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How does the oscilloscope work?

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The oscilloscope is a measuring device that represents an electrical signal in the form of a curve (most often, variation of voltage as a function of time). There are "single trace" oscilloscopes, also called "mono curves", "double trace" or "four traces", allowing one, two or four signals to be studied simultaneously. This device is mainly used to visualize the shape of one or more signals, rather than to take precise measurements. The most recent devices, however, are equipped with very advantageous performance in the field of measurement.

The oscilloscope's filament and cathode produce a source of free electrons, which grids accelerate and concentrate into a beam directed towards the phosphorescent bottom of a cathode ray tube . This beam produces a spot , which is moved on the X axis by the horizontal deflection plates, via the horizontal amplifier, and on the Y axis by the vertical deflection plates, via the vertical amplifier. The beam therefore appears to draw a continuous line, called a trace . The screen of the tube is squared by a graticule of 10 horizontal and 8 vertical divisions.

Simplified block diagram of an oscilloscope. The signal is presented on the CH1 input (channel 1 ) , then it is amplified (or attenuated) thanks to the VOLTS/DIV setting. The TIME/DIV setting allows you to vary the horizontal sweep speed. The X-POS and Y-POS settings are used to move the trace relative to the axes.

The time base is the circuit that initiates the horizontal motion, or sweep. This circuit synchronizes the system by generating a pulse each time the waveform crosses a certain voltage setting value. The time base switch (TIME/DIV) allows you to choose the scan time of the spot from one vertical division to the next.

Consider, for example, a time base of 1 ms/division and a waveform which repeats itself identically every three divisions. The period of this wave is therefore 3 ms and its frequency 333 Hz.

Just as the time base makes it possible to calibrate the horizontal axis of the oscillogram, the vertical attenuator makes it possible to calibrate the vertical axis. It is therefore possible to take voltage measurements on this axis.

For example, if the gain of the vertical attenuator (VOLT/DIV) is set so that a signal of 10 mV peak-to-peak deflects the spot by one vertical division and if there are 6 divisions between the upper peak and the lower peak of the trace, a voltage of 60 mV peak-to-peak is measured.

The dual-trace oscilloscope allows simultaneous measurements to be made on two signals from two different circuits. To obtain the double trace, either the "chopping" mode (CHOP in English), at low frequency, or the "alternate" mode (ALT), at high frequency, is used.

In chopping mode, the two input signals are applied alternately, for a very brief instant, to the deflection plates, therefore several times during the same sweep. In half-duplex mode, switching from signal A to signal B does not occur until a full scan is completed. Switching from one mode to the other is generally automatic.

Some of the many features to consider when choosing a model include:

the bandwidth or bandwidth of the vertical amplifier, which provides information on the frequencies at which waveforms can be observed without distortion

the rise time of the vertical amplifier, which specifies the time taken by the amplifier to go from 10% to 90% of a vertical variation; at 20 MHz, the rise time should be around 18 ns

the sensitivity of the vertical amplifier, which specifies the value, in voltage, of the smallest signal that can be observed (typical value between 1 mV/division and 10 mV/division)

the time base , or sweep rate ; for a 20 MHz model, the fastest speed is usually between 0.1 and 0.5 µs/division.

The price of a budget 20 MHz dual-trace model is around 535 euros. Better and more sophisticated models, with component tester, PC link interface, etc., cost more than 610 euros, or even more. However, there are good quality "mono-curves" for less than 300 euros, probes and accessories included. These simple models, whose bandwidth is limited to 5 or 10 MHz, are excellent learning tools.

A SINGLE-CURVE MODEL OF AN OSCILLOSCOPE

The illustration below shows an entry-level single-channel (1-channel) oscilloscope, intended for maintenance, school labs, and beginners. Its bandwidth is 10 MHz.

1 channel oscilloscope

We recognize on the left the screen and its graticule, delimiting 10 horizontal and 8 vertical divisions.

The red button, at the top left, acts as an On/Off button (on "On", a small LED lights up) and for adjusting the light intensity.

The two central buttons allow, respectively, the adjustment of the vertical sensitivity (Volts/div) and the horizontal sweep speed (time/div), according to a 1-2-5 sequence. Vertical sensitivity is 10 mV to 5 V per division; the slew rate is 0.1 seconds to 0.2 µs per division.

The sockets at the bottom of the device are used to connect the probes, the maximum input voltage being 250 V rms. The left input is the Y (vertical) input, the middle one is GND and the right one is for external synchronization.

A switch authorizes various coupling modes for the Y input: AC, GND and DC. Another switch is used to choose the type of horizontal sweep trigger: internal, TV (synchronous pulses) and external, when the trigger is caused by a signal on the input provided for this purpose (right base).

A device of this type, admittedly a little outdated, is amply suitable for becoming familiar with the oscilloscope and observing a large number of signals from the experimental setups of a high school student or an amateur!

AN INTERESTING ALTERNATIVE: THE OSCILLOSCOPE ON PC

A 1-channel digital oscilloscope at 12 MHz, which can also be used as a spectrum analyzer and transient signal recorder, all for around 175 euros including probes, is that possible? Yes!

