The article will describe in detail how to use an oscilloscope, what it is and for what purposes it is needed. No laboratory can exist without measuring equipment or sources of signals, voltages and currents. And if you plan to design and create various devices (especially when it comes to high-frequency technology, for example, inverter power supplies), then it will be problematic to do anything without an oscilloscope.
What is an oscilloscope
This is a device that allows you to "see" the voltage, or rather, its shape for a certain period of time. With it, you can measure a lot of parameters - voltage, frequency, current strength, phase angles. But what is especially good about this device is that it allows you to visually evaluate the shape of the signal. Indeed, in most cases, it is she who says what exactly happens in the circuit in which the measurement is carried out.
In some cases, for example, the voltage may contain not only a constant, but also an alternating component. And the shape of the second may be far from an ideal sinusoid. Voltmeters, for example, perceive such a signal with large errors. Pointer instruments will give out one value, digital ones will give out much less, and DC voltmeters will give out several times more. The most accurate measurement can be carried out with the help of the device described in the article. And it doesn’t matter if the H3013 oscilloscope is used (how to use it is discussed below) or another model. The measurements are the same.
Device features
To implement this is quite simple - you need to connect a capacitor to the input of the amplifier. In this case, the entrance is closed. Note that low frequency signals below 5 Hz are attenuated in this measurement mode. Therefore, they can only be measured in open input mode.
When the switch is set to the middle position, the amplifier is disconnected from the input connector, and a short to ground occurs. Thanks to this, it is possible to install a scan. Since it is impossible to use the S1-49 oscilloscope and analogues without knowledge of the basic controls, it is worth talking about them in more detail.
Oscilloscope channel input
On the front panel there is a scale in the vertical plane - it is determined by the sensitivity control of the channel through which the measurement takes place. It is possible to change the scale not smoothly, but in steps, using the switch. What values you can set with it, look at the case next to it. On the same axis as this switch is a knob for smooth adjustment (here's how to use the C1-73 oscilloscope and similar models).
On the front panel, you can find a handle with a double-headed arrow. If you rotate it, then the chart of this channel will begin to move in the vertical plane (up and down). Please note that there is a graphic next to this knob, which shows in which direction you need to rotate it in order to change the multiplier value up or down. both channels are the same. In addition, the front panel has knobs for adjusting contrast, brightness, and synchronization. It is worth noting that a digital pocket oscilloscope (we are considering how to use the device) also has a number of graph display settings.
How measurements are taken
We continue to describe how to use a digital or analog oscilloscope. It is important to note that they all have a disadvantage. It is worth mentioning one feature - all measurements are carried out visually, so there is a risk that the error will be high. You should also take into account the fact that the sweep voltages have an extremely low linearity, which leads to a phase or frequency shift of about 5%. To minimize these errors, one simple condition must be met - the graph should occupy approximately 90% of the screen area. When measurements of frequency and voltage are carried out (there is a time interval), the controls for adjusting the signal gain at the input and the sweep speed should be set to the extreme right positions. It is worth noting one feature: since even a beginner can use a digital oscilloscope, devices with a cathode ray tube have lost their relevance.
How to measure voltage
To make a voltage measurement, you must use the scale values in the vertical plane. To get started, do one of these things:
- Connect both input terminals of the oscilloscope together.
- Move the input mode switch to the position that corresponds to the connection to the common wire. Then, with the regulator, next to which a double-headed arrow is shown, ensure that the scan line coincides with the central (horizontal) line on the screen.
Switch the device to the measurement mode and apply the signal to be investigated to the input. In this case, the mode switch is set to any operating position. But how to use a portable digital oscilloscope? A little more complicated - such devices have much more adjustments.
As a result, you can see some graph on the screen. To accurately measure height, use a pen with a horizontal double-headed arrow. Make sure that the top point of the graph falls on the one located in the center. It has a graduation, so it will be much easier to calculate the effective voltage in the circuit.
How to measure frequency
Using an oscilloscope, you can measure time intervals, in particular, the signal period. You understand that the frequency of any signal is always proportional to the period. You can measure the period in any area of the waveform. But it is more convenient and more accurate to measure at those points at which the graph intersects with the horizontal axis. Therefore, before starting measurements, be sure to set the sweep clearly on the horizontal line located in the center. Since it is much easier to use a portable digital oscilloscope than an analog one, the latter have long since sunk into oblivion and are rarely used for measurements.
Further, using the handle indicated by a horizontal double-headed arrow, it is necessary to shift the beginning of the period with the leftmost line on the screen. After calculating the signal period, you can use a simple formula to calculate the frequency. To do this, you need to divide the unit by the period calculated earlier. Measurement accuracy varies. To increase it, you need to stretch the chart horizontally as much as possible.
