Oscilloscope and its principle of operation. Electronics lessons. Use oscilloscope
How does the oscilloscope work?
To understand how the oscilloscope works, consider the block diagram of the averaged instrument. Almost all oscilloscopes are arranged exactly like this.
The diagram does not show only twopower supply : high-voltage source, which is used to generate high voltage supplied to a CRT (cathode-ray tube ) and low-voltage, ensuring the operation of all nodes of the device. And there is no builtcalibrator , which serves to configure the oscilloscope and prepare it for work.
The test signal is fed to the input “Y ”Channel vertical deviation and falls on the attenuator, which is a multi-switch that adjusts the sensitivity. His scale is graded in V / cm or V / div. This refers to one division of the coordinate grid deposited on a CRT screen.The values themselves are also printed there: 0.1 V, 10 V, 100 V. If the amplitude of the signal under study is unknown, we set the minimum sensitivity, for example, 100 volts per division. Then even a signal with an amplitude of 300 volts will not disable the device.
Dividers 1: 10 and 1: 100 are included with any oscilloscope. They are cylindrical or rectangular nozzles with connectors on both sides. Perform the same functions as the attenuator. In addition, when working with short pulses, they compensate for the capacity of the coaxial cable. Here is the external divider from the C1-94 oscilloscope. As you can see, the division ratio is 1:10.
Thanks to the external divider, it is possible to expand the capabilities of the device, since using it makes it possible to study electrical signals with an amplitude of hundreds of volts.
From the output of the input divider signal is fed topreamplifier . Here he forks and entersdelay line and on the sync switch. The delay line is designed to compensate for the response time of the sweep generator with the arrival of the test signal to the vertical deflection amplifier.The final amplifier forms the voltage applied to the plates. ”Y ”And provides the deflection of the beam vertically.
Sweep generator forms a sawtooth voltage that is applied to the horizontal deflection amplifier and the plates “X ”CRT and provides horizontal deflection of the beam. It has a switch graduated as time per division ("Time / del"), and sweep time scale in seconds (s), milliseconds (ms) and microseconds (μs).
The synchronization device ensures the start of the start of the sweep generator simultaneously with the appearance of a signal at the starting point of the screen. As a result, on the oscilloscope screen, we see an image of the pulseunfolded in time . The sync switch has the following positions:
Synchronization from the studied signal.
Synchronization from the network.
Synchronization from an external source.
The first option is the most convenient and it is used most often.
In addition to the complex and expensive models of oscilloscopes, which are used in the development of electronic equipment, our industry has launched the production of the C1-94 compact oscilloscope specifically for radio amateurs.Despite the low cost, he has proven himself to work and has all the functions of an expensive and serious device. Unlike its more "clever" fellows, the S1-94 oscilloscope has a rather small size and is also easy to use. Consider its controls. Here is the front panel of the S1-94 oscilloscope.
To the right of the screen from top to bottom.
These controls can adjust the focus of the beam on the screen, as well as its brightness. In order to extend the life of a CRT, it is desirable to set the brightness to a minimum, but so that the readings are clearly visible.
Net ". Power button.
Mode button "Waiting-Avt ».
This is the button for selecting the standby and automatic scanning mode. When operating in the standby mode, the start and synchronization of the sweep is performed by the signal under study. In automatic mode, the sweep starts without a signal. To study the signal, a standby sweep is often used.
This button selects the trigger pulse polarity. You can choose to start from a pulse of positive or negative polarity.
Set sync button "Internal-External ».
Usually, internal synchronization is used, because to use an external clock signal, a separate source of this external signal is needed. It is clear that in the conditions of the home workshop it is not necessary in the overwhelming case. The external clock input on the front of the oscilloscope looks like this.
Button to select "Open" and "Closed" input.
It's all clear. If we intend to study the signal with a constant component, then select "Variable and constant". This mode is called "Open", since a signal is sent to the vertical deviation channel, which contains a constant component or low frequencies in its spectrum.
At the same time, it should be borne in mind that when the signal is displayed on the screen, it will go up, since the constant component level will be added to the amplitude of the variable component. In most cases it is better to choose the "closed" entrance (). In this case, the constant component of the electrical signal will be cut off and not displayed on the screen.
