Oscillogram of constant voltage at the input and output of a sinusoid. Taking measurements using an oscilloscope. Measuring current using an oscilloscope

Oscilloscope- a device that shows the voltage waveform over time. It also allows you to measure a number of signal parameters, such as voltage, current, frequency, phase angle. But the main benefit of an oscilloscope is the ability to observe the waveform. In many cases, it is the waveform that allows us to determine what exactly is happening in the circuit. In Fig. Figure 1 shows an example of such a situation.

Rice. 1. Oscillogram of a complex signal.

In this case, the voltage contains both direct and alternating components, and the shape of the alternating component is far from sinusoidal. On such a signal, voltmeters give big mistake: pointer voltmeter AC showed a voltage of 2.2 volts, and the digital one was generally 1.99 volts. Voltmeter DC showed 4.8 volts. The correct effective voltage value was shown by the oscilloscope - 5.58 volts (digital oscilloscopes measure voltage and allow you to save the results in a computer format). In addition, the oscillogram allows you to see some properties of the signal:

• the signal is pulsed in nature;
• the signal does not take negative values ​​(measured with the oscilloscope input open);
• the signal changes very quickly from zero to 6.4 volts and back to zero (sensitivity of the vertical deflection channel is 2 V/div);
• The duration of the pulses is more than three times the duration of the pauses.

In general, it is better to see once than to hear a hundred times.

In the vast majority of cases, periodic signals are studied, and we will talk about them.

1. Operating principle of an oscilloscope

The “heart” of the device is a cathode ray tube (CRT), Fig. 2.

Rice. 2. Electrostatically controlled cathode ray tube device.

CRT is vacuum tube, and like all lamps, it is “filled” with vacuum. The cathode emits electrons, and the focusing system forms a thin beam from them. This electron beam hits a screen coated with a phosphor, which glows under the influence of electron bombardment, and a luminous point appears in the center of the screen. Two pairs of CRT plates deflect the electron beam in two mutually perpendicular directions, which can be considered as coordinate axes. Therefore, to observe the voltage under study on the CRT screen, it is necessary that the beam deviate along the horizontal axis in proportion to time, and along the vertical axis - in proportion to the voltage under study.

A scanning voltage is applied to the horizontal beam deflection plates (located vertically). It has a sawtooth shape: it gradually increases linearly and quickly decreases (Fig. 3). Negative voltage deflects the beam to the left, and positive voltage deflects it to the right (as viewed from the screen). As a result, the beam moves across the screen from left to right at a certain constant speed, after which it very quickly returns to the left edge of the screen and repeats its movement. The distance that the beam travels along the horizontal axis is proportional to time. This process is called scanning, and the horizontal line that the beam draws across the screen is called the scanning line (sometimes called the zero line in measurements). She plays the role of the axis of time t graphics. The repetition rate of the sawtooth pulses is called the sweep frequency, but it is not used for measurements. For measurements, you need to know the sweep speed, which will be discussed below.

Rice. 3. Sweep voltage waveform.

If at the same time the test voltage is applied to the vertical deflection plates (located horizontally), then the beam will begin to deflect vertically: with a positive voltage, upward, and with a negative voltage, downward. Movements vertically and horizontally occur simultaneously and, as a result, the signal under study “unfolds” in time. The resulting image is called an oscillogram.

In fact, in addition to linear, there are also circular and spiral scans, as well as Lissajous figures, when one of the signals is a scan for the second. But that's a completely different story...

An important point is the ratio of the scanning frequencies and the signal. If these frequencies are exactly equal, then exactly one period of the signal under study is displayed on the screen. If the signal frequency is twice the sweep frequency, then we will see two periods, if three times, then three. If the signal frequency is half the sweep frequency, then we will see only half the signal period. The scanning frequency (speed) can be adjusted within a wide range. But the image will be stable only if the scanning and signal frequencies exactly match. At the slightest discrepancy in frequencies, each start of movement of the beam across the screen will correspond to a new point of the input signal function, and its graph will be drawn in a new position each time. With a slight discrepancy between frequencies (fractions of a hertz), it will look like a graph “floating” to the left or right. If the frequencies differ by several hertz or more, the oscillogram becomes unreadable (Fig. 4).

Rice. 4. Oscillogram in the absence of synchronization.

But achieving an absolutely exact match of frequencies (especially tens to hundreds of kilohertz) is almost impossible. Therefore, the sweep in the oscilloscope is controlled by a special synchronization circuit. It delays the start of the beam moving across the screen so that the beam begins to move at the moment when the input voltage reaches a certain value. In this case, the beam starts moving (and drawing an oscillogram) each time from the same point on the input signal graph. As a result, each subsequent movement of the beam draws a picture in the same position, even if the frequencies of the signal and the sweep are noticeably different. The image is stable and stable. The signal voltage at which synchronization occurs (sync level) is set by the oscilloscope controls. Visually, a change in this voltage causes a shift in the beginning of the displayed graph relative to the beginning of the signal period, Fig. 5.

Rice. 5. Oscillograms at different levels synchronization

In order to observe several signals simultaneously, multi-beam and multi-channel oscilloscopes are produced. Usually the number of channels is two (otherwise it turns out to be very complicated and expensive). The CRT of dual-beam oscilloscopes operates simultaneously with two beams on a common screen, which allows the two signals to be observed completely independently. But such devices are complex and expensive. Therefore, two-channel oscilloscopes are more common. Their CRTs are very basic, but they have two separate inputs and two independent vertical deflection amplifiers that handle the input signals. In addition, they have a built-in high-speed switch that switches the CRT (vertical deflection plate) from one channel to another very quickly. The signal images are not continuous lines, but consist of many strokes. But on the screen, the strokes merge, and the result is two graphs of input signals. Only when observing high-frequency signals and an unsuccessful scanning frequency can the image become dotted.

