Oscilloscope Measurements, How to Use an Oscilloscope An oscilloscope is an effective modern device designed to measure the frequency parameters of electric current over time and allows you to display them graphically on a 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 shift angle.
Why do I need an oscilloscope?
There is no laboratory that can 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 computer technology, say, inverter power supplies), then the oscilloscope is not a luxury, but a necessity.
It is especially good because it helps to visually determine the shape of the signal. Most often, this form shows well what exactly is happening in the measured circuit.
The center of all oscilloscopes is a cathode ray tube. We can say that it is like a radio tube, inside, respectively, there is a vacuum.
The cathode ejects electrons. The installed focusing system creates a thin beam from the emitted charged particles. A special layer of phosphor covers the entire screen inside. Under the influence of a charged beam of electrons, a glow occurs. Observing from the outside, you can see a glowing dot in the center. The beam tube is equipped with two pairs of plates that control the beam created in this way. The operation of the electron beam is carried out in the directions 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 supplied to the device in certain time intervals. Depending on the type of signal supplied to the oscilloscope, it is possible to measure not only the voltage parameters, but also other values of a particular unit under test.
What they are
Currently, two types of oscilloscopes are common - analog and digital (the latter is more convenient, advanced functions and often more accurate). Both of them work on the same principle, and the following methods of measuring physical quantities can be used on any models of this device.
Correct connection
When making measurements, it is important to correctly connect the device to the measured section of the circuit. The oscilloscope has two outputs with terminals or probes connected to them. One terminal is phase terminal, it is connected to the amplifier of vertical deviation of the beam. 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 clamp, and the ground ends with a small clip of the "crocodile" type (see photo).
On Soviet-made oscilloscopes and some Russian models, both probes are the same, you can distinguish them either by the "earth" icon on the corresponding wire, or by length - the phase wire is shorter. They are connected to the inputs of the oscilloscope, as a rule, with a standard plug (see figure).
If there is no marking, and it was not possible to find out where the probe is by external signs, 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 interference 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 the "ground", the monitor will remain unchanged.
When an oscilloscope is connected to a 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 "earth" to it. If this is not done, then the accuracy of measurements will drop significantly (in some cases, such measurements will be very far from the true values and it will be impossible to trust them).
Oscilloscope voltage measurement
The basis for measuring the voltage is a known value of the vertical scale. Before starting the measurements, both probes of the device should be shorted or the entry regulator should be switched to the position. See the following picture more clearly.
After that, with the vertical adjustment handle, the scan line should be set to the horizontal axis of the screen so that the height can be correctly determined.
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 bar 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 price were 2 volts, then the voltage would be 2.8 volts and so on...
Setting the desired scale is carried out by rotating special adjustment knobs. Current Determination
To recognize the current strength in the circuit using an oscilloscope, a resistor is connected in series, which has a significantly 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 in direct proportion to the period and is calculated by the formula f = 1 / T, where f is the frequency, T is the period.
Before measurement, the scan line is combined with the central horizontal axis of the device. When making measurements, the oscilloscope is connected to the network under study and a graph is observed on the screen.
For greater convenience, using horizontal adjustment knobs, combine the starting point of the period with one of the vertical lines on the oscilloscope screen. Having successfully calculated the number of divisions that make up the period, it should be multiplied by the magnitude of the sweep rate.
Let's consider a specific example in more detail. For example, the period is 2.6 divisions, the sweep is 100 microseconds / division. Multiplying them, we get a period value equal to 260 microseconds (260 * 10-6 seconds).
Knowing the period, we calculate the frequency according to the formula f = 1 / T, in our case the frequency is approximately equal to 3.8 kHz.
Phase shift measurement
Phase shift is a quantity indicating the mutual 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 maximum accuracy of measurements of this indicator if the period is stretched by scaling to 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, you should make their amplitude different. Preparing a phase shift measurement requires precise preparatory operations.
The first thing to do is to install the vertical adjustment knobs of the scan line of both channels on the central axis of the screen without connecting the device to the measured circuit. Then, with the knobs for adjusting the amplification of the vertical deviation channels (smoothly and stepwise), the 1st signal is set with a larger amplitude, and the second with a smaller amplitude. With the control knobs for the scanning speed, its value is set so that both signals on the screen have approximately the same period. After that, adjusting the level Synchronization, combine the beginning of the voltage graph with the time axis.
With the horizontal adjustment knob, set the beginning of the voltage graph in the vertical line to the extreme left. Then, with the sweep speed adjustment knobs, make sure that the end of the voltage graph period coincides with the vertical grid line of the monitor to the far right.
All these preparatory operations are performed in order until the schedule of the stress period stretches to the screen completely. In this case, it should begin and end in the sweep lines (see figure).
After completing the preparatory stage, you should find out which of the parameters is ahead of the other - the current or voltage. A value whose starting point begins earlier in time is ahead, and vice versa. If the voltage is ahead, then the parameter of the phase shift angle 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 magnitude of the monitor's division grid. It is calculated according to the following formula:
In it, the value of N is the number of grid cells that take one period, and α is the number of divisions between the beginnings of periods.
If the current and voltage period graphs share a common start and end point, the phase shift angle is zero.
When repairing radio equipment, fault-finding is carried out by measuring with an oscilloscope the parameters indicated above on individual sections of the electronic circuit or on specific electronic components (for example, microcircuits). Then they are compared with the values specified in the technological catalogs, standard for these components, after which conclusions are drawn about the error-free operation or malfunction of a particular element of the chain.
