Working principle and basic functions of oscilloscope, the difference between oscilloscope and spectrum analyzer

　　An oscilloscope is a widely used electronic measuring instrument. It is the eyes of electronic engineers. It can transform invisible electrical signals into visible images, making it easier for people to study the changing process of various electrical phenomena. The first condition of an oscilloscope is to accurately display waveforms and ensure signal integrity measurement.

　　Function of oscilloscope: An instrument used to measure the shape of alternating current or pulse current waves, consisting 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 transformed into an electrical effect can be observed with an oscilloscope.

How an Oscilloscope Works

　　The oscilloscope uses a narrow electron beam composed of high-speed electrons to hit the screen coated with fluorescent material to produce tiny light spots (this is the working principle of traditional analog oscilloscopes). Under the action of the measured signal, the electron beam is like the tip of a pen, which can draw a curve of the instantaneous value of the measured signal on the screen. The oscilloscope can be used to observe the waveform curve of various signal amplitudes changing with time, and it can also be used to test various electrical quantities, such as voltage, current, frequency, phase difference, amplitude modulation, etc.

　　First of all, oscilloscopes are divided into two types based on design principles: analog oscilloscopes and digital oscilloscopes. The earliest oscilloscopes were analog oscilloscopes, but now due to bandwidth and other issues, analog oscilloscopes have gradually been eliminated. So what is the principle of an analog oscilloscope? The following picture can explain it well:

　　The analog oscilloscope generates a periodic sawtooth signal inside to control the horizontal deflection of the silver flat electron gun, and the measured voltage signal is amplified to control the vertical deflection of the phosphor screen electron gun. In this way, the light spot or bright line is clearly displayed on the phosphor screen, which is the waveform.

　　From the perspective of design concept, analog oscilloscopes have many incomparable advantages, such as no signal waveform loss, no dead time, etc. Although digital oscilloscopes had these problems at the beginning, with the vigorous development of electronic technology, such defects have become smaller and smaller. So what is the design principle of digital oscilloscopes?

　　The waveform first passes through the probe, is amplified by the front-end amplifier, then converted by the analog-to-digital conversion unit, stored in the acquisition memory, and then displayed on the monitor.

　　During this whole process, the waveform is not displayed on the screen in real time, but is displayed on the waveform after being acquired in memory. Therefore, if the entire sampling and conversion time is long, a large dead time will be generated, so the waveform within the dead time cannot be observed. This is why many people still insist that digital oscilloscopes are not as good as analog oscilloscopes.

　　The advantages of analog oscilloscopes are self-explanatory, including good real-time performance, simple principle, and low price. However, the instrument principle itself also contains flaws that will eventually be abandoned by the times. There are roughly the following:

　　1. Limited bandwidth: This is definitely a fatal flaw. The input signal of the analog oscilloscope is amplified and directly controls the deflection of the electron gun of the CRT display. Although the bandwidth of the amplifier can be higher and higher, the deflection speed of the CRT electron gun is limited. For high-frequency signals, the speed of the electron gun cannot keep up with the signal changes. Therefore, it is really difficult to increase the bandwidth of the current analog oscilloscope.

　　Second, it cannot be stored and analyzed: Many experienced engineers know very well that to save waveforms with an analog oscilloscope, you need to take a photo with a camera. If you want to measure amplitude, period, and rise time, you can only do it manually. If you want to measure phase difference and power, for a digital oscilloscope, this can be done by just checking a box, but for an analog oscilloscope, it is simply a physical job.

　　3. Triggering capability is too weak: basically it can only be edge triggered. Want to trigger by pulse width? Slope trigger? It is impossible! Not to mention the triggering method of opening a graphic to do template triggering, which is a very creative way.

　　4. Unstable performance: After all, there are a lot of analog devices. After a long time, the indicators will be unstable, and the temperature drift is much more serious than that of digital oscilloscopes. Since the 1980s, digital oscilloscopes have gradually emerged. Especially with the development of high-speed ADC chips and digital processing technology, digital oscilloscopes have surpassed analog oscilloscopes in bandwidth, triggering, analysis, and display. Now, almost all oscilloscopes on the market are digital oscilloscopes.

　　One point to emphasize here is still the dead time, which depends on the processing and display speed behind the digital oscilloscope. Although real-time processing is still not possible under the current technical level, the faster the processing speed, the fewer waveforms are lost. The performance indicator in this regard is called the waveform refresh rate. For a 200MHz bandwidth oscilloscope, almost all brands will be equipped with a 1G sampling rate, but the waveform refresh rate is one of the more important parameters. The higher the waveform refresh rate, the smaller the dead time of waveform observation.

　　In any case, it is an inevitable trend for digital oscilloscopes to replace analog oscilloscopes. In the stage of rapid development of electronic technology, I believe that the price advantage of analog oscilloscopes will gradually disappear.