2. Connecting an oscilloscope

Since voltage is measured between two points, the oscilloscope input has two terminals. Moreover, they are not equivalent. One terminal, called "phase", is connected to the input of the vertical beam deflection amplifier. The second terminal is “ground” or “housing”. It is called so because it is electrically connected to the body of the device (this is the common point of all its electronic circuits). The oscilloscope shows the phase voltage in relation to ground.

It is very important to know which of the input conductors is phase. In imported devices, specialized probes are usually used, the ground of which has an alligator clip, as it is often connected to the body of the device under test, and the phase ends either with a “needle”, which can be conveniently and reliably “stuck” even into a small-sized contact, or with a clamp ( Fig. 6). In this case, it is basically impossible to confuse the phase and the body.

Rice. 6. Imported oscilloscope probe, “needle” on the left, clamp on the right.

Domestic oscilloscopes are most often equipped with cords that have standard 4 mm plugs for Russia (the name “banana”, which comes from audio equipment, is sometimes applied to them), Fig. 7. In this case, both plugs are the same, and additional features are used to distinguish them. There are several of these signs, and they can occur in any combination:

However, unfortunately, these rules are not always followed. This especially applies to cables that have undergone repair: any available conductor and the first plug that comes across can be installed there. Therefore, there is another way to determine the phase and housing, which gives a 100% guarantee.

Rice. 7. Plug for a domestic oscilloscope.

To determine which of the conductors is a phase and which is the housing, you need to, with the oscilloscope not connected anywhere, grab the contact of one of the input conductors with your hand, while not touching anything with the other hand. If this conductor is a body, then there will be only a horizontal scan line on the screen. If this conductor is a phase, then quite significant interference will appear on the screen, representing a highly distorted sinusoid with a frequency of 50 Hz (Fig. 8).

Rice. 8. Noise on the oscilloscope screen when you touch the phase of the input cable with your hand.

This interference occurs due to the fact that there is capacitance between the human body and the network wires laid in the room. And a current arises flowing through the following circuit: phase of the lighting network AC 220 V 50 Hz - capacitance between the network wires and the human body - human hand - amplifier input (phase of the input cable) - electronic circuit amplifier - oscilloscope housing - capacitance between the housing and the Earth - neutral wire of the network (it is always grounded). The circuit is closed, current flows. The magnitude of this current is 10^-8...10^-6 amperes, but the oscilloscope input has a very high resistance (about 10^6 Ohms), so a fairly large voltage appears on it. The sine wave looks distorted because the capacitive reactance of the network - human body section depends on the frequency: the higher the frequency, the lower the resistance. Therefore, high-frequency components (mains harmonics and interference that penetrates into it) create greater current and greater voltage at the input of the oscilloscope.

Having determined the phase and housing of the input cable, you can connect the oscilloscope to the circuit under study. If there is no clearly defined common wire, then the housing is connected to any of the points, the voltage between which needs to be examined. If there is a common wire in the circuit - a point conventionally taken as zero potential, connected to the device body or actually grounded, then it is better to connect the oscilloscope body to this point. Failure to follow this rule can lead to significant measurement errors (sometimes so large that the measurements cannot be trusted at all).

At its core, an oscilloscope is a voltmeter that displays a voltage graph. However, it can also be used to observe the shape of the current. To do this, a resistor Rt is connected in series with the circuit under study (here the index “t” means current), Fig. 9. The resistance of the resistor Rt is chosen much lower than the resistance of the circuit, then the resistor does not affect its operation and its inclusion does not lead to changes in the operating mode of the circuit. According to Ohm's law, a voltage appears across the resistor:

This voltage is measured by an oscilloscope. And knowing the value of Rt, you can convert the voltage shown by the oscilloscope into current.

Rice. 9. Measuring current with an oscilloscope.

A dual-channel (and dual-beam) oscilloscope can display waveforms of two signals simultaneously. To do this, it has two inputs (channels), usually designated I and II. It should be remembered that one of the input terminals of each channel is connected to the oscilloscope body, therefore The housing terminals of both channels are connected to each other. Therefore, these terminals must be connected to the same point in the circuit, otherwise a short circuit will occur in the circuit (Fig. 10).

Rice. 10. Connecting a two-channel oscilloscope. Input grounds can create a short in the circuit.

In Fig. 10a, circuit points B and D turned out to be closed to each other through the oscilloscope body (the closing conductor is shown by a dotted line). As a result, the circuit configuration changed.

The ability to observe not any two voltages, but only those having a common point, is a disadvantage, but a small one - in electronics, one of the poles of the power source is always a common wire, and all voltages are measured relative to it.

Using a two-channel oscilloscope, you can simultaneously observe both the voltage and current in a circuit. And thus measure the phase shift between current and voltage. The oscilloscope connection diagram in this case is shown in Fig. 11.

Rice. 11. Connecting an oscilloscope to measure phase shift.