Pay attention to one regularity: with an increase in the period, the frequency decreases (the proportion is inverse). Conversely, as the period decreases, the frequency increases. A low margin of error is when it is less than 1 percent. But not every oscilloscope is able to provide such high accuracy. Only on digital ones, in which a linear sweep, can such accurate measurements be obtained.
How phase shift is determined
And now about how to use the S1-112A oscilloscope to measure phase shift. But first, a definition. Phase shift is a characteristic showing how two processes (oscillatory) are located relative to each other for some time. Moreover, the measurement takes place not in seconds, but in parts of the period. In other words, the unit of measurement is the units of the angle. If the signals are equally spaced mutually, then their phase shift will also be the same. Moreover, this does not depend on the frequency and period - the real scale of the graphs on the horizontal (time) axis can be any.
The maximum measurement accuracy will be if the graph is stretched to the full length of the screen. In analog oscilloscopes, the waveform for each channel will have the same brightness and color. To distinguish these graphs from each other, it is necessary to make for each its own amplitude. And the voltage that is supplied to the first channel, it is important to make it as large as possible. This will make it much better to keep the image on the screen in sync. Here's how to use the C1-112A oscilloscope. Other devices differ slightly in operation.
Oscilloscope - multipurpose a device that is used in the study of the shape and measurement of signal parameters, in the study of the characteristics of various electronic devices.
Voltage measurement. Voltage measurement using an oscilloscope can be carried out both by the direct conversion method and by the comparison method.
Method direct conversion(method calibrated deviation) provides for pre-calibration of the Y channel using an amplitude calibrator. This sets the required value of the coefficient of deviation K d . The measured voltage is applied to the Y channel input, and the vertical size of the image on the CRT screen is determined l B (in divisions or in units of length). Knowing the deviation coefficient K d or the sensitivity S u , with a symmetrical (or constant) voltage, you can find its amplitude
When measuring the amplitudes of an asymmetric voltage, it is necessary to fix the initial position of the horizontal line (or light spot) on the oscilloscope screen with the help of a scale grid in the absence of a measured voltage. Then, by applying the measured voltage to input Y and setting a still image, measure the amplitudes of each half-wave separately.
Method comparisons can be realized using a two-beam (two-channel) oscilloscope. To do this, one input, for example, Y 1 , is supplied with the signal under study, and at the input Y 2 - an exemplary voltage, which can be both constant and variable. Then, by changing the value of the reference voltage, it is necessary to achieve alignment of the calibration line created by the reference voltage with the boundaries of the measured section of the oscillogram. The value of the desired voltage is determined by the value of the reference voltage.
Measurement of time intervals can be carried out by the method direct conversion(method calibrated sweep factor) is similar to the case of voltage measurement. Before measurement with the help of a time calibrator, the required value of the sweep coefficient is set, which is the price of division of the scale horizontally. In this case
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Where l x are the dimensions of the investigated section of the oscillogram.
Frequency measurement an alternating signal can be produced by measuring the period. The frequency is found as the reciprocal of the period.
When using a dual-beam (two-channel) oscilloscope, the frequency measurement can be made by comparisons investigated oscillations with oscillations of known frequency. At the same time, it is carried out simultaneous fixation on the oscilloscope screen of two oscillations. The disadvantage of this method is the low accuracy.
More accurate are the modifications of the comparison method: the method Lissajous figures(method interference figures) and method circular sweep. When implementing these methods, the oscilloscope performs the functions of an indicator of the equality or multiplicity of the measured f X and the reference frequencies f 0 and practically does not introduce errors into the measurement result f X.
To obtain Lissajous figures, an unknown frequency signal is applied to the Y input of the oscilloscope. The internal sweep of the oscilloscope is turned off and a sinusoidal voltage is applied to the horizontal deflection plates from a high-precision measuring generator. In this case, the beam on the CRT screen makes a complex movement. The frequency of the measuring oscillator is selected so that a still image is obtained on the oscilloscope screen ( Lissajous figure). This occurs with an integer ratio between the frequencies of the two input signals, and the form of the Lissajous figure depends on the multiplicity f X /f 0 , the ratio of the voltage amplitudes and the phase shift between them. The frequency ratio is found as the ratio of the number of points of intersection of the figure on the screen with the horizontal n X and vertical m Y reference lines (the ratio of the number of touches of the figure with the horizontal and vertical axes superimposed on the screen).
On fig. examples of Lissajous figures are shown for various values of the frequency ratio f X /f 0 .
If the voltage of the measured frequency f X is applied to the input Y of the oscilloscope, and the voltage of the known frequency f 0 is applied to the input X, we obtain the relation
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from which the frequency value f X can be determined.
Usually they try to choose the frequency of the exemplary generator equal to the measured frequency, since in this case the figure has the simplest form - a straight line, a circle, an ellipse.
The method, characterized by high accuracy, is simple, convenient and economical. Its disadvantage is the difficulty of deciphering the figures with a frequency ratio of more than 10 and, therefore, the measurement error increases by establishing the true frequency ratio. This method is expedient to use only with a relatively small multiplicity of the measured and known frequency, usually not exceeding 6–8.
