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This article was first published in 2004.
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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?
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....
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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.
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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'.
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) |
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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.
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.
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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.
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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.
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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.
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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
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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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