Channel I measures voltage and channel II measures current. This inclusion is most optimal, because the voltage dropped across the resistor Rt and supplied to channel II is 30...100 times less than in channel I, therefore, it is more susceptible to interference and synchronization from low voltage is not as good. In addition, the design of most oscilloscopes is somewhat “single-ended” - the synchronization from the channel I signal is usually better and more stable. Thus, connecting channel I to voltage provides a more stable waveform image.

Connection error in fig. 11b is that the housing terminals of both inputs are not connected at one point. As a result, the resistor Rt is short-circuited through the oscilloscope body. The most unpleasant thing is that the voltage on the resistor Rt is not equal to zero - due to the fact that the resistance of the input cable wires (through which this resistor is closed) is not zero. Therefore, with such a connection, you may not notice this error (after all, the oscilloscope shows something), and the result of measuring the current will be incorrect.

The inclusion shown in Fig. 11c is unsuccessful in that channel I of the oscilloscope does not measure the voltage in the circuit under study, but the sum of the voltages in the circuit and across the resistor Rt (the voltage is measured not at the load, but at the source). The voltage on Rt, although small in magnitude, still introduces an error in the voltage measurement.

The oscilloscope connection shown in Fig. 11a not only provides the greatest measurement accuracy, but also allows in some cases to use a resistor Rt with a fairly high resistance. This is important when measuring small currents: if both the current in the circuit and the resistance Rt are small, then the voltage arising at Rt may be so small that the sensitivity of the oscilloscope is not enough to display it.

When measuring the phase shift, it is necessary to invert the signal in channel II, since channel II is connected opposite to channel I.

Let's look at the front panel of a two-channel oscilloscope S1-83 (Fig. 12).

Rice. 12. Front panel of the S1-83 oscilloscope.

A - control of channel I.
B - channel display control.
B - control of channel II.
G - adjustment of beam brightness, focusing and screen backlight.
D - scan control.
E - synchronization control.

It is clearly visible that the oscilloscope screen is divided into cells. These cells are called divisions, and are used in measurements: all vertical and horizontal scales are attached to them. The vertical scale is volts per division (V/div or V/div), the horizontal scale is seconds (milli- and microseconds) per division. Typically an oscilloscope has 6...10 divisions horizontally and 4...8 divisions vertically. The central vertical and horizontal lines have additional marks dividing the division into 5 or 10 parts (Fig. 13, also visible in Fig. 12). Risks serve for more accurate measurements; they are shares division.

Rice. 13. Oscilloscope screen divisions.

Control of both channels is the same. Let's consider it using channel I as an example (Fig. 14).

Rice. 14. Channel I controls.

1. Input mode switch. In the upper “” position, both direct and alternating voltage are supplied to the input. This is called "open input" - that is, open to direct current. In the lower “~” position, only alternating voltage passes to the input, this allows you to measure a small alternating voltage against the background of a large constant one, for example in amplifiers. This is implemented very simply: the amplifier input is connected through a capacitor. This is called "closed entry". Please note that when the input is closed, very low frequencies (below 1...5 Hz) are greatly attenuated, so they can only be measured with the input open. In the middle position of switch 1, the oscilloscope amplifier input is disconnected from the input connector and shorted to ground. This allows you to use knob 7 to set the scan line to the desired location.

2. Channel input connector.

3, 4, 5, 6. Sensitivity regulator for the vertical deflection channel (vertical scale). Switch 4 sets the scale in steps. The values ​​it sets are shown next to it. The selected value is indicated by a 5 mark on the switch. In the figure it indicates a value of 0.2 volts/division. Knob 3, located coaxially with the switch, allows you to smoothly reduce the scale by 2...3 times. In the extreme right position (in Fig. 14 the knob is “smoothly” located in this position), this knob has a lock, then the vertical scale is exactly equal to that set by switch 4. The scale values ​​highlighted by bracket 6 are indicated in millivolts per division - this is indicated inscription " mV" inside the bracket.

7. The handle has two functions. When rotated, it moves the channel graph vertically up or down. When “pulling”, it sets the vertical scale multiplier: the elongated handle (Fig. 15) sets the x1 multiplier, and the recessed x10 multiplier. The recessed and extended positions are symbolically shown above and below the handle.

Rice. 15. The vertical scale multiplier knob is pulled to the “x1” position.

Channel II (Fig. 16) is similar to channel I:

1 - input mode switch;
2 - input connector;
3 - scale smoothly;
4 - scale in steps;
5 - vertical beam movement and scale multiplier.

Rice. 16. Channel II controls.

But the second channel has an additional switch 6, which allows you to invert its input signal. In the pressed position, the channel works as usual, but in the extended position it is inverted, that is, when the input signal is negative, the beam moves up, and when it is positive, it moves down. This is necessary when measuring, for example, phase shift.

In Fig. Figure 17 shows the channel display control, which is determined by pressing one of the buttons.

Rice. 17. Channel display control.

1 - Only channel I is working, channel II is disabled.

2 - Both channels are displayed simultaneously (the beam switches between channels very quickly) and the relative position of the waveforms of both channels is correct. In this mode, phase shift can be measured.

3 - The oscilloscope shows the sum or difference of signals in the channels (the sign of the second channel is determined by the position of knob 6 in Fig. 16).

4 - The signals of both channels are displayed, but they are independent in time, so no comparison of the signals with respect to time and phase shift can be made.

5 - Only channel II works, channel I is disabled.