Basic functions 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, that 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.

　　Functions of the Oscilloscope Panel Knobs

　　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 function 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.

The Difference Between Oscilloscope and Spectrum Analyzer

　　The differences in the analysis performance indicators of oscilloscopes and spectrum analyzers are compared from four aspects: real-time bandwidth, dynamic range, sensitivity and power measurement accuracy.

　　1 Real-time bandwidth

　　For an oscilloscope, bandwidth is usually its measurement frequency range. For a spectrum analyzer, bandwidth is defined as intermediate frequency bandwidth, resolution bandwidth, etc. Here, we will discuss the real-time bandwidth that can perform real-time analysis on the signal.

　　For spectrum analyzers, the bandwidth of the final analog intermediate frequency can usually be used as the real-time bandwidth of its signal analysis. The real-time bandwidth of most spectrum analyzers is only a few megahertz, and the wider real-time bandwidth is usually tens of megahertz. Of course, the widest bandwidth FSW spectrum analyzer can reach 500 megahertz. The real-time bandwidth of an oscilloscope is the effective analog bandwidth of its real-time sampling, which is generally hundreds of megahertz and can reach several thousand megahertz.

　　It should be pointed out here that the real-time bandwidth of most oscilloscopes may not be consistent when the vertical scale is set differently. When the vertical scale is set to the most sensitive, the real-time bandwidth usually decreases.

　　In terms of real-time bandwidth, oscilloscopes are generally superior to spectrum analyzers, which is particularly beneficial for certain ultra-wideband signal analysis, especially in modulation analysis where they have incomparable advantages.

　　2 Dynamic Range

　　Dynamic range varies depending on its definition. In many cases, dynamic range is described as the level difference between the maximum and minimum signals measured by the instrument. When the measurement settings are changed, the instrument's ability to measure large and small signals is different. For example, the distortion caused by measuring large signals by a spectrum analyzer is different when the attenuation settings are different. Here, we discuss the instrument's ability to measure large and small signals at the same time, that is, the optimal dynamic range of an oscilloscope and spectrum analyzer under appropriate settings without changing any measurement settings.

　　For spectrum analyzers, without considering proximal noise and spurious signals such as phase noise, the average noise level, second-order distortion, and third-order distortion are the main factors restricting the dynamic range. Calculated based on the technical indicators of mainstream spectrum analyzers, the ideal dynamic range is about 90dB (limited by second-order distortion).

　　Most oscilloscopes are limited by their AD effective sampling bits and noise floor. The ideal dynamic range of traditional oscilloscopes usually does not exceed 50dB. (For R&S RTO oscilloscopes, at 100KHz RBW, its dynamic range can be as high as 86dB)

　　In terms of dynamic range, spectrum analyzers are better than oscilloscopes. But it should be pointed out that this is true for spectrum analysis of constant signals. However, the spectrum of an oscilloscope is the same frame of data, while the spectrum of a spectrum analyzer is not the same frame of data in most cases. Therefore, for transient signals, a spectrum analyzer may not be able to measure them. The probability of an oscilloscope finding transient signals (when the signal meets the dynamic range) is much greater.

　　3 Sensitivity

　　The sensitivity discussed here refers to the minimum signal level that an oscilloscope and spectrum analyzer can test. This indicator is closely related to the instrument settings.

　　For an oscilloscope, when the Y-axis is set to the most sensitive level, usually 1mV/div, the minimum signal that the oscilloscope can test. Leaving aside factors such as port mismatch, the noise generated by the oscilloscope's signal channel and the noise caused by unstable track are the most important factors limiting the oscilloscope's sensitivity.

　　4 Power measurement accuracy

　　For frequency domain analysis, power measurement accuracy is a very important technical indicator. Whether it is an oscilloscope or a spectrum analyzer, there are many factors that affect power measurement accuracy. The main factors are listed below:

　　For an oscilloscope, the factors that affect power measurement accuracy include: reflection caused by port mismatch, vertical system error, frequency response, AD quantization error, calibration signal error, etc.

　　For spectrum analyzers, the influencing factors of power measurement accuracy include reflection caused by port mismatch, reference level error, attenuator error, bandwidth conversion error, frequency response, calibration signal error, etc.

　　In addition, within the frequency range, the frequency response index of the oscilloscope is also very good, not exceeding 0.5dB within the 4GHz range. From this point of view, the performance of the oscilloscope is even better than that of the spectrum analyzer.

　　In general, oscilloscopes and spectrum analyzers have their own strengths in frequency domain analysis performance. Spectrum analyzers are superior in technical indicators such as sensitivity, and oscilloscopes are better than spectrum analyzers in real-time bandwidth. When measuring different types of signals, you can choose according to the test requirements and the different technical characteristics of the instrument.