The oscilloscope.

What is an oscilloscope?

The oscilloscope is a very useful piece of test equipment. It is a measuring device (if you paid many thousands of pounds for it), giving accurate measurements to a high degree.

It is also an indicating device, (with a more modest budget) giving a good indication (with reasonable accuracy) as to what is going on.

For the purposes of this article, I will use the terms “measuring” and “indicating” interchangeably.

So what can it measure?  The simple answer is “voltage”, and time, although modern equipment can have an on-screen display of other things, like frequency.

The first thing that you would notice is the screen, which has a grid superimposed on it. (See figure 1). This grid may, on older equipment, have consisted of 1cm squares. This could then give rise to a setting of, say, one volt per centimetre. That is to say, the electronically generated line going across the screen would move upward one cm for every volt of input signal. (if the signal was positive). Equally, a negative one volt would cause the line to move downward.

Figure 1. No input signal 

An oscilloscope would normally be supplied with a number of “input sensitivities”. i.e. 1mV per cm, (1mV/cm) 10mV, 100mv, etc, up to maybe 50V/cm. So if you wanted to measure a 7.5Vdc, and you had a range where it was 5V/cm, you would select this, and expect to see the line move either 1.5 grid squares up, or two down, according to which way around you connected the voltage. Note that modern screens may have a different physically sized grid, but it is still in the main referred to 1cm, so although the actual grid may be half that on a small screen, it would still have a switch that is set to, say, 5V/cm, and the line would move almost two squares.  (See Figure 2).

If you measured 7.5V from a battery as above, the line would move, and it would stay there forever. 7.5V DC does not vary, it is 7.5V now, and in an hour’s time (or whatever). So the oscilloscope is giving you a good indication of two things:

                             1) The voltage.

                             2) The fact that it is DC. 

Note that the screen has vertical lines as part of the superimposed grid. Again, nominally at 1cm intervals.  These represent TIME. And there will be a switch (the “timebase”) that can select 1mS/cm. That means that the line (which in older equipment was drawn by a moving “dot” going left to right) took 1 millisecond to move 1 cm. (Or square).

Given the above, the oscilloscope can show what the voltage was at a particular time. In the case of the DC example above, the time made no difference, the battery supplied a continuous, steady 7.5V, with the line deflected up, or down as appropriate.

Figure 2.

Figure 2. Here, the trace across the screen has gone up 1.5 squares. As the oscilloscope is set to 5V/cm, this represents a DC voltage of 7.5. Note that in reality, this is a very small screen, and the lines are less than 1cm apart. It still holds good that the oscilloscope is set to 1.5 squares per volt.

But what happens if we put an alternating current (AC) voltage into it? We will see a “wavy” line that goes up, then down, then up etc. as it goes across the screen. (See figure 3). You now have a good indication of four things:

1) The maximum positive voltage that the wave goes up to.

2) The maximum negative voltage that the wave goes down to.

3) The fact that you are looking at a signal that is AC. 

4) The time taken to complete one complete waveform. (i.e. from any particular point on the wave, to the next identical point further to the right).

With the knowledge gained from 4), you can then work out the frequency. Let us suppose that you have the timebase switch set to 1mS/cm, and you have “quite a large” screen, upon which you can just fit in one complete wave. If that screen has enough vertical lines to show 20 squares (21 vertical lines), then that complete wave has taken 20mS to make. And that is one fiftieth of a second. So every second, you would get 50 of these waves. And that wave therefore has a frequency of 50Hz, our mains electricity frequency. In figure 3, each square is 5mS wide, and each complete wave has taken four squares, or 20mS. So it is a (rough) 50Hz waveform. (Notice 5mS in the small blue rectangle at the bottom of the screen, this is what the timebase has been set to. (Per square).

As you can measure the time taken, and also how big the wave is from the highest part to the lowest, you now know its frequency and voltage characteristics.

Figure 3.

Figure 3. This is an example of an AC waveform. If you merely touch the centre of the probe, with the sensitivity set high, your body is picking up the signals your body has collected! Here, it is a very rough 50Hz signal. Remember touching the input to an audio amplifier and getting a loud hum? Well this is the signal you gave the it!

Uses for an oscilloscope.

The oscilloscope can do so much more, even a very simple one. Suppose you look at a waveform, and it has nice curves at the input of an amplifier. You are checking, because the amplifier is producing distortion. As you work from the input of the amplifier, towards its output, you would probably be looking at the collector connection of the transistors. And if your waveform suddenly looked a lot different from that you are applying to the input, you have found the part of the amplifier producing distortion.

 From there, a simple multimeter will probably show that the DC voltages around that transistor are not what they should be, and it becomes simple to find what has gone open/short circuit, or with an ohm meter, check that a resistor has not become markedly different from the value it should be.