The scan control panel (Fig. 18) is similar to the control panel for the vertical beam deflection channel. It contains knob 4, which allows you to shift the image left and right and a combined regulator (1 - stepwise, 3 - smoothly) of the scan speed (horizontal scale). Mark 2 on the switch shows the set value. As with the vertical channels, the sweep speed switch has different units: seconds s , milliseconds ms , microseconds µs . The extended/recessed knob 4 “” sets the scan speed multiplier x0.2 and x1, respectively. Please note: in fig. 18, knob 3 for adjusting the sweep speed is not “smoothly” set to the extreme right position. This means that the scan speed is not equal to the value specified by switch 1, but less than it (the speed of the beam is less, and the time/division value is greater!).

Rice. 18. Sweep controls

On the synchronization control panel (Fig. 19) the following is set:

Rice. 19. Synchronization controls.

1 - Source of internal synchronization: the voltage of which channel synchronizes the movement of the beam. This synchronization is produced by the input signal and is therefore called internal. This mode is used for most measurements. The options here are: either synchronization only with the signal of channel I. Or an attempt to synchronize from channel I, and if that does not work, then synchronization is performed with the signal of channel II. The first option sometimes works a little better, so you should try to keep the signal of the first channel large enough for stable synchronization. In the vast majority of cases, for normal operation, you should select this particular synchronization mode by turning on the “I” button.

2 - External synchronization. The movement of the beam is synchronized by pulses supplied from a special external source to the synchronization input of the oscilloscope. This mode is sometimes required to study specific signals. If there is no external synchronization source, then it is impossible to obtain a stable image. The “0.5-5” and “5-50” buttons set the range of input voltages from an external synchronization source. The “X-Y” button together with the “II X-Y” button for controlling the display of channels (Fig. 17) supplies the signal of channel II to the horizontal scan plates. In this mode, you can observe Lissajous figures.
3 - “Synchronization level” knob. Sets the synchronization voltage (Fig. 5). When this knob is pressed (as in the figure), the scan is automatic. In this case, the beam will move even if synchronization does not occur. The beam is delayed at the beginning of movement for some time until the moment of synchronization, but after some time it still begins to move. This is a “soft” mode, more convenient for work, since the beam always remains visible. When the handle is extended, the standby sweep is activated. In this mode, the beam will not start moving until synchronization occurs. If synchronization does not occur, the beam does not move. This mode is well suited for observing non-periodic signals. The effect of this pen on the image is shown in Fig. 4 and 5.

4 - “Polarity” of synchronization. In fact, the signs “+” and “-” mean something slightly different. In the “+” position, synchronization occurs along the front, i.e. at the moment when the input voltage reaches the value specified (with the “Synchronization Level” knob) as the input voltage increases (changes from “-” to “+”), Fig. 20. In the “-” position, synchronization occurs on a decline - when the input voltage decreases (changes from “+” to “-”). In an oscilloscope, two different circuits are used in the synchronization circuit: one determines whether the input voltage is equal to the specified one and, if equal, triggers the movement of the beam. This voltage is set with the “Synchronization Level” knob. The second circuit determines how the input voltage changes - increases or decreases. And accordingly allows the first scheme to work.

5 - Sync input mode. Applies to both external and internal synchronization. In the “~” position, the input is closed, and synchronization occurs only from alternating voltage. In the “” position, the input is open, and both alternating voltage and direct voltage act on the synchronization circuit. The “LF” mode is the same, but the signal enters the synchronization circuit through a filter low frequencies, cutting off high-frequency interference. This mode is not available in all oscilloscopes.

6 - Input for supplying an external synchronization signal.

Rice. 20. “Polarity” of synchronization.

4. Oscilloscope measurements

Measurements are made visually and their error is quite high. In addition, the sweep voltage has low linearity, so the error in measuring frequency and phase shift can reach 5%. To minimize the error, the image should have a size of 80...90% of the screen size. When measuring voltage and frequency (time intervals), the knobs for smooth adjustment of the input signal gain and sweep speed must be set to the extreme right position.

4.1. Voltage measurement

Used to measure voltage known value vertical scale. Before starting the measurement, it is necessary to short-circuit the input terminals of the oscilloscope (or set the input mode switch to position) and use the handle to set the scan line to the horizontal line of the screen grid so that it is possible to correctly determine the height of the oscillogram, Fig. 21a.

After this, the signal under study is supplied to the input (or the input mode switch is set to one of the operating positions). A graph of the signal function appears on the screen, Fig. 21b.

Rice. 21. Voltage measurement (screenshot of a digital oscilloscope): a - preparation; b - measurement.

In order to more accurately measure the height of the graph, the oscillogram is shifted with the handle so that the point at which the amplitude is measured falls on the central vertical line, which is graduated in fractions of a division (Fig. 22). We get: the sensitivity of the vertical deflection channel = 1 V/div, the size of the oscillogram is 2.6 divisions, therefore the signal amplitude is 2.6 volts.

Rice. 22. Determination of signal amplitude.

Let's demonstrate measuring voltage on the oscilloscope itself. The maximum voltage has a value of 3.4 divisions (Fig. 23). The definition of vertical scale is shown in Fig. 24. The handle is “smoothly” set to the extreme right position. The mark on the sensitivity switch shows 0.5 volts/div. The scale multiplier is set to x10 (recessed). Therefore the measured voltage is:

Rice. 23. Determination of amplitude on an oscilloscope S1-83.

Rice. 24. Determining the vertical scale on the S1-83 oscilloscope.