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Measurement of phase shifts
For a harmonic signal U(t) = Uo sin(t + 0), the expression (t + 0) is the phase of the sine argument, where 0 is the initial phase of oscillations. The phase value depends on the chosen time reference, therefore the phase shift or the phase difference 1 - 2 of two signals with the same frequencies has physical meaning (Fig. Fig. 5 .15a). The phase is measured in angular units - radians or degrees. The method for measuring phase shift using a two-channel oscilloscope is overlay method, which consists in obtaining on the oscilloscope screen and combining the waveforms of the voltages U 1 and U 2 supplied to input A and output B (Fig. 5.9). From fig. Rice. 5.15a it is clear that in this case
The phase difference of the two signals can be determined from the time shift. On the screen, a fixed picture of two oscillograms is obtained (Fig. Fig. 5.15b). Since the entire period T corresponds to an angle of 360, the phase difference is determined from the relation = 360T/T . In this case, the important question is which of the signals is ahead "in phase" of the other signal. On Fig. 5.15b, the voltage U 1 leads the voltage U 2 in phase by \u003e 0, since the signal U 1 reaches its maximum earlier than the signal U 2 (the signal U 1 also reaches its minimum earlier than the signal U 2).
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The phase shift can also be determined from the interval Т 1, but if during the measurements one signal, for example U 2, on the oscilloscope screen will be slightly shifted vertically down, as shown in Fig. Rice. 5.15b, then the measurement of the phase shift by the time shift T 1 turns out to be incorrect. This becomes obvious if we take into account that Т 1 is not equal to the time shift between the same signals, cut off by the horizontal straight line, to the right of Т 1 .
Phase shift measurement can also be carried out on a single-beam oscilloscope ellipse method. The ellipse is a special case of the Lissajous figure for f 1 = f 2 . Let voltages U x = U 0 sint and U y = U 0 sin(t + φ) be applied to horizontally and vertically deflecting plates. With equal amplitudes and frequencies of the signals at the inputs Y and X of the oscilloscope, a change in the phase shift leads to a change in the shape of the Lissajous figure from a straight line (φ = 0) through an ellipse to a circle (φ = 90 °), as shown in Fig. Rice. 5.16.
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In the general case, the phase shift can be determined from the ellipse as follows. The vertical and horizontal deflection gains are selected so that the ellipse fits into the square (Fig.). The value of the phase shift is found as the ratio of the parameters of the ellipse according to the formula
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The disadvantage of this method is its ambiguity. Measurement results φ are unambiguous only within 0–180 o, further (within 180–360 o), the figures will be repeated, but the direction of the beam will change.
To measure the phase difference, a circular sweep created by the voltage U 1 as a reference can also be used. In this case, the angular position of the luminous semicircle created by the voltage U 2 is measured when it is applied to the Z channel input of the CRT.
Divisions are marked on the monitor. Divisions allow you to visually evaluate the signal parameters. The divisions plotted along the horizontal axis allow you to measure time parameters. The divisions marked along the vertical axis allow you to measure the voltage.
Graphs displayed on the monitor are called oscillograms. The simplest oscilloscope only displays voltage waveforms. This form of display shows voltage versus time. There are devices that display the dependence of amplitude on frequency - spectrum analyzers. Such devices are used in measuring noise/vibration levels, as well as in analyzing the spectral composition of the signal. Graphs displayed by such instruments are called spectrograms.
By viewing voltage oscillograms and spectrograms, you can identify faults in electrical circuits in operating mode without disassembling them. Voltage oscillograms can be used to detect malfunctions of sensors, actuators and electrical wiring in the electronic systems of vehicles.
Zero line.
If no voltage source is connected to the oscilloscope input, the waveform will look like a flat horizontal line. Such a line is called the "zero line", since it displays the level corresponding to a voltage of 0 volts at the input of the oscilloscope.
A:– voltage value at the moment of time indicated by the marker. In this case, it corresponds to the zero line voltage, which is 0 Volts.
If the oscilloscope input is connected to a constant voltage source, for example, to a car battery, then the resulting waveform will also have the form of a flat horizontal line, but its vertical position on the screen will differ from the position of the zero line.
A:– voltage value at the moment of time indicated by the marker. In this case, it corresponds to the voltage of a car battery and is equal to ~ 12.3 Volts.
The difference between the positions of the received waveform and the zero line is directly proportional to the voltage value.
Most waveforms of signal voltages have a shape other than a flat horizontal line. The position of the zero line on the oscilloscope screen can be changed vertically - raise it higher or lower it. The need to change the position of the zero line (higher or lower) depends on the shape of the signal under study, and also arises in the case of using a multichannel oscilloscope.
