Oscilloscope. Basics

I will be maximum consecutive; only the most significant will be mentioned in this entry!

What is the oscilloscope? Which oscilloscope to buy?

An oscilloscope is a tool, which draws a graph:

a. X (horizontal) axis – time;

b. Y (vertical) axis – the value of the signal at the current moment of time.

Sounds simple? Just that simple, the oscilloscope works! Everything else – additional “features” for the user to see the exact moment, the exact abnormality of the signal in a more convenient and transparent way.

First (analog) oscilloscopes used the electron tube. Hardware managed the flow of the dots (electrons):

a. Horizontal deflection “moves” the dot from the left side to the right side; when it reached, swiftly “returns” to the initial point, and a new cycle starts;

b. Vertical deflection “moves” the dot up/down, depending on the signal value at the current moment.

The most simple oscilloscope has two basic management buttons:

a. with one button (time/div), the time units of measurements are changed. For the slowly changing signal, the time interval, which is seen on the screen, can be “stretched”; for a swiftly changing signal – the time can be “compressed.” 

b. with a second button (V/div), the amplitude of the measured signal can be changed. By increasing the sensitivity, it is possible to see signals with a smaller amplitude; when decreasing the sensitivity – see “more powerful” signals.

Why did I talk about history? Little patience, please! From the first analog oscilloscopes, we swiftly move to the first digital oscilloscopes.

First digital oscilloscopes tried to repeat the working principle of the analog oscilloscopes. The memory buffer of the tool corresponded to the resolution of the screen. For example, if the screen resolution was 600×800 dots, the “length” of the oscilloscope memory was 800 points. Sounds logical – each dot of the screen (horizontally) has its memory cell! The value of the signal recorded in each cell – a full-fledged “image” appears on the screen. Where the problem hides?

The switching speed of the analog oscilloscope’s horizontal deflection (as strange as it may sound) actually was impressive – during a second, they redraw the “graphic” for several thousand, even tens of thousands of times. Their Horizontal deflection managed to move the dot from left to right at an impressive speed!

Hardware possibilities of the first digital oscilloscopes were limited – they were redrawing the image for only several tens of times during a second. Also, the reaction ability of the vision is not unlimited – there is no point in trying. But – is it so? Why this slow refresh rate is a problem?

When looking at the swiftly changing signals, in the thousand part of the second, the image is “drawn” and outputted on the screen of the oscilloscope. Excellent! But after this “heroic” work comes to the pause, when the hardware “takes a rest” – performs different service procedures; serves the keyboard, or simply waits. Till the next procedure of the screen redrawing, a long time passes. The main problem – if any important event happens during the time of the “pause,” the oscilloscope will not show it!

So:

a.the analog oscilloscope displays the signal for the longest part of the time (for only a short time – when the Horizontal deflection moves the beam from the “end” to the “start” – it is “offline”);

b. in simple digital oscilloscopes, the “offline” time can be for even 99,99% of all time!

Note: in more expensive analog oscilloscopes, the analog “memory” was created for the diagnostics specialist to better notice swiftly changing signals. The electron tube was coated with a luminescent coating, which continued to glow for several seconds after its first lighting. Even if the signal appeared for a concise moment (mili or even microseconds), for several more seconds, it was lighted on the screen. Wisely!

Why long offline time is a huge problem?

If the signal under study is periodical (stable, repeats regularly) – it is enough to check only a small part of the whole signal. If we see stage 12 of the signal, stage 13, 53, 455 will be the same. Unfortunately, this is the simplest case. Most often, the defects of the signal are sporadic – it means they suddenly appear and just as unexpected disappear!

In the area of car diagnostics, sporadic problems count the most part of all cases. Slipup of the sensor. Wire short circuits or bad connections – all these problems hide very well!

A quick example: let’s imagine a typical problem of the ABS sensor. One of the sensors sometimes sends not really the correct signal, so the DSC module thinks that some of the wheels are slipping. There are no error messages in the DSC module; DSC does not recognize such a jittering of the sensor signal. Will we notice such a defect with a simple digital oscilloscope?

Unlikely! For example, if 1 of 30 tooth wheel tooths is damaged, and the oscilloscope for “only” 99% of all time is offline, the possibility that this defect will be noticed is 1/3’000. If the defect manifests only in each 100-the turn of the wheel, the possibility that this defect will be noticed reduces to 1/300’000!

With such a cheap digital oscilloscope, you can observe the signal for hours but will not even notice the defect!

Yes, the manufacturers of the oscilloscopes are aware of this problem. As soon as the hardware possibilities allowed, the best manufacturers radically changed the way, how the oscilloscope work. What was changed?

a. much larger memory buffers were introduced in the oscilloscopes; for example, 1M points mean that the data of 1000(!) screens, not of only 1, are stored in the oscilloscope memory! So – the oscilloscope works also as the data storage unit/logger. At any moment, the user can press the Stop button and stop the recording of new data to view the information from the unit memory. There is a 1000 times larger amount of data at the disposal of the user as it was previously! This feature is described by the parameter “memory depth; M points”;

b. offline time is reduced to zero; it means l the large memory buffer is used by the FIFO principle. The situation that/when some data are “lost” is not possible anymore! 

c. the software takes care that (not taking into account the limited display and reaction time of the human vision), any “non-standard” changes of the signal will be seen very well. The software is especially highlighting these changes, paying attention to the diagnostics specialist to them. This ability is (not directly) described by the parameter ”waveforms/sec”. Not taking into account the physical redrawing of the screen for several tens of times in seconds (and also the inertia of the human vision, which reaches tens of seconds), the best oscilloscopes are able to display even 100.000+ waveforms/sec!

Here, a demo video of Agilent:

The idea of the test: (only) each 40’000th signal has a defect. Comparing different and their settings, it is easy to make sure that even powerful oscilloscopes can fail. The competitor of Agilent is a powerful and expensive oscilloscope, but even it failed. Cheaper “simple” digital oscilloscopes would not notice such a defect at all!

The essence of this entry: if you are looking for an oscilloscope, which can do more than detect if there is a signal/there is no signal, critical parameters (they should be): 

a. memory depth (>1M points);

b. waveforms/sec (>10.000/sec).

Cheaper digital oscilloscopes are characterized by:

a. memory depth 1..4K points;

b. refresh rate: 10..30 waveforms/sec.

As we see, both the memory and the refresh rate is for some 1000(!) worse. Unfortunately, these oscilloscopes are of low value. An interesting obstacle – for example, in the price range of 200 .. 300 EUR, you can find the USB oscilloscopes with a memory of 8..16+M points, but the most part is with the memory of 1..4K points and prolonged refresh rate. It is your choice – to buy a useless toy or a valuable measuring tool.

An example. USB oscilloscopes of the cheapest range of Picoscope.

https://www.picotech.com/oscilloscope/2000/picoscope-2000-overview

Here, in the configurator choosing 25MHz, spending 198 EUR, you can get an oscilloscope with a memory depth of 16K points;

Choosing the band of 50MHz, spending 299 EUR, you can get an oscilloscope with a memory depth of 32M points.

The memory buffer of the second configuration is 2000(!) times larger!

Yes, the first configuration will do in the case of simple applications, but for diagnostics – the second configuration is fundamentally the best choice.