It is actually a "scope" module that connects to a PC compatible computer via the LPT (printer) port. Once the box is plugged in, the computer is transformed ipso facto into an oscilloscope, with an interface very similar to a normal device, the difference being that the commands are carried out with the mouse. What's more, this original solution makes it possible to record the screens obtained on hard disk.

If you have an "old" PC (under Windows 95 , anyway), here's an interesting alternative: recycle it, at a lower cost, into an oscilloscope! The whole thing, it is true, will not be very easy to move, but that is, so to speak, its only fault. (That said, you can disconnect the module and reconnect it to another PC...).

The " PC-scope " is made by a well-known Belgian company and distributed by mail order and specialized shops. It exists in two versions: 2 channels at 50 MHz (about 500 euros) or 1 channel at 12 MHz (about 175 euros).

IS AN OSCILLOSCOPE REALLY USEFUL?

A modern oscilloscope is undoubtedly a fairly intimidating instrument at first glance... Its front panel has an impressive number of adjustment buttons, marked with not very explicit inscriptions. However, there is no need to worry: just read the manufacturer's instructions to master the beast...

The oscilloscope, as we have said, is most certainly the most useful of the instruments available to the electronics engineer, both in the laboratory and in the workshop. Its role essentially consists in drawing a V/t curve, that is to say that of a voltage (on the Y axis) evolving over time (on the X axis). This curve, the operator can visualize it at leisure, in real time, on the screen. We therefore see exactly what is happening in the entrails of the capacitor or the integrated circuit, as if we were giving it an X-ray!

In practice, the oscilloscope will prove its usefulness when used to compare signals at the input and output of a function block, ensuring that these signals agree with those that the we wait. It is thus possible to test a complex assembly, proceeding block by block.

Let's add that a modern oscilloscope is certainly an "oversized" device for a beginner or an amateur, who may never use certain advanced functions. The complexity of the device is therefore more apparent than real, since we can simply ignore the many functions intended for experienced operators.

X/Y

To the right of the POWER button, there is a 2-position button designated XY . This button is in its normal position when not pressed. It is pressed in some special cases, for example to draw the characteristic of a component

X-POS

The X-POS button allows lateral movement of the trace.

HOLD OFF

The HOLD OFF button is used to introduce a delay in relation to the moment of triggering. In most cases, a beginner will be content to leave this setting at a minimum.

MS VT

The TV-separation adjustment can occupy three positions. It is used when you want to work on a television set. Attention! The presence of very high voltages in television sets makes this operation dangerous. It is therefore strictly reserved for qualified personnel. The correct position of this setting is therefore OFF

TIME/DIV

Here is now one of the essential settings of the oscilloscope: the TIME/DIV knob . It allows the sweep time to be varied from 0.2 seconds to 0.5 µs.

If you choose a setting of 0.2 s/DIV, the spot will take 2 seconds to cross the 10 divisions. On the 0.1 s/DIV position, it will only take 1 second. From a value of 10 ms/DIV, the spot is no longer visible in a punctual manner: it gives way to a continuous line, due to retinal persistence.

TRIG

The switch at the bottom, to the left of TIME/DIV, allows you to choose between different trigger options ( TRIGGER ). In general, the correct position will be AC ​​.

The other positions (DC, HF for High Frequency, LF for Low Frequency and ~ for a frequency of 50 Hz) are only used for measurements that are of no interest to a beginner.

The rectangular TRIG LED illuminates when a trigger point has been detected.

AT/NORM

To the right of TIME/DIV, there is a group of buttons that allow you to synchronize the display of the scope with the signal you want to study.

When AT/NORM is not pressed, triggering is automatic. This is the most common position.

If you press AT/NORM, then you use the LEVEL knob to view the signal.

The EXT button is only pressed if the trigger is caused by an external signal presented on the TRIG INP (trigger input). In all other cases, this button should not be pressed.

In summary, the scope is powered up using the M/A button, XY is left in the OUT position (not pressed), HOLD-OFF at the minimum, TV-SEP on OFF, TRIG on AC, AT/NORM on OUT (not pressed), and all that remains is to choose the TIME/DIV setting. So it wasn't so rocket science...

Now let's move on to the lower part of the control panel:

Y-POS

Each channel has a Y-POS setting , respectively Y-POS I and Y-POS II. This button allows, like its counterpart X-POS, to move the trace vertically, up or down. Since it is an AC signal, Y-POS will be adjusted so that the center line of the screen is 0 V.

If two signals are displayed simultaneously, the two settings are independent.

INVERT

When the INVERT button is pressed, the corresponding signal is inverted, from bottom to top, on the screen. Sounds like a gimmick...

CH I

At the bottom of the front panel, there are the BNC sockets for the CH I and CH II inputs . This is where the input signals are connected, using the probes. Small jacks on the side provide additional 0V or GROUND inputs.