An example of displaying several signals on the screen of a multi-channel oscilloscope at the same time with individual adjustment of the position of the zero line for each channel.
Gain.
The graph on the oscilloscope screen displays the dependence of the voltage value on time. The greater the amplitude of the signal under study, the greater the vertical deviation of the signal on the oscilloscope screen. Depending on the amplitude, a suitable gain is selected for clarity of the signal display. Gain value is measured in Volts per division
The ability to change the gain value allows the oscilloscope to display both signals with a very small voltage amplitude and signals with a very large voltage amplitude. The required gain value depends on the amplitude parameters of the signal under study.
The same signal will be displayed differently, depending on the selected gain value. A larger Volt/division value is chosen when the entire signal in amplitude needs to be displayed on the screen.
A smaller Volt/division value is chosen when it is necessary to study in detail the shape and amplitude parameters of individual sections of the signal. In this case, only part of the signal in amplitude is displayed on the screen.
The above examples demonstrate how the display of the waveform of the same signal on the oscilloscope screen changes when the gain value changes.
Scan.
The oscilloscope traces the voltage graph from left to right, starting from the left side of the screen. The speed at which the graph is drawn is called sweep. The timebase is measured in Seconds per division. The sweep value can be changed using the time/div switch.
The same signal will be displayed differently, depending on the selected sweep value. A smaller time/division is chosen when it is necessary to study in detail the shape and temporal parameters of individual sections of the signal. In this case, a shorter fragment of the signal is displayed on the screen.
Oscillogram of the injector control signal voltage at a smaller sweep value. In this case, the time base is 0.2 milliseconds/div.
If it is necessary to display a fragment of the oscillogram that is longer in time on the screen, for example, to identify individual pulses with an irregular signal shape or skipped pulses, select a larger time / division.
Oscillogram of the injector control signal voltage at a larger sweep value. In this case, the time base is 1 millisecond/division.
The above examples demonstrate how the display of the waveform of the same signal on the oscilloscope screen changes when the sweep value changes.
Synchronization.
For convenient and visual display of periodic (cyclically repeating) signals, synchronization is used. Synchronization ensures that individual pulses are drawn, always starting from the same point on the screen, thereby creating the effect of a still or relatively stable image. When trigger is off, the oscilloscope traces the voltage graph from left to right, starting at the far left of the screen until the screen is full, at which point it starts again at the far left of the screen, which is inconvenient for displaying relatively fast periodic waveforms.
To set the trigger, you must select the trigger level (voltage value at which the oscilloscope starts drawing the waveform) and the signal front (falling or rising voltage).
If a multi-channel oscilloscope is used, it is also necessary to specify which channel signal to use for synchronization.
analog signal.
The voltage value of most analog signals changes with time. If the changes are cyclically repeated, then such a signal is called periodic, for example, a nozzle control signal. If the voltage waveform of a periodic signal crosses the zero line, then such a signal is called variable. If the voltage waveform of a periodic signal does not cross the zero line, then such a signal is called constant. An example of a complex analog DC signal is a lambda probe signal.
Oscillogram of the output voltage of the BOSCH lambda probe
(based on zirconium oxide).
A:– voltage value at the moment of time indicated by the marker. In this case, it corresponds to the maximum voltage of the output signal of the lambda probe and is equal to ~ 840 millivolts;
A-B:– the value of the voltage difference between the two specified markers in time. In this case, it corresponds to the output voltage swing of the probe signal and is ~740 millivolts.
sinusoidal signal.
The simplest example of a variable analog voltage is a sine wave. Such a signal is characterized by only two parameters - amplitude and frequency. The zero line of a sinusoidal alternating voltage is located exactly in the middle of the signal.
It should be noted that most AC voltage signals are significantly different from pure sinusoidal. In automotive electronics, signals generated by magnetic gear position sensors are close to sinusoidal.
A:– voltage value at the moment of time indicated by the marker;
A-B:– the value of the voltage difference between the two specified markers in time.
Similar signals are generated by some sensors for the speed of rotation of the crankshaft, camshaft, wheel speed ...
digital signal.
Digital signals differ from analog signals in the presence of only two voltage levels - "high"/"low", "on"/"off", "1"/"0". Such digital signal voltage levels are referred to as "logic levels". In most cases, digital signal logic levels have exact voltage values, such as +5 volts and 0 volts.
A:– voltage value at the moment of time indicated by the marker. In this case, it corresponds to the voltage of the high level of the digital signal and is +5 Volts.
Digital signals are generated by keys (switches). The role of the keys is performed by transistors that switch between the "open" / "closed" states. Sometimes digital signals are generated by mechanical switches - mechanical switches, switches, electromechanical relays ... Examples of digital signals for automotive electronics are a Hall sensor, throttle extreme position sensors, active crankshaft / camshaft position / speed sensors ...
But mainly, digital signals are used in computer technology, including in digital control units for electronic systems of cars.
