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OSCILLOSCOPE: oscilloscope icon Deepen

with some EXAMPLEs

This part of the guide (or tutorial or primer how they say) is dedicated to beginners, to better understand situations that may happens while using the oscilloscope.

Warm up time

Each instrument starts to warm up when turned on, only after the warm up time is elapsed the measuring errors will be within the declared tolerance limits. This is as importan as higher is the accuracy. The oscilloscope too warms up but, as hinted in introduction section, since its accuracy is not very high the warm up time also does not assume great importance. The only noteworthy thing, really obvious, refers to the 15 or 20 seconds needed to the warm up of cathode into the CRT (cathode ray tube). Like into the old valve radios and old televisions, it begins to emit electrons only when it comes up to its right working temperature. Then at power up be patient.

Click to zoom Triangular wave Peak and peak to peak measurement

As it seems, the peak to peak value indicate the maximun vertical range of a waveform that is the difference between extreme levels (max positive and max negative). The peak value instead is the half since it indicates the maximum value from zero. The peak to peak value of triangular wave in the picture is two divisions. Supposing to have setted a 1V/Div the resulting value is 2 Vpp (peak to peak) which of course means 1 Vp (peak). This concept applies to every shape of waveform signal.

Coupling of entering signal

It can be a direct current coupled (DC) or an alternating current coupled (AC).
  • AC coupled means that the input signal pass through a (series) capacitor which blocks the continuous component. So just the alternating part can pass to the input or better still the quick variations given that it is created a high frequencies pass filter.
  • When DC coupled instead means that the input signal enters directly, without any filter at all, then pass through and it is observed the signal continuous component too.
Suppose now to observe the ripple signal outgoing from a power supply. This is usually a little oscillation superimposed to a continuous voltage which compared is much bigger. For example could be a 0,05 Volt peak to peak signal placed on top of a continuous 12 Volt. With DC coupling and setting a 2 V/Div it is drawn a line that stay 6 division tall from zero but the ripple is just observable and certainly not measurable.

To be able to measure the ripple I have to change the range for example at 20 mV/Div but the trace would go over the top of screen so forcing me to AC couple the input signal.

When the de-coupling begins it happens a sudden movement of trace that in a short time settles around the zero. It is due to the charging of the coupling capacitor which has to reach the continuous voltage to be blocked. After that, moving the probe from 12 V to ground (the zero) happens the opposite phenomenon due to the discharge of the same capacitor.

How to make measurement easier

To perform measurement the X and Y potentiometers are handy to move the trace so that crosses exactly a grid's line and that become the starting point to count divisions. Have a better explanation watching the picture. Click to zoom Measuring a distorted sinusoidal waveform To measure the peak to peak amplitude "V" of that signal move the Y position so that the lower peak leans on a grid's row. Next move the X position to center an upper peak on the graded vertical grid's line (the central one). Now it is easily readable the "V" amplitude of 4 divisions and two fifth. Supposing to have 1 Volt per division the signal value is 4.4 Vpp.

The same method applies to "X" temporal axis. Move the trace horizontally to cross a vertical line in a steep zone of the signal, as steep is as high the reading accuracy is. Now move the Y position to bring the point at the graded X axis and read the "T" duration that is three divisions and three fifth. If time base is 2ms/Div the period become 7.2ms, it is not hard, is it ?

Last advice, in the example there are on screen two whole periods and more, then is possible to halve the time base to 1ms/Div so that a period fills the double (in picture the two "T" added together). In such case the reading has better accuracy, it is easier to read seven divisions and a fifth. Of course the result will be 7.2ms also in this case.

Rise and fall times

To measure rising and falling times of a step signal we have to know their definition, that is the time to travel from 10 to 90 percent of the amplitude. This excludes overshoot and undershoot from measurement because they are not connected to rise and fall times but for example to reflections of a signal, unstabilities of a circuit and so on.
So it is about a transition time measurement between two voltage levels defined as ratio compared to a peak to peak value, therefore it is not essential to know the absolute amplitude.

Measurement execution

It is handy to uncalibrate the gain to bring the peak to peak amplitude between +2.5 and -2.5 vertical division respect the central zero row. Indeed just there on the grid we find the two signs for 0 and 100 percent. Then of course the +2 and -2 divisions are respectively the 90% and the 10% of the peak to peak value. Now, where they crosses the signal define the two measurement points for the rise or fall time. Note that the time base must not be uncalibrated.

Rise time multiplied by Bandwidth = 0.35

Multiplying the bandwidth by the rise time of an analog oscilloscope will get the 0.35 constant. The right one would be 0.339 but in practice is used 0.35 because it is easier to remember and introduces a little tolerance margin. To give an example for a 120MHz bandwidth oscilloscope applying the formula
Rise time = 0.35 / Bandwidth
results : T = 0.35 / 120 Mhz = 2.91 ns
This means that the instrument allows to observe and to measure signals with rise time bigger than 2.9 ns (that means just the slower ones, see note). However we have to keep in mind that it is a method to give an idea of where the limit is, not an iron severe rule. Indeed in the above example, a rise time of 2.95 ns rather than 2.80 ns does not mean that in one case we see very well and in the other we see nothing at all. It is due to the fact that analogue oscilloscopes have a gaussian frequency responce.
To understand where this formula came from you can read at this link "Base RC circuit", near the bottom you will find the answer. At the moment it is in Italian language but the formulas are understandable.

In modern digital oscilloscopes instead (known as DSO) things are changed. Here the constant is between 0.4 and 0.5 but it depends on make and model, then must be necessarily declared by manufacturer. However generally talking, from this point of view the digital models are better than analog ones.

Note: Be careful to never confuse the rise time with the frequency of a signal. They are two well different measure unit not necessarily bound one another as instead the repetition period that is always the inverse of frequency. For example it is possible to have very low frequencies with very fast rise times (thinking to square waves).

Analog and digital oscilloscope

The memory

It is obvious that storage oscilloscopes are very handy to observe non-repetitive events. Conceptually does not matter what kind of instrument is, analogue or digital it is important that has the memory.

The trigger explanation has depicted the three possible modes, AUTO, NORMAL and SINGLE. Well, between them the SINGLE mode is perfect to establish the moment when store a scan.

Certainly modern digital oscilloscopes are easier and powerful to use but the basic concepts do not change.

Analogue or digital

Into digital oscilloscopes is implicit the presence of memory. However must be aware that signals are sampled and the sampling frequency can produce a beating with the input signal frequency. To cut a long story short practically this means that for example a high frequency sinusoidal signal when observed with slow time base could be shown as a slow frequency sine. Then it leads into error if not aware of the problem. The analog oscilloscopes with memory do not have such problem but by now are rares and expensives. I am afraid that there are not more on the market. However the storing was directly on the visualizing screen by analog way. To conclude the digital oscilloscopes have supplanted the analog ones, indeed the former are cheaper and with better performances.


At the end if you have
  • corrections to point out
  • other questions about oscilloscope
  • broadening suggestions to this guide
  • and more...
write to me at the following address bbaba at tiscali dot it and you will have my gratitude beside my answer. That could be used for example to realize a F.A.Q. section.

PS: To do that you have to change the words "at" and "dot" with corresponding symbols. This trick helps to reduce spam creating more problems for programs made to search e-mail addresses.

internet site = http://www.bbaba.altervista.org
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Last modify : January 2, 2010