Introduction to Oscilloscopes

Introduction to Oscilloscopes

An oscilloscope is an instrument used to measure the shape of alternating current or pulse current waves. It consists of a tube amplifier, a scanning oscillator, a cathode ray tube, etc. In addition to observing the waveform of the current, it can also measure the frequency, voltage intensity, etc. Any periodic physical process that can be converted into an electrical effect can be observed with an oscilloscope.

Oscilloscopes are divided into digital oscilloscopes and analog oscilloscopes. Analog oscilloscopes use analog circuits (oscilloscope tubes, which are based on electron guns). The electron guns emit electrons to the screen, and the emitted electrons are focused to form electron beams and hit the screen. The inner surface of the screen is coated with fluorescent substances, so that the points hit by the electron beam will emit light. Digital oscilloscopes are high-performance oscilloscopes made by a series of technologies such as data acquisition, A/D conversion, and software programming. Digital oscilloscopes generally support multi-level menus, which can provide users with a variety of choices and multiple analysis functions. Some oscilloscopes can also provide storage to save and process waveforms.

The working principle of the oscilloscope is: the relative size of the waveform amplitude displayed on the oscilloscope is used to reflect the relative size of the maximum voltage applied to the Y deflection plate of the oscilloscope, thereby reflecting the maximum size of the alternating electromotive force generated in electromagnetic induction. Therefore, with the help of an oscilloscope, the relationship between the induced electromotive force and its generation conditions can be studied. The oscilloscope is an electronic measuring instrument with a wide range of uses. It can transform electrical signals that are invisible to the naked eye into visible images, making it easier for people to study the changing process of various electrical phenomena. The oscilloscope uses a narrow electron beam composed of high-speed electrons to hit the screen coated with fluorescent materials to produce tiny spots of light. Under the action of the measured signal, the electron beam is like the tip of a pen, which can draw a curve of the change of the instantaneous value of the measured signal on the screen. The oscilloscope can be used to observe the waveform curves of various different electrical signal amplitudes changing with time, and it can also be used to test the electrical quantities of various different signals, such as voltage, current, frequency, phase difference, amplitude modulation, etc.

The dual-trace oscilloscope is composed of two channels of y-axis preamplifier circuit, gate circuit, electronic switch, mixing circuit, delay circuit, y-axis post-amplifier circuit, trigger circuit, scanning circuit, x-axis amplifier circuit, z-axis amplifier circuit, calibration signal circuit, oscilloscope and high and low voltage power supply circuits.

When observing the signal waveform, the measured signals UA and UB are input into the oscilloscope through the two input terminals of CHA and CHB, and are first sent to the y-axis preamplifier circuits yA and yB for amplification. Because both channel yA and channel yB are controlled by electronic switches, the UA and UB signals are alternately sent to the mixing circuit, delay circuit, and y-axis post-amplifier circuit at the back, and added to the vertical deflection plate of the oscilloscope.

In order to meet various testing needs, the electronic switch can have five different working states, namely CHA, CHB, alternating, intermittent, ADD, etc. These five working states are controlled by the display mode switch.

When the display mode switch is in the alternate position, the electronic switch is a bistable circuit. It is controlled by the gate signal from the scanning circuit, so that the two front channels of the y-axis work alternately as the gate signal of the scanning circuit changes. The number of alternating conversions per second is related to the repetition frequency of the scanning signal generated by the scanning circuit. The alternating working state is suitable for observing the measured signal with a not too low frequency.

When the display mode switch is set to the intermittent position, the electronic switch is a self-excited multivibrator circuit with an oscillation frequency of about 200kHz. Two rectangular signals with opposite phases are output from its two output terminals. The preamplifier circuits CHA and CHB are controlled by the above two rectangular signals and work in turn. In this way, two signals can be displayed stably. This intermittent working state is suitable for observing the measured signal with a low frequency.

When the display mode switch is set to CHA or CHB, the electronic switch is a monostable circuit. The preamplifier circuit CHA or CHB can work independently. At this time, the dual-trace oscilloscope can be used as an ordinary single-line oscilloscope.

When the display mode switch is in the ADD position, the electronic switch is in the non-working state. At this time, the CHA and CHB channels work simultaneously, so the display of the addition or subtraction of the two signals can be obtained. However, whether the two signals are added or subtracted is selected by the polarity action switch of the CHA channel.

In order to observe the waveform of the tested signal changing with time, a linear scanning voltage (sawtooth voltage) must be applied to the horizontal deflection plate of the oscilloscope. This scanning voltage is generated by the scanning circuit. When the trigger signal is added to the trigger circuit, the trigger scanning circuit generates a corresponding scanning signal. When no trigger signal is added, the scanning circuit does not generate a scanning signal.

There are two trigger modes: internal trigger and external trigger. The trigger source selection switch is used to select the trigger mode. When the switch is in the internal position, the trigger signal comes from the measured signal sent in through the y-axis channel. When the switch is in the external position, the trigger signal is sent in from the outside. This signal should be in an integer ratio with the frequency of the measured signal. In the use of oscilloscopes, most of them adopt the internal trigger working mode.

The scanning circuit generates a scanning signal (sawtooth wave circuit), which is connected to the x-axis amplifier circuit through the x-axis selection switch and sent to the x-axis deflection plate of the oscilloscope after amplification.

The Z-axis amplifier circuit regulates the brightness of the light spot on the fluorescent screen and erases unnecessary light spot tracks. When the gate signal of the scanning circuit reaches the z-axis amplifier circuit, the z-axis amplifier circuit outputs a positive brightness enhancement pulse signal and adds it to the control electrode of the oscilloscope. That is to say, during the positive process of the scanning signal, the light spot on the fluorescent screen is enhanced. During the switching process of the electronic switch, the electronic switch circuit will also add the output pulse signal to the z-axis amplifier circuit. At this time, the z-axis amplifier circuit outputs a negative pulse signal and adds it to the control electrode of the oscilloscope. In this way, during the switching process of the electronic switch, the excessive light spots when the two channels work alternately are eliminated to improve the clarity of the displayed waveform.

The correction signal generating circuit generates a rectangular signal with a certain frequency and amplitude, which is used to correct the sensitivity of the y-axis amplifier circuit and the scanning speed of the x-axis.

High and low voltage power supply, the high voltage is supplied to the oscilloscope display system, and the low voltage is supplied to the oscilloscope circuits at all levels.