Frequency.
Frequency is the number of cycles of a periodic signal that repeats in a given period of time. If one second is taken for such a period of time, then the number of cycles of a periodic signal repeated over this period of time is called Hertz (Hz). In automotive electronics, the number of engine revolutions is usually calculated over a period of time equal to one minute (RPM).
From the oscillogram of the voltage of a periodic signal, you can easily measure the pulse repetition rate. To do this, it is necessary to measure the duration of the complete cycle of the signal - the period. Further, the obtained value of the time interval can be converted into frequency using the appropriate formula.
Calculate the pulse repetition rate of the crankshaft position sensor signal.
The sensor whose output voltage waveform is shown above generates one voltage pulse per revolution of the crankshaft. The time interval between two nearest such impulses is called a period. In this case, two consecutive pulses are 7.4 divisions apart on the oscilloscope screen horizontally. To display this signal on the screen, a sweep was selected (the time interval between each division on the oscilloscope screen horizontally) of 10 milliseconds / division, that is, 0.01 seconds. By multiplying the number of divisions corresponding to the period by the sweep value, you can get the numerical value of the signal repetition period in Seconds:
0.01*7.4=0.074 Seconds.
Knowing the value of the duration of the signal repetition period, it is possible to calculate how many such periods will follow in one second, that is, the frequency of the signal in Hertz. To convert the period into frequency, it is necessary to divide the selected time interval (in this case 1 Second) by the signal repetition period (for this signal 0.074 Seconds):
1/0.074=13.5 Hz.
If in this case we calculate how many such periods will follow in one minute, then the resulting value will correspond to the crankshaft speed in revolutions per minute. To convert the period into frequency, it is necessary to divide the selected time interval (in this case, 60 seconds) by the signal repetition period (for this signal, 0.074 seconds):
60/0.074=810 rpm.
This calculation can be done with any oscilloscope, but some oscilloscopes are able to calculate and display the signal frequency in Hertz or RPM in automatic or semi-automatic mode.
rpm:– current engine speed in RPM.
Pulse duration.
The pulse duration is the time interval during which the signal is in an active state. The active state is the voltage level that turns on the actuator (actuates the mechanism). Depending on the switching circuit of the actuator, the active state can have different voltage levels, for example 0 Volt, +5 Volt, +12 Volt ... For example, the voltage of the active state of the electromagnetic injector control signal in most engine control systems is theoretically 0 Volt, but in practice it can fluctuate in the range of 0 ... + 2.5 Volts or more.
impulse width:- pulse duration.
For the above waveform, the injector opening pulse duration is 4.4 divisions on the oscilloscope screen horizontally, which at 1 millisecond/div corresponds to 4.4 milliseconds.
Duty cycle.
Duty cycle is the percentage of time from the repetition period when the signal is in an active state. The duty cycle is one of the parameters of PWM (Pulse Width Modulation) signals.
duty cycle:- duty cycle of the signal. The signal is in the active state 67% of the time (in this case, the voltage value of the signal's active state is ~1 Volt);
Frequency:is the pulse repetition rate. In this case, it is ~100 Hertz.
PWM signals are used to control some actuators. For example, in some engine management systems, a PWM signal activates an idle solenoid valve. In addition, some sensors generate a PWM signal by converting the value of the measured physical parameter into a duty cycle.
EMF of self-induction.
EMF (Electro-Motive Force) of self-induction is a voltage that occurs due to a change in the value of the magnetic field and / or its direction around an electrical conductor. In the case of a high rate of change in the magnitude of the magnetic field inside the solenoid (the winding of an electromagnetic relay, an electromagnetic nozzle, an ignition coil, an electromagnetic speed sensor), the self-induction EMF voltage can reach tens/thousands of volts. The magnitude of the self-induction EMF voltage depends mainly on the inductance of the winding and the rate of change in the magnitude of the magnetic field. For electromagnetic actuators, the magnitude of the magnetic field changes most rapidly when it is destroyed, that is, when the solenoid supply voltage is quickly turned off.
In some cases, the effect of self-induction EMF is undesirable, and measures are taken to reduce / eliminate it. But some electrical circuits are designed to get the maximum surge of self-inductive EMF, such as the ignition system of a gasoline engine.
A:– voltage value at the moment of time indicated by the marker. In this case, it corresponds to the EMF voltage of the self-induction of the secondary winding of the ignition coil, limited by the breakdown voltage of the spark plug and corresponds to 8.3 kilovolts.
Some ignition systems with a supply voltage of 12 volts are capable of developing self-induction EMF voltage up to 40-50 thousand volts.
Oscilloscope - is an effective modern device designed to measure the frequency parameters of electric current in time and allows you to display them in graphical form on the monitor, or record them using self-recording devices. It allows you to measure such characteristics of the electric current inside the circuit as its strength, voltage, frequency and phase angle.
Why do you need oscilloscope ?