As a further example, an amplifier could be suffering from low gain. Again, working through the circuit observing waveforms will show which transistor is not making the signal bigger, and from there, maybe a coupling capacitor, or an emitter resistor bypass capacitor has gone open circuit, or as a result of “drying out”, has become much less in value that it should be.

The oscilloscope therefore allows you to “see” signals in a way that a multimeter cannot.

As this article started with mentioning equipment costs, it may surprise you to know that an oscilloscope capable of reasonable performance at audio (and just beyond) frequencies may be built from a (Chinese) kit for less than £20, including a case! Most of the time, this is what I use in preference to my somewhat more comprehensive oscilloscope .

 Meet the “DSO138 Mini”.

There are currently several versions of the kit available, and also beware that there is a “DSO138” which is not the same, and does have a few issues. It is an older design.

For the DSO138 Mini, the latest one is to version “J”. Check the pictures in this document,  where you can see the “range switches”. These will either appear as slide switches that are ABOVE the display, (“H” version) with the switch toggle facing you, (and if in a case, there will be switch extensions in use), or the switches will stick out the rear, with no extensions, your fingers move the switch toggles directly. (“J” version). The DSO138 Mini pictured is the “J” version.

  Another clue: “H” versions use switches whose pins stick out the opposite side from the toggle. “J” version switches have pins that go out 90 degrees to the toggle. Also, there are two trimmer capacitors at the rear of the board, behind the switches, green on the left, blue right for the “J” version. On the “H” version, both capacitors are green, mounted one behind the other, and between two switches… Useful to know if you can only see certain details on ebay. Both versions are available, I recommend the “J” version

The kit is supplied as two printed circuit boards, and a number of basic standard components. There are surface mounted components on both boards, already soldered (“welded” in “Chinglish”!).

First thing to do is plug the display board into USB. (It uses a micro USB connector). This is important, you should see the whole thing boot up (start), and get a display that looks like figure 1. If this does not happen, then prove your USB lead with something else, and if the lead/5V supply is OK, you must return the whole kit for replacement. I am not aware of anyone having this difficulty though.

The supplied instructions are quite good, and in keeping with most electronic construction, start off with placing the smaller components first. Of course, the instructions are not perfect…

This is a very easy kit to assemble. Note however that the case is a swine to assemble. But it does fit, and protects the finished article really well.

A small jack is supplied for a battery to be connected, see later. An identical jack is supplied to connect the input signal, along with a plug, and a couple of test probes. I do NOT recommend you fit this jack (J1), opting instead for the BNC connector. You will then be able to use a standard oscilloscope probe. Since this oscilloscope only works to 200kHz, the cheapest probe you can buy is way more than adequate. On ebay, you can get two for less than a tenner!

The supplied BNC connector (check your kit for its inclusion before purchase) is a large piece of metal. You do need a fairly powerful soldering iron to attach it to the board. (It is better to have a powerful iron that can supply a lot of heat rapidly. A quick joint reduces the chances of heat damage. Leaving a smaller iron in contact with the board will eventually heat many things up, which is not good…).

A small iron is necessary for all the other components, however. What temperature you set and type of solder is down to choice, I prefer leaded solder, and set 272 degrees Centigrade.

The components. 

Most components prove no difficulty. But even though I have been “into” electronics for 60 years, I struggled with the resistors in this kit. The makers do suggest you “meter” the resistors before soldering. I had to…

Identifying the resistors.

The reason for the resistors being hard to identify is that in the first instance, they are VERY small. Also, some of the colours are indistinct. In addition, they are “five band” types. That is to say, there are four colours indicating the value, and a “tolerance” band.

The supplied resistors are to a tolerance of 1%. That means that the tolerance band is brown. And it is at one end of the resistor. Some values also start with a 1. Also brown. Which means that you have some components with a brown ring at each end of the resistor. 

Some 1.1k (1k1, or 1100 ohm) resistors, 1%, are used.

Brown Brown Black Brown Brown. Whichever way you start, it reads the same. 

For the remainder of this guide, the “multiplier” in resistor values will be used as the decimal point, in keeping with current practice. That is to say 1100 ohms is actually 1.1 X 1000, or 1.1k The decimal point is easy to miss. Written as 1k1, it is very clear.

Examples:

R = When behind the number, the number is the value. (390R = 390 ohms). When used in front of the number, denotes a resistor value less than 1. (i.e. 0R56 = 0.56 ohms. 0R1 is 0.1 ohms.).

1k = 1000

1M = 1,000000

You may have heard the term “preferred value”. This is the range of values that resistors are commonly manufactured to. You will not be able to buy a 132 ohm resistor. (Unless you have them specially made). But you can buy a 130 ohm resistor, and you can buy a 100k resistor, both are “preferred value” resistors.

Brown Black Black, Orange, Brown   =  1  0  0    000    1%   

Now read it “backwards”… 130  ohms, 1%. Unfortunately, this error results in a preferred value, but it is wrong.