The terminal "case" is used to ground the case of the device. This is done for security purposes.In the home workshop, it is sometimes not possible to ground the instrument case. Therefore, we have to work without grounding. It is important to remember that in the on state there may be voltage potential on the oscilloscope case. When touching the case, it can "pull". It is especially dangerous to touch the oscilloscope body with one hand and the radiators or other working electrical appliances with the other hand. In this case, the dangerous potential from the body will pass through your body ("hand" - "hand") and you will get an electric shock! Therefore, when operating an oscilloscope without grounding, it is advisable not to touchmetal body parts. This rule holds true for other electrical devices with a metal casing.
On the center of the front panel switch "scan" -Time / Div . It is this switch that controls the operation of the sweep generator.
Just below is the input divider switch (attenuator) -V / div . As already mentioned, when studying a signal with an unknown amplitude, it is necessary to set the maximum possible value of V / div. So for the oscilloscope S1-94 you need to set the switch to position 5 (5V / div. ).In this case, one cell on the coordinate grid of the screen will be equal to 5 volts. If a divider with a division factor of 1 to 10 (1: 10) is connected to the "Y" input of the oscilloscope, then one cell will be equal to 50 volts (5V / div. * 10 = 50V / div.).
Also on the oscilloscope panel are available:
Nowadays, with the development of digital technology, digital oscilloscopes have become widely introduced. In fact, it is a hybrid of analog and digital technology. The attitude towards them is ambiguous, like a meat grinder with a processor or a coffee grinder with a display.
Analog equipment has always been reliable and easy to use. In addition, it is easily repaired. The digital oscilloscope is much more expensive and very difficult to repair. Of course there are many advantages. If the analog signal is converted to digital form using an ADC (analog-to-digital converter), then you can do anything with it. It can be recorded in memory and at any time displayed on the screen for comparison with another signal, put in phase and antiphase with other signals. Of course, analog technology is good, but with digital electronics, the future.
Since the voltage is measured between two points, the oscilloscope input has two terminals.And they are not equivalent. One terminal, called “phase”, is connected to the amplifier input of the vertical beam deflection. The second terminal is “ground” or “body”. 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 relative to earth.
It is very important to know which of the input conductors is the phase. In imported devices, specialized probes are usually used, the ground of which has a crocodile clip, as it often connects to the device’s body, and the phase ends with either a “needle”, which can be conveniently and reliably “stuck” even into a small contact, or a clip ( Fig. 6). In this case, it is impossible to confuse the phase and the housing.
Fig. 6. The probe of an import oscillograph, at the left "needle", on the right a clip.
Domestic-made oscilloscopes are most often equipped with cords that have 4-mm plugs that are standard for Russia (the name “banana” sometimes comes from audio equipment), 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:
Ground wire is longer;
The earth wire is brown (standard) or black;
On the body of the plug of the ground wire marked with the symbol "body"
Unfortunately, however, these rules are not always followed. This especially applies to cables that have been repaired: any conductor that is available and the first plug to be connected can be put in there. Therefore, there is another way to determine the phase and case, giving one hundred percent guarantee.
To determine which of the conductors is a phase, and what kind of housing, it is necessary to hold the contact of one of the input conductors when the oscilloscope is not connected anywhere, while not touching the other hand. If this conductor is a body, then only a horizontal scanning line will be on the screen. If this conductor is a phase, then quite significant interference will appear on the screen, representing a highly distorted 50 Hz sinusoid (Fig. 8).
Fig. 8. Interference 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 a capacitance between the human body and the wires of the network that is laid in the room.And there is a current flowing through this circuit: the phase of the lighting network AC 220 V 50 Hz - the capacitance between the wires of the network and the human body - the human hand - the amplifier input (input cable phase) - the electronic circuit of the amplifier - the oscilloscope body - the capacitance between the housing and the Earth - neutral wire network (it is always grounded). The circuit is closed, the current flows. The magnitude of this current is 10 ^ -8 ... 10 ^ -6 amperes, but the input of the oscilloscope has a very high resistance (about 10 ^ 6 Ohms), so a sufficiently large voltage appears on it. A sinusoid appears distorted because the capacitance of the network — the human body depends on the frequency: the higher the frequency, the lower the resistance. Therefore, the high-frequency components (the harmonics of the network and the noise penetrating into it) create a higher current and a higher voltage at the input of the oscilloscope.