There is no laboratory that could function for a long time without measuring instruments or sources of signals, currents and voltages. If you plan to design or create high-frequency devices (especially serious computing equipment, say, inverter power supplies), then oscilloscope This is by no means a luxury, but a necessity.
It is especially good because it helps to visually determine the shape of the signal. Most often, it is this form that shows well what exactly is happening in the measured circuit.
The center of any oscilloscope is a cathode ray tube. We can say that it is like a radio tube, inside, respectively, a vacuum.
The cathode ejects electrons. The installed focusing system creates a thin beam of emitted charged particles. A special layer of phosphor covers the entire screen inside. Under the influence of a charged electron beam, a glow occurs. Looking outside, you can see a luminous dot in the center. The beam tube is equipped with two pairs of plates that control the beam created in this way. The work of the electron beam is carried out in directions that are perpendicular. As a result, two control systems are obtained that create a sinusoid on the screen, in which the vertical indicates the magnitude of the voltage, and the horizontal indicates the period of time. Thus, it is possible to observe the parameters of the voltage applied to the device in certain time intervals. Depending on the type of signal supplied to the oscilloscope, it can be used to measure not only voltage parameters, but also other values of a particular unit under test.
What are they like
Currently, there are two types of oscilloscopes - analog and digital (the latter is more convenient, has more features and is often more accurate). Both of them work on the same principle, and the following methods for measuring physical quantities can be used on any model of this device.
Correct connection
When carrying out measurements, it is important to correctly connect the device to the measured section of the circuit. Oscilloscope has two outputs with terminals or probes connected to them. One terminal is phase, it is connected to the beam vertical deflection amplifier. The other is the ground connected to the body of the device. On most modern devices, the phase wire ends with a probe or a miniature clip, and the ground ends with a small alligator clip (see photo)
On Soviet-made oscilloscopes and some Russian models, both probes are the same, you can distinguish them either by the “ground” icon on the corresponding wire, or by length - the phase wire is shorter. They are connected to the oscilloscope inputs, as a rule, with a standard plug (see figure)
If there is no marking, and by external signs it was not possible to find out where which probe is, then a simple test is carried out. One hand touches one probe, while the other hand is held in the air without touching anything. If this probe goes to the phase input, then noticeable noise will appear on the monitor (see figure). They are a significantly distorted sine wave with a frequency of 50 Hertz. If the probe goes to ground, then the monitor will remain unchanged.
When connecting the oscilloscope to the measured section of the circuit that does not have a common wire, the ground probe can be connected to each of the measured points. If there is a common wire (this is a point connected to the body of the device or grounded and conditionally having a “zero” potential), then it is preferable to connect the “ground” to it. If this is not done, then the measurement accuracy will drop significantly (in some cases, such measurements will turn out to be very far from the true values and it will be impossible to trust them).
Measuring voltage with an oscilloscope
The voltage measurement is based on the known value of the vertical scale. Before starting measurements, it is necessary to short-circuit both probes of the device or switch the input regulator to the position. See the following picture for a clearer picture.
After that, the vertical adjustment knob should set the scan line to the horizontal axis of the screen so that you can correctly determine the height.
After that, the device is connected to the measured section of the circuit and a graph appears on the monitor. Now it remains only to calculate the height of the graph from the horizontal line and multiply by the scale. For example, if in the graph below one cell is counted as 1 volt (respectively, it is divided into dashed divisions of 0.2, 0.4, 0.6, and 0.8 volts), then we get a total voltage of 1.4 volts . If the division value were 2 volts, then the voltage would be 2.8 volts, and so on ...
Setting the desired scale is carried out by rotating the special tuning knobs.
Current strength determination
To determine the current strength in the circuit using an oscilloscope, a resistor is connected in series with it, which has a much lower resistance than the circuit itself (such that it practically does not affect its proper operation).
After that, the voltage is measured according to the principle indicated above. Knowing the nominal resistance of the resistor and the total voltage in the circuit, it is easy, using Ohm's law, to calculate the current strength.
Frequency measurement with an oscilloscope
The device allows you to successfully measure the frequency of the signal, based on its period. The frequency is directly proportional to the period and is calculated by the formula f = 1 / T, where f is the frequency, T is the period.
Before measurement, the sweep line is aligned with the central horizontal axis of the instrument. During measurements, the oscilloscope is connected to the network under study and a graph is observed on the screen.
For greater convenience, using the horizontal adjustment knobs, combine the start point of the period with one of the vertical lines on the oscilloscope screen. Having successfully counted the number of divisions that make up the period, you should multiply it by the sweep speed.
Let's take a closer look at a specific example. For example, the period is 2.6 divisions, the time base is 100 microseconds/div. Multiplying them, we get the value of the period equal to 260 microseconds (260 * 10-6 seconds).
Knowing the period, we calculate the frequency using the formula f = 1 / T, in our case, the frequency is approximately equal to 3.8 kHz.