4.2. Frequency measurement

An oscilloscope allows you to measure time intervals, including the signal period. The frequency of a signal is inversely proportional to its period. The signal period can be measured in various parts of the oscillogram, but it is most convenient and accurate to measure it at the points where the graph intersects the time axis. Therefore, before measurement, the scan line must be set to the central horizontal line of the screen grid (Fig. 21a).

Rice. 25. Measuring the signal period.

Using the handle, the beginning of the period is aligned with the vertical grid line, Fig. 25 (it is best to combine the beginning of the period with the leftmost vertical line of the screen, then the accuracy will be maximum). The period of the signal shown in Fig. 25 is equal to 6.8 divisions. Sweep speed - 100 µs/div (since the Greek letter µ, meaning "micro", is not always available for display, it is often replaced by the Latin letter u , similar in outline). Then the signal period

and its frequency:

Please note that Figures 22 and 25 show the same signal, but with different meanings sweep speed. Frequency determination according to Fig. 22 gives higher value errors (exact frequency value 1.459 kHz). Therefore, the most accurate measurements are obtained by stretching the image horizontally as much as possible. And one more thing. In Fig. 25 the duration of the signal period is slightly longer than 6.8 divisions. Since the period is longer, the signal frequency is actually a little less than the one we received: it’s actually 1.459 kHz, but ours is 1.47 kHz. In fact, a measurement error of less than one percent is high accuracy. This accuracy is ensured by a digital oscilloscope whose sweep is linear. In an analog oscilloscope, the frequency measurement error would most likely be higher.

4.3. Phase shift measurement

The phase shift shows the relative position of two oscillatory processes in time. But it is measured not in units of time (which are plotted along the horizontal axis), but in fractions of the signal period (i.e., in units of angle). In this case, the same relative position of the signals will correspond to the same phase shift, regardless of the period and frequency of the signals (i.e., regardless of the actual scale of the graphs along the time axis). Therefore, the greatest measurement accuracy is obtained if the signal period is stretched across the entire screen.

Since in an analog oscilloscope the signal graphs of both channels have the same color and the same brightness, in order to distinguish them from each other, it is recommended to make them of different amplitudes. In this case, it is better to make the voltage measured by channel I of the device larger - in this case, the synchronization will better “hold” the image. Preparation for measurements is carried out as follows (see Fig. 26, for greater clarity, voltage and current are shown in different colors):

Using the knobs of both channels, their scan lines are set to midline screen grid (in the absence of signals at the inputs). Using the knobs for adjusting the gain of the vertical deflection channels (stepwise and smoothly), the signal of the 1st channel is set to a large amplitude, and the signal of the 2nd channel is set to a smaller amplitude. The sweep speed adjustment knobs set the sweep speed so that approximately one signal period is displayed on the screen. Use the “Synchronization Level” knob to ensure that the voltage graph starts from the time axis (from the scan line) - point A. Use the knob to ensure that the voltage graph starts from the leftmost vertical line of the screen grid - point A. Use the “Sweep Speed” knobs (stepwise and smoothly) ensure that the period of the voltage graph ends on the rightmost vertical line of the screen grid. Repeat steps 4...6 until the period of the voltage graph is stretched across the entire screen, and its beginning and end must coincide with the scan line (Fig. 26).

Before measuring the magnitude of the phase shift, it is necessary to determine which of the signals (voltage or current) is leading and which is lagging. The sign of the phase shift angle φ depends on this. In Fig. 26a, the current lags behind the voltage - the beginning of its period is located in time later than the beginning of the voltage period (the beginning of the voltage period at point A, and the current period at point B). The current starts later, therefore, it lags behind, and the voltage leads. This situation corresponds to positive phase angle values. In Fig. 26b the current is leading and the voltage is lagging. Since the beginning of the current period is not displayed on the screen, the ends of the first half-cycle are compared: the graph that began earlier will be the first to return to zero (point D occurs earlier in time than point B). The phase shift angle is negative in this case.

Rice. 26. Current lags behind voltage, φ>0 (a); current leads voltage, φ<0 (б).

The modulus of the phase shift angle φ is the distance between the beginnings or between the ends of the period (positive half-cycle) of signals in screen grid divisions (Fig. 27). Next, the value of the modulus φ is found from the proportion, taking into account that one full period of any oscillation is equal to 360 degrees:

here N is the number of grid divisions occupied by one signal period,
α is the number of grid divisions between the beginnings of periods (the ends of the positive half-period).
In the example in Fig. 18 module φ in both cases is equal to:

It should be taken into account that

Rice. 27. Measurement of phase shift angle.

In principle, the magnitude of the phase shift can be measured at the end of the period (points D and E in Fig. 26), but on the right side of the screen the linearity of the sweep voltage is the worst, so the measurement error will be maximum.
If the phase shift is zero (there is only an active load in the circuit or resonance occurs), then the voltage and current will begin and end simultaneously, Fig. 28.

Rice. 28. Oscillogram with a phase shift equal to zero.

▌Old article about an analog oscilloscope
Sooner or later, any novice electronics engineer, if he does not give up his experiments, will grow up to circuits where it is necessary to monitor not just currents and voltages, but the operation of the circuit in dynamics. This is especially often needed in various generators and pulse devices. There is nothing to do here without an oscilloscope!

Scary device, right? A bunch of knobs, some buttons, and even a screen, and it’s not clear what’s there or why. No problem, we'll fix it now. Now I will tell you how to use an oscilloscope.

In fact, everything is simple here - an oscilloscope, roughly speaking, is just... voltmeter! Only a cunning one, capable of showing a change in the shape of the measured voltage.