Having determined the phase and case of the input cable, you can connect the oscilloscope to the circuit under study. If there is no clearly defined common wire in it, then the case is connected to any of the points between which the voltage is required to be investigated. If there is a common wire in the circuit - a point, conventionally taken as a zero potential, connected to the device case or actually grounded, then the oscilloscope case should be connected to this point. Failure to comply with this rule can lead to significant measurement errors (sometimes so large that measurements cannot be trusted at all).
At its core, the oscilloscope is a voltmeter showing a voltage graph. However, it can be used to observe the form of the current. To do this, in series with the circuit being investigated, include a resistor Rт (here the index "t" means current), fig. 9. The resistance of the resistor Rт is chosen much smaller than the resistance of the circuit, then the resistor does not affect its operation and its inclusion does not lead to changes in the mode of operation of the circuit. On the resistor according to Ohm’s law voltage arises:
This voltage is measured by an oscilloscope. And knowing the value of RT, one can convert the voltage shown by the oscilloscope into a current.
Fig. 9. Current measurement by oscilloscope.
A dual-channel (and dual-beam) oscilloscope can display waveforms of two signals simultaneously. For this, it has two entrances (channels), usually denoted by I and II. It should be remembered that one of the input terminals of each channel is connected to the body of the oscilloscope, therefore,terminals "body" of both channels are interconnected. Therefore, these terminals must be connected to the same point of the circuit, otherwise a circuit will close (Fig. 10).
Fig. 10. Connecting a two-channel oscilloscope. "Earth" inputs can create a short circuit in the circuit.
In fig. 10a points of the circuit B and D turned out to be closed between themselves through the body of the oscilloscope (the closing conductor is shown by a dotted line). As a result, the circuit configuration has changed.
The ability to observe not any two voltages, but only having a common point, is a disadvantage, but small - in electronics one of the poles of the power supply is always a common wire, and all voltages are measured relative to it.
Using a two-channel oscilloscope, you can simultaneously monitor both voltage and current in a circuit. And thus measure the phase shift between current and voltage. The connection diagram of the oscilloscope in this case is shown in Fig. eleven.
Fig. 11. Connect the oscilloscope to measure the phase shift.
Channel I measures voltage, and Channel II measures current. Such an inclusion is optimal, since the voltage falling on the resistor Rт and supplied to channel II is 30 ... 100 times less than in channel I, therefore, it is more susceptible to interference and low-voltage synchronization is not so good.In addition, the design of most oscilloscopes is somewhat "asymmetrical" - the synchronization from the signal of channel I is usually better and more stable. Thus, connecting the channel I to the voltage provides a more stable waveform image.
Connection error in fig. 11b consists in that the terminals of the case of both inputs are not connected at one point. As a result, the resistor RT is shorted through the body of the oscilloscope. The most unpleasant thing is that in this case the voltage across the resistor Rт is not zero - due to the fact that the resistance of the wires of the input cables (through which this resistor closes) is not zero. Therefore, with such a connection, you can not notice this error (because the oscilloscope shows something), and the result of the current measurement will be wrong.
The inclusion shown in fig. 11c is unfortunate that the oscilloscope channel I measures not the voltage in the circuit under study, but the sum of the voltages in the circuit and on the resistor RT (the voltage is measured not on the load, but on the source). The voltage on RT, although small in size, still makes an error in the measurement of voltage.
The oscilloscope connection shown in fig.11a not only provides the greatest accuracy of measurements, but also allows in some cases to use a resistor Rт with a rather large resistance. This is important when measuring small currents: if both the current in the circuit and the resistance Rt are small, then the voltage generated on RT can be so small that the sensitivity of the oscilloscope is not enough to display it.