By now, you can probably see why the kit maker suggests measuring them first ☺

R1, R13, 100k. Brown Black Black, Orange, Brown.

R2, 1M8. Brown Grey Black Yellow Brown.

R3, R15, 200k. Red, Black Black Orange Brown.

R4, 2M, Red Black Black Yellow, Brown.

R5, 20k. Red Black Black Red Brown

R6, R14, R17, 300R. Orange Black Black Black Brown.

R7, R11, 180R. Brown Grey Black Black Brown.

R8, R12, 120R Brown Red Black Black Brown

R9, R10, R16, 1k1. Brown Brown Black Brown Brown

Chokes.

L1 and L2 LOOK like resistors. Close inspection will show that they are different and, have four coloured bands. Brown Black Brown silver,  (100uH, 10%).

Capacitors.

Straightforward, the ceramic capacitors are identified as follows:

C1, 0.1uF. Marked 104  (10 and four more zeros = 100,000pF or 0.1uF).

C2, 220pF. Marked 221 (22 and one zero = 220pF).

C3, 3pF. Marked simply “3”.

C5, H version only, not recommended, 1pF Would be marked simply “1”.

C7, J version 500pF. Marked 501 (50 and one more zero = 500pF). H version 120pF, marked 121.

Polarity sensitive components.

These include a diode, LED, and electrolytic capacitors. The instructions are quite clear, so no difficulty here.

When mounting parts, don’t throw away all the little wire ends, you will need to make a small loop of wire. This is the “test loop”. When you want to use some form of test equipment, it is always useful to know that it is actually working, like seeing zero ohms on a meter by touching the probes together. If you connect the test probe to the test loop, you will see a signal that the oscilloscope itself is generating. If you cannot see this, something is wrong… If you can, then you should be able to see any signals that the equipment you have under test is producing. See figure 4 for an example of the test signal. It is a square wave, and what you would expect to see if trying to measure square waves.

Figure 4. This shows the test signal. The probe (a black “thing”) is connected to the test loop at the top right of the image . This signal is a square wave at a frequency of 1kHz,  and approximately 3V3. (3.3 volts). When looking at the screen, the waveform was complete to the naked eye, but the speed of the photograph has picked up the fact that the trace is not yet complete, the right half not yet having been electronically redrawn.

Final checks and first power up.

When you have completed the lower circuit board, check, and double check that you fitted everything where it should be, and all joints are soldered, with no short circuit solder “splashes”. At the time you were fitting the resistors, making sure they are all round the right way (electrically unnecessary) is useful. i.e. the first band is to the left. (Or right, your choice). Likewise, the first band up, or down. When you are satisfied all is well, plug the display board onto the top of the analogue board that you constructed.

There are a number of tests that can be made with a digital multimeter, and the voltages that are to be expected are all mentioned clearly in the instructions.  If all these voltages are correct, you can be reasonably sure it is working OK, and move onto calibrating the probe. The instructions are quite clear on this, and involve adjusting the variable capacitors to see a nice “square wave” (the test signal). If the edges of the waves are either rounded, or have a “spike”, adjustment is needed. In high end equipment, the adjustment is part of the probe itself. The test signal will also be provided on higher spec equipment, all oscilloscopes have it.

The controls.

When making adjustments to the oscilloscope, the rear mounted switches are involved, as are the buttons below the screen. For the buttons, there is a “SEL” button, and this is used to select the different screen settings available. Looking very carefully, you will see that when a setting has a blue box around it, (or, for the smaller icons, they have changed to blue) the other buttons are used to alter that setting. 

From left to right, at the bottom of the screen, the first two icons cannot be selected, as they show the settings of the rear switches. 

The timebase setting determines how much time is taken for the trace to move one square left to right. The higher the frequency you are measuring, the lower you will need to set this, and vice-versa. It can be set from 10uS to 500S (500 seconds. Very slow!)

The “trigger” of an oscilloscope is essential to a stable display. The idea is that the trigger is set to a specific voltage level by the user, and then when the incoming waveform reaches that value, the oscilloscope is “triggered”, and begins to make a measurement.

Auto trigger will make the oscilloscope trigger, even if the trigger conditions set are not being met. 

Norm trigger gives a display only when the conditions are met.

Single will allow a single trigger event and then ignore further signals. (This can be useful if you want to see the start of something, when further signals follow).

Trigger direction. This determines whether the trigger occurs as the voltage rises, or falls. i.e. if set to rising  (the icon looks like a flat “S”, with a horizontal line going through it), if the trigger is set at 1V, then the trigger will occur as the voltage rises to 1V, but will not trigger if a signal drops from, say, 2V down to 1V. The reverse applies when the trigger is set to falling. (The icon looks like a flattened “Z” with a horizontal line going through it).