As always, I’ll explain with an abstract example.
Imagine that you are standing in front of the railway, and an endless train consisting of completely identical cars is rushing past you at breakneck speed. If you just stand and look at them, you will see nothing but blurry garbage.
Now we’ll put a wall with a window in front of you. And we begin to open the window only when the next carriage is in the same position as the previous one. Since our cars are all the same, you don’t necessarily need to see the same car. As a result, pictures of different but identical cars will pop up before your eyes in the same position, which means the picture will seem to stop. The main thing is to synchronize the opening of the window with the speed of the train, so that the position of the car does not change when opening. If the speed does not match, the cars will “move” either forward or backward at a speed depending on the degree of desynchronization.

Built on the same principle strobe- a device that allows you to look at fast moving or rotating crap. There, too, the curtain opens and closes quickly.

So, An oscilloscope is the same strobe, only electronic. And it doesn’t show cars, but periodic voltage changes. For the same sinusoid, for example, each subsequent period is similar to the previous one, so why not “stop” it, showing one period at one time.

Design
This is done through ray tube, deflection system and scan generator.
In the beam tube, a beam of electrons hitting the screen causes the phosphor to glow, and the plates of the deflection system allow this beam to be driven across the entire surface of the screen. The higher the voltage applied to the electrodes, the more the beam is deflected. Feeding onto plates X sawtooth voltage we create a scan. That is, our beam moves from left to right, and then sharply returns and continues again. And on the plates Y we apply the voltage being studied.

Operating principle
Then everything is simple, if the beginning of the appearance of the saw period (the beam is in the extreme left position) and the beginning of the signal period coincide, then in one scanning pass one or more periods of the measured signal will be drawn and the picture will seem to stop. By changing the sweep speed, you can ensure that only one period remains on the screen - that is, during one period of the saw, one period of the measured signal will pass.

Synchronization
You can synchronize the saw with the signal either manually, adjusting the speed with the handle so that the sine wave stops and possible by level. That is, we indicate at what input voltage level we need to start the sweep generator. As soon as the input voltage exceeds the level, the sweep generator will immediately start and give us a pulse.
As a result, the scan generator produces a saw only when needed. In this case, synchronization is completely automatic. When choosing a level, you should take into account such a factor as interference. So if you take the level too low, then small needles of interference can start the generator when it is not needed, and if you take the level too high, then the signal can pass under it and nothing will happen. But here it’s easier to turn the knob yourself and everything will immediately become clear.
The synchronization signal can also be supplied from an external source.

The theory is out of the way, let's move on to practice.
I will show you the example of my oscilloscope, stolen a long time ago from the defense enterprise Design Bureau "Rotor" :). An ordinary oscil, not very sophisticated, but reliable and simple as a sledgehammer.

So:
Brightness, focus and illumination of the scale are, I think, self-explanatory. These are the interface settings.

Amplifier U and up and down arrows. This knob allows you to move the signal image up or down. Adding additional offset to it. For what? Yes, sometimes the screen size is not enough to accommodate the entire signal. We have to drive it down, taking the lower limit, rather than the middle, as zero.

Below goes toggle switch switching input from direct to capacitive. This toggle switch in one form or another is found on all oscilloscopes without exception.

Important thing! Allows you to connect the signal to an amplifier either directly or through a capacitor. If you connect directly, it will work both constant component and variable. And it goes through the conduit variable only.

For example, we need to look at the noise level of the computer's power supply. The voltage there is 12 volts, and the amount of interference can be no more than 0.3 volts. Against the background of 12 volts, these measly 0.3 volts will be completely unnoticeable. You can, of course, increase the gain by Y, but then the graph will go off the screen, and the offsets along Y not enough to see the top. Then we only need to turn on the capacitor and then those 12 volts of constant voltage will settle on it, and only the alternating signal will pass into the oscilloscope, those same 0.3 volts of interference. Which can be enhanced and seen in full height.

Next comes the coaxial connector for connecting the probe. Each probe contains signal and ground. The ground is usually placed on the negative or on the common wire of the circuit, and the signal wire is poked according to the circuit. The oscilloscope shows the voltage on the probe relative to the common wire. To understand where the signal is and where the ground is, just grab them with your hand one by one. If you take the general one, then the corpse’s pulse will still be on the screen. And if you take up the signal signal, you will see a bunch of crap on the screen - interference to your body, which is currently serving as an antenna. On some probes, especially modern oscilloscopes, Built-in voltage divider 1:10 or 1:100, which allows you to plug the oscilloscope into an outlet without the risk of burning it. It turns on and off with a toggle switch on the probe.

Still on almost every oscilloscope there is a calibration output. Where you can always find rectangular signal with a frequency of 1 KHz and a voltage of about half a volt. Depending on the oscillator model. It is used to check the operation of the oscilloscope itself, and sometimes it comes in handy for testing purposes :)

Two hefty knobs: Gain and Duration

Gain serves to scale the signal along the axis Y. It also shows how many volts per division it will ultimately show.
Let's say, if you have 2 volts per division, and the signal on the screen reaches a height of two cells of the dimensional grid, then the amplitude of the signal is 4 volts.

Duration determines the sweep frequency. The shorter the interval, the higher the frequency, the more high-frequency signal you can see. Here the cells are already graduated in milli and microseconds. So by the width of the signal you can calculate how many cells it is, and by multiplying it by the scale along the axis X You will get the duration of the signal in seconds. You can also calculate the duration of one period, and knowing the duration it is easy to find the frequency of the signal f=1/t

Twisted top allows you to change the scale smoothly. I usually have it on a click so that I always clearly know what scale I have.

