Functions of digital oscilloscopes Oscilloscope frequency calculation method

　　Oscilloscopes are electronic instruments that are frequently used by electronic engineers for daily testing and measurement. From the perspective of development history, oscilloscopes have gone through the era of analog oscilloscopes and digital oscilloscopes. Nowadays, analog and digital oscilloscopes can handle most applications. Since oscilloscopes are so important, what is the role of oscilloscopes in our daily electronic testing and measurement? What important role does it play? Let's take a look together.

　　1. The role of oscilloscope

　　1. Can measure the voltage amplitude of DC signal and AC signal

　　2. The period of the AC signal can be measured and used to convert the frequency of the AC signal.

　　3. Can display the waveform of AC signal.

　　4. Signal measurement can be performed using two channels separately.

　　5. The waveforms of two signals can be displayed on the screen at the same time, which is the dual-trace measurement function. This function can measure the phase difference between the two signals and the difference in shape between the waveforms.

　　2. The function of the oscilloscope panel knob

　　1. The scan speed knob can change the speed at which the oscilloscope scan line moves from left to right.

　　2. The voltage selection knob can change the input voltage to make the scan line deflect in the Y-axis direction of the oscilloscope screen.

　　3. Use the up and down adjustment knob and the left and right adjustment knob to change the position of the scan line in the up, down, left and right directions on the screen.

　　4. The state where the voltage standard knob reaches the maximum value in the clockwise direction is the standard state. Other positions are non-standard states.

　　5. The state where the scanning speed standard knob reaches the maximum value in the clockwise direction is the standard state. Other positions are non-standard states.

　　6. It is the synchronization knob, which can stabilize the waveform of the oscilloscope.

　　7. The function selection keys are CH1 channel selection, CH2 channel selection, and dual-trace function selection.

　　8. The function selection key is CH1 signal synchronization and CH2 signal synchronization.

　　9. The measurement selection switch can make the measurement in three states: DC, AC, and GHD. When in DC state, both DC and AC signals can be measured. When in AC state, a capacitor is connected in series inside the oscilloscope measurement interface. At this time, the DC component in the signal is blocked by the capacitor, while the AC component can be measured through the capacitor.

　　When in the grounded state, the measurement interface of the oscilloscope is short-circuited to the ground inside the oscilloscope, and external signals cannot enter the oscilloscope.

　　10. This is the brightness adjustment knob, which can adjust the brightness of the image.

　　11. Adjust the focus knob to make the image more detailed.

　　3. Oscilloscope frequency calculation

　　There are many ways to measure signal frequency using an oscilloscope. The following are two basic and commonly used methods.

　　1. Periodic method

　　For any periodic signal, the aforementioned time interval measurement method can be used to first determine the time T of each cycle, and then use the following formula to calculate the frequency f: f=1/T

　　For example, if the waveform being measured displayed on the oscilloscope has a cycle of 8 div, the "t/div" switch is set to the "1μs" position, and its "fine adjustment" is set to the "calibration" position. Then its cycle and frequency are calculated as follows:

　　T = 1us/div& TI mes; 8div = 8us

　　f = 1/8us = 125kHz

　　Therefore, the frequency of the measured waveform is 125kHz.

　　2. Lissajous figure method to measure frequency

　　Set the oscilloscope to XY working mode, input the measured signal into the Y axis, and the standard frequency signal into "X external", and slowly change the standard frequency until the two signal frequencies become integer multiples, for example, fx:

　　If fy=1:2, a stable Lissajous figure will be formed on the fluorescent screen.

　　The shape of the Lissajous figure is not only related to the phase of the two deflection voltages, but also to the frequency of the two deflection voltages. The tracing method can be used to draw Lissajous figures at various frequency ratios of ux and uy and different phase differences. Several Lissajous figures with different frequency ratios are shown in Figure 5-15.

　　By using the relationship between Lissajous figures and frequency, accurate frequency comparison can be performed to determine the frequency of the measured signal. The method is to draw horizontal and vertical lines through the Lissajous figure respectively, and the drawn horizontal and vertical lines should not pass through the intersection of the figure or be tangent to it. If the number of intersections between the horizontal line and the figure is m, and the number of intersections between the vertical line and the figure is n, then

　　fy / fx = m / n

　　When the standard frequency fx (or fy) is known, the frequency fy (or fx) of the signal under test can be obtained from the above formula. Obviously, in actual testing work, when using Lissajous figures for frequency testing, in order to make the test simple and correct, if conditions permit, the frequency of the known frequency signal is usually adjusted as much as possible to make the figure displayed on the screen a circle or ellipse. At this time, the frequency of the signal under test is equal to the frequency of the known signal.

　　Since the two voltages applied to the oscilloscope have different phases, the graph on the screen will have different shapes, but this has no effect on the determination of the unknown frequency.

　　The Lissajous figure method is quite accurate in measuring frequency, but it is time-consuming to operate. At the same time, it is only applicable to measuring signals with lower frequencies.