The timebase in an oscilloscope is mainly used set to “INT”. That is to say, the timebase used is being generated by the oscilloscope itself. If set to “EXT”, an external signal can be used as a timebase. Probably only useful for more complex measurements. Possible on the DSO138 Mini, but a modification is required.

Bottom right of the display is the trigger voltage level, referred to above. This cannot be selected, but to the right of the screen is a very small square that will change colour to blue. This can be selected by the “SEL” button, and then using the “+” and “-“ buttons, the trigger voltage level may be altered. The new value will appear in the bottom right of the screen.

This oscilloscope is actually a storage oscilloscope. It can store 1024 readings, and using the “SEL” button to access the bar along the top of the display, the “+” and “-“ buttons may be used to see the waveform before and after that displayed on the screen.

Finally, using the “SEL” button once more, the very small square on the far left of the display may be selected, and this allows the display to be centred on the vertical axis using the “+” and “-“ buttons. Mostly, the display with no input signal would be adjusted so that the horizontal line was always in the middle of the screen, although advanced users may have a reason to offset it vertically from centre.

The rear switches.

These switches set the sensitivity of the oscilloscope., and select the type of signal expected.

The “DC AC GND” switch selects whether the input signal is expected to be AC, DC, or to internally connect the input to ground for calibration purposes. If AC is selected, then a DC signal (from a battery for example) will not cause the display to deflect vertically. This is useful when you wish to see a signal on, say, the collector of a transistor, when there is also a DC voltage. Particularly useful when the signal is a few millivolts, but the collector has 6 volts or so on it.

DC. Here, you can measure the voltage of a DC signal, and if there is a small AC component, this will be seen. Effectively, the only difference between using “AC” and “DC”, is that on AC, a capacitor is switched in series between the probe, and the oscilloscope amplifiers.

The remaining two switches set the sensitivity of the oscilloscope. The results of these settings are shown on the screen, bottom left. One switch is marked “10mV, 100mV, 1V”. This sets the amount that the trace will move vertically, and if, say, 100mV is selected, then if a 100mV signal is applied, the trace will move one square up (or down). 10mV needs a smaller signal to move one square, and 1V needs a larger signal.

The other switch is a “multiplier” switch. Assume that the previous switch is set to 1V. If the multiplier switch is set to 1V, then it will take an input of 1V to move up or down one square. If set to X2, it will take 2V to move a square, and X5, 5V per square.

Note that there is a maximum voltage that may be applied to the input of any oscilloscope, which is 50V for this device. High end machines can handle larger signals.

Note that it is easily possible to obtain a “X1  X10” probe. The cheapest will be X10 only. That means you already cut the input signal to 1/10 of its amplitude BEFORE it got to the BNC socket on the oscilloscope itself. Using such a probe means that all measurements need to be multiplied by 10.

i.e. You set the switches to 1V per square, and the other switch to X2. The oscilloscope itself will need an input of 2V to make the trace deflect vertically by one square. If you then use a 10X probe, you will need a 20V signal to cause the same deflection. This oscilloscope can handle a signal up to 50V without damage. But what about the probe? Be careful what you measure. My probes can handle 500V. I have directly connected to the 230Vac mains and observed the waveform. Having set my probe to X10, I was putting 23V into the oscilloscope.

The information provided with the kit is quite good. As an example, it shows how you can turn on the “on screen” display by selecting the timebase, and then holding down the “OK” button for three seconds. Various waveform parameters will be displayed, including frequency!

Modifications.

As a small portable oscilloscope, it is a great, and very inexpensive addition to your test equipment. As supplied, it will run when powered by a 5V USB supply. It can be run from a battery, however. Small 3.7 Li-on batteries are available that will fit inside it, and this will require the purchase of a battery charge controller. This is known as a “JYE118” and is available from Banggood. It sits on a small header on top of the analogue board. (Tip: solder the header to the JYE118, and then solder in place onto the mainboard).

The battery connects to J6 on the mainboard, (install that facing inward). JP4 should NOT be linked with solder. With this configuration, the oscilloscope will power up on the battery when switched on by SW4. However, when connected to USB power, the oscilloscope will run regardless of SW4, and the battery will receive a charge.

 If you want the oscilloscope NOT to run while being charged, then remove D2 from the underside of the display board. This is a factory fitted diode, surface mount. Then, you will always need to switch it on using SW4, USB or battery.

Troubleshooting.

Not a lot that I can add to the information available on the manufacturer’s web site:

Quite an interesting read!

Conclusion.

I hope you have as much fun making it as I have. It certainly works OK, and for me, if I make a long distance European trip in my classic car again, it will be going with me, as if I run into trouble, it could be invaluable in tracing signals from the various engine sensors.

It is also very useful checking audio signals as previously mentioned, and no doubt will find many uses.

73 de Stan, G4EGH.

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