There is also input X to which you can send your signal, instead of a sweep saw. Thus, an oscilloscope can serve as a TV or monitor if you assemble a circuit that will form an image.

Twist with the inscription Scan and the left and right arrows allow you to move the graph left and right across the screen. It is sometimes convenient to adjust the desired area to the divisions of the grid.

Synchronization block.

Level knob— sets the level from which the saw generator will start.
Switch from internal to external, allows you to apply clock pulses to the input from an external source.
Switch labeled +/- switches level polarity. Not available on all oscilloscopes.
Handle stability— allows you to manually try to select the synchronization speed.

Quick start.
So, you turned on the oscil. The first thing you need to do is to short-circuit the signal probe to your own earthenware crocodile. In this case, “Corpse Pulse” should appear on the screen. If it doesn’t appear, then turn the stabilization and offset and level knobs - maybe it just hid behind the screen or didn’t start due to insufficient level.

As soon as the band appears, use the offset knobs to set it to zero. If you have an analog oscill, especially if it’s an ancient one, then let it warm up. After turning it on, mine floats for another fifteen minutes.

Set it further voltage measurement limit. Take extra if you need to reduce anything. Now, if you attach the ground wire of the oscilloscope to the minus of the battery, and the signal wire to the plus, you will see how the graph jumps by one and a half volts. By the way, old oscilloscopes often begin to falsify, so using a reference voltage source is useful to see how accurately it displays the voltage.

Choosing an oscilloscope.
If you've just started, then anyone will suit you. Extremely preferably if he will two-channel. That is, it will have two probes and two Gain knobs, for the first and second channel, which allows you to simultaneously obtain two graphs.
The second most important criterion for an oscilloscope is frequency. The maximum frequency of the signal that it can pick up. 1MHz was enough for me so far I didn’t aim for more. Those oscilloscopes that are sold in stores already have a frequency of 10 MHz and higher. The cheapest oscilloscope I saw cost 5 thousand rubles - . A two-channel one already costs 10 thousand, but I set my sights and got it for a kilobuck. Different requests - different toys. But, I repeat, 1 MHz is enough for a start, and will last for a long time. So find yourself at least some kind of oscilloscope. And then you will understand what you need.

An oscilloscope is an instrument used to observe the waveform of a voltage over time. It might look something like this:

Here we see a screen on which the signal is displayed. The waveform on an oscilloscope is called an oscillogram.

Below in the picture you can see a probe for an oscilloscope.

If a multimeter's probe consists of a simple wire, then an oscilloscope's probe consists of a cable. And the cable contains two probe wires, which branch at the end. This cable is capable of measuring high frequency voltages without interference. The little pin in the middle is the signal probe, and the screen is the ground or ground probe. Electronics engineers call it differently, but that’s what I’m used to. At the end of the probe, a white crocodile clip is the ground, and a signal clip has a needle.

We connect the cable to the connector. My oscilloscope has two connectors. In my case, the oscilloscope is two-channel. On some cool oscilloscopes you can even see 4 or more channels.

There is a situation when you need to identify a signal wire; to do this, take one of the wires, touch it with your finger and look at the oscilloscope display. If the signal is not distorted, it is ground. If it is distorted, it is a signal signal. The photo below is an example of defining a signal wire.

How to use an oscilloscope

With an oscilloscope we can only measure the voltage waveform; we cannot measure the current directly! If only indirectly, using . In order to measure the magnitude of DC voltage, we need a DC voltage source. This could be a simple battery or power supply. In my case this is power unit. For clarity, we set it to 1 Volt.

The oscilloscope unit of measurement is the side of the square on the display. In order to measure on a 1:1 scale, we set the nutcracker Y to 1.

We cling to the ground on the “minus” of the power supply, and the signal to the “plus” of the power supply. We see this picture:

The line has moved up 1 square. This means that over time the signal from the power supply is always 1 Volt.

But how can we measure signals that are, say, 100 Volts? This is why the nutcracker was invented according to U :-). Leave 1 Volt on the power supply and click on the “2” mark.

What does it mean? This means that the received signal on the display must be multiplied by 2.

Here comes the signal

On the oscillogram we see the value of Y = 0.5. We multiply this value by the one on the oscilloscope and get the desired value. That is, 2x0.5 = 1 Volt.

But this is the signal if we set the nutcracker to 5.

5x0.2=1 Volt.

If we apply the probes the other way around, then nothing bad happens. For example, we set 2 Volts on the power supply. The oscilloscope ground is to the “plus” of the block, and the signal ground is to the “minus” of the block - that is, everything is connected in reverse. Our line just went down, but that doesn’t change anything. 2 Volts remain as they are.

But for practice, as I already said, you need to know the signal shape. Electronics uses 90% periodic signals. This means that they are repeated after a certain period of time. Very often you need to find out the period and frequency of an alternating signal. This is what our electron beam device is used for.

In order not to burn the oscilloscope, I took . Thanks to the step-down transformer, at the output I have a voltage amplitude (this means from zero to the highest or lowest peak) within 1.5 Volts, and a voltage of 220 Volts enters the primary winding.

We attach the oscilloscope probes to the secondary winding of the transformer and display the readings on the display.