Phase shift measurement
Phase shift is a value indicating the relative position of two oscillatory processes over time.
It is measured not in seconds, but in fractions of the period (T) of the signal. It is possible to achieve the maximum measurement accuracy of this indicator if the period is extended by scaling to the full screen.
In a modern digital oscilloscope, absolutely each of the signals has its own color, which is very convenient for measurements. In the old analog versions, their brightness and color, unfortunately, are the same, so for greater convenience, their amplitude should be different. The preparation of a phase shift measurement requires precise preparatory operations.
The first thing to do is, without connecting the device to the circuit being measured, set the vertical adjustment knobs of the sweep lines of both channels to the central axis of the screen. Then, using the gain adjustment knobs of the vertical deflection channels (smoothly and stepwise), the 1st signal is set with a larger amplitude, and the second with a smaller one. The sweep speed control knobs set its value so that both signals on the screen have approximately the same period. After that, by adjusting the synchronization level, the beginning of the voltage graph is aligned with the time axis. The horizontal adjustment knob sets the beginning of the voltage graph in the leftmost vertical line. Then, the sweep speed adjustment knobs ensure that the end of the period of the voltage graph coincides with the rightmost vertical line of the monitor grid.
All these preparatory operations are carried out in order until the graph of the voltage period is completely stretched onto the screen. At the same time, it must begin and end in the scan lines (see figure).
After completing the preparatory stage, you should find out which of the parameters is ahead of the other - current or voltage. A quantity whose period start point begins earlier in time is a leading quantity, and vice versa. If the voltage is leading, then the phase angle parameter will be positive, if the current is negative. The phase shift angle (modulo) is the distance between the beginnings and ends of signal periods in the value of the monitor division grid. It is calculated using the following formula:
In it, the value N is the number of grid cells that one period occupies, and α is the number of divisions between the beginnings of periods.
If the graphs of the periods of current and voltage have a common start and end point, then the phase angle is equal to zero.
When repairing radio equipment, troubleshooting is carried out by measuring the parameters indicated above with an oscilloscope in certain sections of the electronic circuit or in specific electronic components (for example, microcircuits). Then they are compared with the values indicated in the technological catalogs, standard for these components, after which conclusions are drawn about the error-free operation or malfunction of one or another circuit element.
LABORATORY WORK 10 CLASS.
Introduction to the digital oscilloscope interface.
Measuring current with an oscilloscope
1. Remember that before removing a flash drive from a USB port, you always turn off the power on that port using the "Safely Remove" option.
Be careful with the USB port of the computer, a short circuit of its contacts can lead to failure not only of the port, but of the entire computer!!!
One of the computer's USB ports will serve as a source of direct current in works on electrodynamics. Connect the USB port switching unit to the electrical circuit (hereinafter current source) to one of the USB ports. Connect the oscilloscope voltage sensor to the second USB port with a cable (hereinafter oscilloscope). Connect the oscilloscope probes to the output terminals of the DC source.
If you are having problems setting up your oscilloscope or other sensor, you may have started the program before installing the sensor driver, interrogate the sensor again
(button) or restart the program.
2. Launch the Digital Lab program. In the opened window with the list of works, select the work scenario 3.1 "Introduction to the oscilloscope interface". The window with the list of jobs can be called up by pressing the button in the top menu of the program.
3. Oscilloscope - a device that allows you to measure the voltage of direct and
time-varying electrical signal. Using the button, open the computer settings window (Fig. 1)
Fig.1 Review the contents of the nested lists of settings in each of the
parameter setting window. The oscilloscope can simultaneously measure the voltage in two sections of the circuit through two channels. Check the box for selecting the "red" channel (Channel No. 1). Operating mode "auto" and sweep "5 ms/div", sensitivity of Channel #1 "1 V/div", zero line position "0", signal type "Constant" * , check the box "Signal display" and
* The “Variable” option in the “Signal type” window when setting the registration parameters of an oscilloscope sensor allows you to cut off a constant or slowly changing (with a characteristic time of about 0.1 s) voltage component and display only a rapidly changing signal (with a characteristic time of 0.05 s or less ). In the set of works “Digital Lab. Baseline" option is not used anywhere.
"Zero line display". The parameters in the remaining windows can not be changed yet. Fix the selected parameters (button )
4. Start the measurements in the "Digital Laboratory" program (button) and after writing the zero line with the red line, connect the oscilloscope leads in the "red" braid to the terminals of the current source. Notice which way the signal shifts when the blue-tipped cable is connected to the source terminal.
"+", and with a red tip - to the "minus" terminal. Stop measurements (button )
and with the left mouse button set the yellow vertical marker on the working field at the first horizontal division. Pay attention to the numerical voltage values
and time in the upper left corner (or the bottom of the window) of the registration window. Time
is counted from the green vertical marker located on the left border of the working field. You can move the green marker with the right mouse button. Right-clicking outside the left border of the registration window returns the green marker to the left edge of the field.
