functioning versions of the software, so you can play before you buy the associated hardware.

Scope Selection So you now know that you want a digital scope with the highest possible sampling rate, widest possible bandwidth, greatest memory and highest bit-number ADC! Trouble is, that model could easily cost you as much as your car... At the low price end of town scopes are likely to have specs like 1MHz bandwidth and 10 mega samples/second sampling rate. So what frequencies are these useful up to? Digital scope manufacturer Tektronix has two rules: Rule 1: The scope sampling ratio divided by 10 = the highest frequency waveform that can be accurately shown. Rule 2: The scope bandwidth divided by 5 = the highest frequency component of the signal that can be measured. So on the scope mentioned above, the highest frequency waveform that will accurately show is (10 mega samples/second divided by 10) = 1MHz. But with a quoted bandwidth of only 1MHz, this scope is limited to 0.2MHz, or 200KHz. (1MHz divided by 5 = 0.2MHz). So is a 200kHz actual bandwidth adequate for car use? The answer to that is: mostly. A 200KHz frequency has a period of 1/200,000ths of a second, or 5 micro seconds. If you think that 5 micro-seconds is a pretty short time, you're right. In most cars a bandwidth of this sort is required only when looking at ignition waveforms and high-speed on-board communications systems like CAN bus. But even with a relatively slow scope, waveforms for speed sensors, injectors, most crankshaft position sensors and so on will usually appear fine.

Some Scopes

The Velleman HPS10 personal handheld LCD scope costs just US$150. It has a 1Mhz (2MHz with some signals) bandwidth, 10 mega samples/second sampling rate and 8 bit ADC (6 bit on screen). The screen is 128 x 64 pixels. Slightly more upmarket, the company's HPS40 handheld digital scope has a 40 mega samples/second sampling rate and a wider 12MHz bandwidth. The backlit LCD is 192 x 112 pixels and the unit costs about US$290. No buffer memory specs are quoted for either scope.

The Velleman HPS10 is available in Australia from Jaycar Electronics, for AUD$349 - cat no QC-1916. We will be reviewing this scope in more detail in Part 3 of this series.

Pico Technology, a major UK manufacturer of plug-in modules that turn laptop PCs into digital scopes, sells a highly regarded automotive diagnostics kit that uses their ADC-212 scope adaptor. This design has 12-bit resolution, a bandwidth of 1.5MHz and a sampling rate of 3 mega samples/second. It also has a long 32 kilo-sample buffer memory. The ADC-212 incorporates some multimeter functions and can record long term changes. The ADC-212 costs about US$500.

Dutch company TiePie Engineering manufactures laptop PC scope adaptors; their Handy Scope 3 is being used in some advanced automotive workshops. This scope adaptor has much better specifications than the Pico - up to 16-bit resolution, 50 MHz bandwidth and a sampling rate of up to 100 mega samples/second. It has a very long 128 kilo-sample buffer memory and uses a USB link to the host PC. The Handyscope 3 costs about US$770.

Fluke is amongst the best known companies producing handheld digital scopes. Their Scopemeter 123 model has a bandwidth of 20MHz and a sampling rate of up to 1.25 giga samples/second. However its ADC is only 8-bit and it has a very small 512 sample memory. The Scopemeter incorporates multimeter functions and a trend recorder that can record data in line graph form from 120 seconds - 16 days. Its inputs are highly protected and it costs about US$1200.

Conclusion

Even the cheapest handheld digital scope will show you the waveform of most car input and output signals - so straight away you'll be ahead of what you'd be seeing with a multimeter. However, as you go higher in specs - especially in sampling rate and bandwidth - you can be more confident of seeing a better representation of the original waveforms. But one thing's for certain - the price of digital scopes is sure to keep on falling and so it's only a matter of time before all people modifying cars use a scope almost as much as they currently use a multimeter.

Next: working with scopes on cars

Velleman www.vellemanusa.com

Pico Technology www.picotech.com

TiePie Engineering www.tiepie.nl

Fluke www.fluke.com

Used? Digital scopes are a fast-developing area of technology, so is it worth looking secondhand? In some cases yes, and in other cases, no. Plug-in laptop PC adaptor scopes like the Pico and TiePie systems rarely come up secondhand. However standalone LCD-based handheld instruments like the Fluke range of Scopemeters can be easily found on auction sites such as www.ebay.com. That's the upside - the downside is they tend to hold their value fairly well! One advantage when looking used is that a scope that's regarded as a bit slow for pure electronics work (eg 10Mhz bandwidth) will work very well in automotive applications. So if you want a handheld digital scope, it's certainly worth chasing the used option.

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