An oscilloscope is a device that displays current, voltage, frequency and phase shift of an electrical circuit. The device displays the ratio of time and intensity of an electrical signal. All values are depicted using a simple two-dimensional graph.
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What an oscilloscope is for
An oscilloscope is used by electronics technicians and radio amateurs to measure
- amplitude of an electrical signal - voltage to time ratio;
- analyze the phase shift;
- to see the distortion of an electrical signal;
- Calculate the frequency of the current from the results.
Despite the fact that the oscilloscope demonstrates the characteristics of the analyzed signal, more often it is used to identify the processes occurring in the electrical circuit. Thanks to the oscillogram, specialists receive the following information:
- the shape of the periodic signal;
- value of positive and negative polarity;
- the range of signal variation in time;
- the duration of the positive and negative half-period.
Most of this data can be obtained with a voltmeter. However, then you would have to make measurements with a frequency of a few seconds. In this case the percentage of error in the calculations is high. Working with an oscilloscope saves a lot of time in obtaining the necessary data.
Operation of an oscilloscope
An oscilloscope takes measurements with an electron-beam tube. This is a lamp that focuses the analyzed current into a beam. It hits the screen of the instrument, deflecting in two perpendicular directions:
- vertical - shows the voltage being analyzed;
- Horizontal - shows the elapsed time.
Two pairs of electron-beam tube plates are responsible for the beam deflection. Those that are vertical are always energized. This helps to distribute the different pole values. The positive attraction deflects to the right, the negative attraction deflects to the left. Thus, the line on the screen of the instrument moves from left to right with a constant velocity.
There is also an electric current acting on the horizontal plates, which deflects the demonstrating beam voltage indicator. The positive charge is up, the negative charge is down. So a linear two-dimensional graph, called an oscillogram, appears on the display of the device.
The distance the beam travels from the left to the right edge of the screen is called the sweep. The horizontal line is responsible for the measurement time. In addition to the standard linear two-dimensional graph, there are also circular and spiral sweeps. However, they are not as convenient to use as classic oscillograms.
Classification and types
There are two main types of oscilloscopes:
- Analog - apparatuses for measuring average signals;
- digital - the devices convert the measured value into a "digital" format for further transmission of information.
According to the principle of operation there are the following classification:
- Universal models.
- Special equipment.
The most popular are universal devices. These oscilloscopes are used to analyze different types of signals:
- harmonic;
- single impulses;
- Pulse packs.
Universal instruments are designed for a variety of electrical devices. They allow you to measure signals in the range of a few nanoseconds. Measurement error is 6-8%.
Universal oscilloscopes are divided into two main types:
- monoblock - have a general specialization of measurements;
- with interchangeable units - adapted to a particular situation and type of device.
Special devices are designed for a particular type of electrical equipment. So there are oscilloscopes for radio signal, television broadcasting or digital technology.
Universal and special devices are divided into:
- high-speed - used in fast-acting devices;
- storage - devices that store and reproduce previously made readings.
When choosing a device, you should carefully study the classifications and types in order to purchase a device for a specific situation.
Device and basic technical parameters
Each device has a number of the following technical characteristics:
- The coefficient of possible error when measuring voltage (most devices have this value does not exceed 3%).
- The value of the sweep line of the device - the greater this characteristic, the longer the time interval of observation.
- Synchronization characteristic containing: frequency range, maximum levels and system instability.
- Parameters of the vertical deviation of the signal with the input capacity of the equipment.
- The values of the transient response, showing the rise time and overshoot.
In addition to the above basic values, oscilloscopes have additional parameters in the form of amplitude-frequency response, which shows the dependence of the amplitude on the frequency of the signal.
Digital oscilloscopes also have an internal memory value. This parameter is responsible for the amount of information that the unit can record.
How the measurements are made
The oscilloscope screen is divided into small squares, which are called divisions. Depending on the instrument, each square will equal a certain value. The most popular designation: one division - 5 units. Also on some instruments, there is a knob to control the scale of the graph, so that users can take measurements more conveniently and accurately.
Before starting a measurement of any kind, the oscilloscope must be connected to an electrical circuit. The probe is connected to any of the free channels (if there is more than 1 channel in the device) or to the pulse generator, if there is one in the device. After connection, different signal images will appear on the display of the device.
If the signal received by the device is abrupt, the problem is in the connection of the probe. Some of them are equipped with miniature screws that need to be tightened. Also in digital oscilloscopes the automatic positioning fixture solves the problem of a stray signal.
Current Measurement
When measuring current with a digital oscilloscope, you should know what type of current type of current you need to observe. Oscilloscopes have two modes of operation:
- Direct Current ("DC") for direct current;
- Alternating Current ("AC") for alternating current.
DC current is measured when the "Direct Current" mode is on. The probes of the apparatus should be connected to the power supply in direct correspondence with the poles. The black crocodile is connected to the minus, the red one to the plus.
A straight line will appear on the screen of the device. The value of the vertical axis will correspond to the DC voltage parameter. The current can be calculated according to Ohm's law (voltage divided by resistance).
Alternating current is a sine wave, because the voltage is also alternating. Therefore its value can only be measured in a certain period of time. The parameter is also calculated using Ohm's law.
Voltage Measurement
To measure signal voltage you will need the vertical coordinate axis of a linear two-dimensional graph. Because of this all attention will be paid to the height of the oscillogram. Therefore you should adjust the screen more conveniently for the measurement before starting the observation.
Then switch the device to the DC mode. Connect the probes to the circuit and observe the result. A straight line will appear on the device display, the value of which will correspond to the voltage of the electrical signal.
Frequency Measurement
Before you understand how to measure the frequency of an electrical signal, you should know what a period is, because the two concepts are interrelated. One period is the smallest interval of time in which the amplitude begins to repeat.
Seeing the period on an oscilloscope is easier with the horizontal time coordinate axis. You just need to notice after what interval of time the line graph begins to repeat its pattern. The beginning of the period is better to consider the point of contact with the horizontal axis, and the end of the repetition of the same coordinate.
To more conveniently measure the period of the signal, the sweep speed is reduced. In this case, the measurement error is not so high.
Frequency is a value inversely proportional to the analyzed period. That is, to measure the value, you need one second of time divided by the number of periods occurring during this interval. The resulting frequency is measured in Hertz, the standard for Russia is 50 Hz.
Measuring phase shift
Phase shift is considered to be the mutual arrangement of two oscillatory processes in time. The parameter is measured in fractions of the signal period, so that regardless of the nature of the period and frequency, the same phase shifts have a common value.
The first thing to do before measuring: find out which signal is lagging behind the other and then determine the value of the parameter sign. If the current is ahead, the angle shift parameter is negative. In the case where the voltage is ahead, the sign of the value is positive.
To calculate the degree of phase shift follows:
- Multiply 360 degrees by the number of grid cells between the beginning of the periods.
- Divide the result by the number of divisions occupied by one period of the signal.
- Choose a negative or positive sign.
Measuring phase shift in an analog oscilloscope is inconvenient because the plots displayed on the screens have the same color and scale. For this kind of observation, either a digital device or dual-channel instruments are used to place different amplitudes on a separate channel.
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