Ideally, we should have a pure sine wave delivered to our sockets. Russia, what else can I say))). Oh well. I think the socket in your house has a cleaner sinusoid than mine :-).

Signal period and frequency

In a periodic signal, such parameters as the frequency of the signal and its shape are important to us. Therefore, to determine the frequency, we must know the period. T – period, V – frequency. They are interconnected by the formulas:

Let's determine the period of the signal. The period is the time after which the signal is repeated again.

We count the sides of the squares according to X. I counted 4 sides of the square.

Next, we look at the X-axis rotator, which is responsible for the time sweep. The risk is worth 5. The price of this division is written at the top - msec/div. That is, it turns out 5 milliseconds on one side of the square.

Millie is a thousand. Therefore 0.005 sec. We multiply this value by our counted sides of the squares. 0.005x4=0.02. That is, one period lasts 0.02 seconds or 20 milliseconds. Knowing the period, we find the frequency of the signal using the formula above. V= 1/0.02=50 Hz. The voltage frequency in our outlet is 50 Hz, which is what needed to be proven.

Currently I have already bought myself

You can read more about a digital oscilloscope.

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 if we are talking about high-frequency technology, for example, inverter power supplies), then doing anything without an oscilloscope will be problematic.

What is an oscilloscope

This is a device that allows you to “see” the voltage, or more precisely, its shape over a certain period of time. With its help, you can measure many parameters - voltage, frequency, current, 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 speaks about what exactly is happening in the circuit in which the measurement is being carried out.

In some cases, for example, 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 one value, digital ones - much less, and DC voltmeters - several times more. The most accurate measurement can be carried out using the device described in the article. And it doesn’t matter whether the H3013 oscilloscope is used (how to use it is discussed below) or another model. The measurements are the same.

Features of the device

This is quite simple to implement - you need to connect a capacitor to the amplifier input. In this case, the entrance is closed. Please note that in this measurement mode, low-frequency signals with a frequency less than 5 Hz are attenuated. 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 circuit occurs to the housing. Thanks to this, it is possible to install a sweep. 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 using the sensitivity regulator of the channel along which the measurement takes place. It is possible to change the scale not smoothly, but stepwise, using a switch. What values ​​can be set using it, look on the case next to it. On the same axis with this switch there is a regulator for smooth adjustment (here's how to use the S1-73 oscilloscope and similar models).

On the front panel you can find a handle with a double-headed arrow. If you rotate it, the chart of this channel will begin to move in the vertical plane (down and up). Please note that there is a graphic next to this knob that shows which way you need to turn it to change the multiplier value up or down. both channels are the same. In addition, on the front panel there are knobs for adjusting contrast, brightness, and synchronization. It is worth noting that a digital pocket oscilloscope (we are discussing how to use the device) also has a number of settings for displaying graphs.

How are measurements taken?

We continue to describe how to use a digital or analog oscilloscope. It's important to note that they all have a flaw. One feature worth mentioning is that 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 sweep voltages have extremely low linearity, which leads to a phase or frequency shift of approximately 5%. To minimize these errors, one simple condition must be met - the graph should occupy approximately 90% of the screen area. When measuring frequency and voltage (there is a time interval), the input signal gain and sweep speed adjustment controls 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 measure voltage, you must use scale values ​​in the vertical plane. To get started, you need to do one of these steps:

1. Connect both input terminals of the oscilloscope to each other.
2. Move the input mode switch to the position that corresponds to the connection to the common wire. Then, use the regulator next to which there is a bidirectional arrow to ensure that the scan line coincides with the central (horizontal) line on the screen.

Switch the device to measurement mode and apply the signal to the input that needs to be examined. In this case, the mode switch is set to any working position. But how to use a portable digital oscilloscope? It’s a little more complicated - such devices have a lot more adjustments.

As a result, you can see a 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. There is a graduation on it, 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. Period measurements can be made in any area of ​​the oscillogram. But it is more convenient and more accurate to measure at those points where the graph intersects the horizontal axis. Therefore, before starting measurements, be sure to set the scan exactly to a horizontal line located in the center. Since using a portable digital oscilloscope is much easier than using an analog one, the latter have long since sunk into oblivion and are rarely used for measurements.

Next, using the handle indicated by the horizontal double-headed arrow, you need to shift the start of the period with the leftmost line on the screen. After calculating the period of the signal, you can use a simple formula to calculate the frequency. To do this, you need to divide the unit by the previously calculated period. The measurement accuracy varies. To increase it, you need to stretch the graph horizontally as much as possible.

Pay attention to one regularity: as the period increases, the frequency decreases (the proportion is inverse). And vice versa - as the period decreases, the frequency increases. A low margin of error is when it is less than 1 percent. But not every oscilloscope can provide such high accuracy. Only with digital ones, in which the scan is linear, can such accurate measurements be obtained.

How is phase shift 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 over a period of time. Moreover, the measurement occurs not in seconds, but in parts of a period. In other words, the unit of measurement is angle units. If the signals are equally positioned relative to each other, then their phase shift will also be the same. Moreover, this does not depend on the frequency and period - the actual scale of the graphs on the horizontal (time) axis can be anything.

The maximum accuracy of measurement will be if you stretch the graph to the entire length of the screen. In analog oscilloscopes, the signal graph for each channel will have the same brightness and color. To distinguish these graphs from each other, it is necessary to make each one have its own amplitude. And it is important to make the voltage supplied to the first channel 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 S1-112A oscilloscope. Other devices differ slightly in operation.