5. Return to the oscilloscope setup window, change the Channel 1 voltage sensitivity and time base. Enable registration on Channel No. 2 by setting in the signal type window (Fig. 1) - "Constant". After accepting the parameters, check how the oscilloscope readings have changed on the working field. After replacing the Channel #1 (red) probes with the Channel #2 probes, check how Channel #2 works, then remove the signal from the source of both channels, connecting the channel terminals so that the signal from them is of opposite polarity.
6. Assemble an electrical circuit consisting of a series-connected resistor with a resistance of 200 ohms, a variable resistance (its resistance varies from 0 to 100 ohms), an LED, a key, and a current source. Connect the oscilloscope Channel 1 terminals to the output terminals of the current source, and the Channel 2 terminals to the ends of the 200 Ohm resistor (Fig. 2). By closing the key and turning the variable resistance knob, make sure that the readings at the terminals of the current source do not change, and the voltage across the 200 Ohm resistor changes synchronously with the brightness of the LED (the LED will light only if the correct polarity of the supplied voltage is observed). Stop recording at the maximum brightness of the LED and measure the voltage across the 200 ohm resistor.
resistance Rsh \u003d 10 Ohm (Fig. 3), leaving the oscilloscope probes on a 200 Ohm resistor. Close the circuit, start recording, and after stopping recording, check that the voltage across the 200 ohm resistor and the brightness of the LED have not changed. A 10 ohm resistor with a small resistance compared to the total resistance of the circuit will be called shunt. The shunt in this circuit reduces the current strength by about 5%, that is
does not affect the voltage on the elements in the circuit and the brightness of the LED. Including it in the section of the circuit through which you want to measure the current strength, measuring the voltage on it, measure the current strength, since Ohm's law I \u003d U / R is fulfilled for the resistor.
8. Remove the LED from the circuit (Fig. 3). Switch the oscilloscope Channel #1 probes to
current source, on the shunt. Open the "Initial data" tab (button ) and enter in
shunt resistance value table Rsh\u003d 10 Ohm (Fig. 4).
Fig.4 Select the polarity of the oscilloscope sensor connection so that
a positive signal was recorded for each of the channels. Start the acquisition and, after receiving a signal from both channels of the oscilloscope, stop the acquisition. By placing a yellow marker on the screen. Go to the "Table" tab of the "Processing" window and select a cell in the "U, B" column (Fig. 5).
(blue braid of the oscilloscope cable and blue color of the signal on the screen) of the oscilloscope into the selected cell of the Table. To fill in the column with the voltage on the shunt, select the cell in the “Ush, V” column (Fig. 5) and press the red button - the voltage value measured on Channel No. 1 (red braid and red signal on the screen) will go to the corresponding cell of the Table. Calculate the value of the current through the shunt Ish and enter it in the cell at the bottom of the table (Fig. 5). After entering the "Initial data" this "gray" cell becomes "yellow", when the correct value is entered Ish- "green", when entering an erroneous value - "red". With a "green" cell, further calculations of the value Ish and the filling of the corresponding cells in the Table is carried out automatically (Fig. 6).
9. Start registration and, by changing the position of the variable voltage resistor knob, achieve a change in the voltage across the 200 Ohm resistor and the current (and, accordingly, the voltage at the shunt) in the circuit. When stopping recording, register several voltages across the resistor and shunt. Without filling in several lines in the Chart Construction Table (see clause 10), it will not be carried out.
ATTENTION! We remind you that the increase in the number of rows in the Table is carried out by pressing the button on the keyboard when filling in at least one cell in the previous row.
10. Go to the tab “Graph U(Ish) of the dependence of the voltage across the 200 Ohm resistor on the current through the resistor (it is equal to the current through the shunt) and analyze the resulting graph. Having selected the function Y=AX in the function selection window to describe the experimental graph (selection of the best straight line is carried out by pressing the button next to the function type selection window, Fig. 7), make sure that Ohm's law U=RI is fulfilled, and the proportionality coefficient A corresponds to
the value of the resistance of the resistor R 200 Ohm.
11. Enter in the Report (button ) one of the screens with the oscilloscope signal, the contents of the "Initial data" and "Table" tabs, the resulting U (I) graph, as well as a photo of the last electrical circuit on which measurements were taken, made using WEB - cameras, and a screenshot of the oscilloscope settings window (shortcut Alt-PrtScr), at which measurements were taken.
ATTENTION! Copying to the Report of the contents of any tab of the "Processing" window and a video frame with the installation, recorded by the WEB camera, is carried out to the place indicated not by the keyboard cursor, but by the MOUSE CURSOR. The content of the tab IS NOT INSERTED INTO THE REPORT UNLESS YOU HAVE OPENED this tab.