VOLTS/DIV

Each channel has an independent adjustment of the vertical scale, namely that of VOLTS / DIV . This is an adjustment of prime importance, on which we will very often have to intervene. Positions range from 20 V to 5 mV per division.

DC/AC/GND

A DC/AC/GND slide switch allows you to choose, for each channel:

  • DC : the input signal is connected directly to the vertical amplifier (this is the setting that is suitable in most cases)

  • AC : a capacitor is inserted, so that DC voltages are blocked, only AC voltages are displayed

  • GND : used to control the 0 V position on the screen

DUAL

In the center, at the bottom of the lower control panel, is a group of three buttons which allow you to choose which trace(s) will be visible on the screen. It is thus possible to obtain 8 different displays: a single signal (CH1 or CH2), both simultaneously, one after the other, etc. Refer to the device manual.

Finally, we still have to see the three functions available at the bottom of the facade, under the screen:

X-MAG

When the X-MAG button is pressed, the horizontal scale is multiplied by 10. If for example TIME/DIV is set to 1 ms/div, the scale changes to 0.1 ms/div.

CAL

These two CAL outputs deliver square signals of amplitude 0.2 V and 2 V at 50 Hz, respectively. These signals are used to verify that the scope is correctly calibrated.

component tester

Some scopes, like this one, are equipped with a component tester , which allows the display of the characteristic of a component. To do this, press the button. In all other cases, this button should not be pressed. Refer to the device manual.

Here we have gone through the controls and settings available. It must be recognized that the abundance of buttons and sliders, frightening for a neophyte, actually hides a relative simplicity when one intends to limit oneself to the most common functions...

USE AN OSCILLOSCOPE

Getting started

We are now going to power up the scope and learn how to use it...

First of all, it should be ensured that all the adjustments are in the correct position; it's a good habit to get into, especially if the device is used by other people. The "correct position" is that indicated in the instructions for use of the device. Most often, buttons are out (not in), slide switches in the up position, and fine adjustments in the middle position.

Now set the TIME/DIV switches to the 1 V/DIV position and VOLTS/DIV to 0.2 s/DIV, which is its lowest adjustment value.

The machine is turned on by pressing the big POWER button. The green indicator LED lights up and after a moment a light spot crosses the screen.

Try the Y-POS I, INTENSITY and FOCUS controls. Adjust these settings so that the spotlight is centered in the middle of the screen. The spotlight should be bright but not dazzling, and as sharp as possible.

Now see the effect when the TIME/DIV knob is moved from the 0.2 s/DIV position to a higher sweep speed. The spot crosses the screen faster and faster.

The VOLTS/DIV setting of channel 1 determines, as we have said, the scale of the vertical axis, that of the volts. Place it on 1 V/DIV: each vertical division then corresponds to a voltage of 1 volt. Make sure that Y-POS I is centered, that INVERT (if your model has this button) is in the normal position, that the AC/DC/GND slider is on AC, and that the three CH1/CH2 adjustment knobs , DUAL and ADD are not pressed. In this configuration, only the trace of signal 1 is displayed.

CALIBRATION TEST

calibration

We are now going to check the calibration of the scope, using the internal CAL source provided for this purpose (it is located under the screen).

To do this, we will first connect the BNC plug of the probe to the CH1 input (we push it in, then we turn it to the right).

The other end of the probe cable splits into two wires, one red and one black, terminated with "crocodile" clips.

The red wire's alligator clip should be connected to the bottom CAL connection, labeled 2V. The black wire's clip is not connected.

This test consists, neither more nor less, of presenting on the CH1 input a square wave signal whose amplitude is 2 V and frequency 50 Hz. Use the VOLTS/DIV and TIME/DIV settings to obtain a faithful representation of the signal , as below:

screen (calibration)

You can fine-tune the display by slightly manipulating the Y-POS 1 and X-POS buttons. Observe the effect (and usefulness) of these settings. Remember that the graduated axes allow you to measure precise values ​​(amplitude in volts, frequency or period)!

USE PROBES

A probe is a coaxial cable (similar to a TV cable), terminated at one end by a BNC type plug, and at the other by two wires, one red and one black, connected to crocodile clips or sometimes to touches.

probe

The BNC connector must be inserted into the scope socket (CH1 or CH2, as the case may be); we push, then we turn. The alligator clip of the black wire must be connected to 0 V or GND. We then use the test probe (or the crocodile clip of the red wire, whichever is more practical) to test the different points of the circuit.

A good way to become familiar with the oscilloscope is to test a known circuit, preferably very simple, for example a 555 mounted as a multivibrator. It is thus easy to compare the result obtained on the screen (waveform, amplitude, frequency, etc.) and that obtained by the calculation. We will then have every interest in varying a parameter (value of R, or of C) to observe its influence.

Once you have understood the basic notions with the display of a single trace, you will make the best use of the possibilities of the device by displaying two traces simultaneously.

scope: 2 traces

Display of two traces simultaneously. It is thus possible to compare two signals.

With experience, the use of the "scope" will quickly become second nature...

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