What you must know before buying an oscilloscope, learn the basic parameters of the oscilloscope

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What you must know before buying an oscilloscope, learn the basic parameters of the oscilloscope [Copy link]

The oscilloscope is one of the most important and commonly used electronic testing tools. With the development of electronic technology, the capabilities of oscilloscopes are constantly improving. Their performance and prices are unique, and there are many varieties on the market. When purchasing an oscilloscope, you should fully consider the following factors: the type of signal to be captured and observed, whether the signal itself has complex characteristics, whether the signal to be detected is a repetitive signal or a single signal, and the signal transition process, bandwidth or rise time to be measured, etc.

Analog oscilloscopes may have the panel control buttons you are familiar with, and they are cheap. However, as the speed of A/D converters increases year by year and the price continues to decrease, as well as the increasing measurement capabilities and various practical functions of digital oscilloscopes, especially the improvement of the functions of capturing instantaneous signals and memorizing signals, digital oscilloscopes are becoming more and more popular. Therefore, when purchasing, you should take appropriate measures and make reasonable choices.

Let's take a look at the parameters that should be considered when choosing an oscilloscope:

bandwidth

Bandwidth is generally defined as the frequency width when the amplitude of a sinusoidal input signal decays to -3dB, which is 70.7% of the average amplitude. Bandwidth determines the basic measurement capability of an oscilloscope for a signal. As the frequency of the measured signal increases, the oscilloscope's ability to accurately display the signal will decrease. If there is not enough bandwidth, the oscilloscope will not be able to distinguish the changes in high-frequency components. The amplitude will be distorted, the edges will become smooth, and the detailed parameters will be lost. If there is not enough bandwidth, all characteristics and parameters of the signal cannot be obtained.

When selecting an oscilloscope, multiply the highest frequency component of the signal to be measured by 5 as the bandwidth of the oscilloscope. This will achieve an accuracy of more than 2% in the measurement. For example, if you want to measure the color subcarrier of a TV, the frequency is 4.43MHz. An oscilloscope that takes 4.43MHz five times, which is about 22MHz, can meet the precise measurement requirements.

In some applications, the signal bandwidth is unknown, but its fastest rise time is needed. The frequency response of most digital oscilloscopes can be used to calculate the equivalent bandwidth and rise time of the instrument: Bw=0.35/fastest rise time of the signal.

There are two types of bandwidth: repetitive (or equivalent time) bandwidth and real-time (or single-shot) bandwidth. Repetitive bandwidth only applies to repetitive signals, showing samples from multiple signal acquisition periods. Real-time bandwidth is the highest frequency that can be captured in a single sample of the oscilloscope, and is quite demanding when the captured signal does not occur very often. Real-time bandwidth is closely related to the sampling rate.

Since wider bandwidth often means higher price, comprehensive consideration should be made based on cost, investment and performance.

Sampling rate

The sampling rate is the number of samples per second, which refers to the frequency at which a digital oscilloscope samples a signal. The faster the sampling rate of the oscilloscope, the higher the resolution and clarity of the displayed waveform, and the lower the probability of losing important information and random signals.

If you need to observe slow-changing signals over a longer time frame, the minimum sampling rate becomes more important. In order to maintain a fixed number of waveforms in the displayed waveform record, you need to adjust the horizontal control knob, and the displayed sampling rate will also change with the adjustment of the horizontal adjustment knob.

The method for calculating the sampling rate depends on the type of waveform being measured and the method used by the oscilloscope to reproduce the signal.

In order to accurately reproduce the signal and avoid confusion, the Nyquist theorem stipulates that the sampling rate of the signal must be greater than twice the highest frequency component of the measured signal . However, the premise of this theorem is based on infinite time and continuous signals.

Since no oscilloscope can provide an infinite time record length, and since low-frequency interference is by definition discontinuous, using a sampling rate twice that of the highest frequency component is usually not sufficient for a digital oscilloscope.

In reality, the exact reproduction of a signal depends on its sampling rate and the interpolation method used to space the signal's sample points. A useful rule of thumb when comparing sampling rate and signal bandwidth is: If the oscilloscope interpolates (filters to regenerate between sample points), the ratio (sampling rate/signal bandwidth) should be at least 4:1. Without sinusoidal interpolation, a ratio of 10:1 should be used.

Waveform capture rate

All oscilloscopes flicker. That is, the oscilloscope captures the signal a certain number of times per second, and no measurements are taken between these measurement points. This is the waveform capture rate, also called the screen refresh rate, expressed as waveforms per second (wfms/s). The sampling rate is how often the oscilloscope samples the input signal within a waveform or cycle; the waveform capture rate is how quickly the oscilloscope acquires waveforms. The waveform capture rate depends on the type and performance level of the oscilloscope and can vary widely. Oscilloscopes with high waveform capture rates will provide more important signal characteristics and greatly increase the probability that the oscilloscope will quickly capture transient anomalies such as jitter, runt pulses, low-frequency interference, and transient errors.

Digital storage oscilloscopes (DSOs) use a serial processing structure to capture 10 to 5,000 waveforms per second. DPO digital phosphor oscilloscopes use a parallel processing structure to provide a higher waveform capture rate, some as high as millions of waveforms per second, greatly improving the possibility of capturing intermittent and difficult-to-capture signals, and can more quickly discover instantaneous signals.

Storage Depth

Memory depth, also called record length, is a measure of how many sampling points an oscilloscope can store. If you need to capture a pulse train without interruption, the oscilloscope must have enough storage space to capture the signals that occur occasionally during the entire process. The required memory depth can be calculated by dividing the length of time to be captured by the sampling speed required to accurately reproduce the signal.

Effective triggering at the right location to capture a signal can often reduce the amount of memory actually required by the oscilloscope.

The memory depth is closely related to the sampling speed. The memory depth depends on the total time span to be measured and the required time resolution.

Many oscilloscopes allow the user to select the record length to optimize for detail in some operations. Analyzing a very stable sinusoidal signal may require only a 500-point record length, but resolving a complex digital data stream may require a record length of one million points or more.

Trigger function

The trigger of the oscilloscope can start the horizontal synchronous scanning of the signal at the correct position, which determines whether the display of the signal waveform is clear. The trigger control button can stably and repeatedly display the waveform and capture a single waveform.

Most general-purpose oscilloscope users use only edge triggering, especially for troubleshooting new product designs. Advanced triggering can isolate the desired signal, making the most effective use of sampling speed and memory depth.

Many oscilloscopes today have advanced trigger capabilities: they can trigger based on pulses defined by amplitude, pulses defined by time (pulse width trigger), and pulses described by logic states or graphics (logic trigger). The combination of extended and conventional trigger functions also helps display video and other hard-to-capture signals. Such advanced trigger capabilities provide a great degree of flexibility when setting up the test process, and can greatly simplify the measurement work, bringing great convenience to use.

Number of channels of the oscilloscope

The number of channels of an oscilloscope depends on the number of signals observed simultaneously. In the development and maintenance of electronic products, a dual-channel oscilloscope or dual-trace oscilloscope is needed. If you need to observe the relationship between multiple analog signals, you will need a 4-channel oscilloscope. Many scientific research environments that work with both analog and digital signal systems also consider using 4-channel oscilloscopes.

Other considerations when purchasing an oscilloscope

When purchasing an oscilloscope, you should choose one based on the test needs. For general testing, a single-channel oscilloscope is sufficient. When testing two related signals, a dual-channel oscilloscope is required. A dual-channel oscilloscope has two independent signal processing channels and can input two signals at the same time, so that the phase, amplitude, waveform and other parameters of the two signals can be compared and measured. In addition, when testing the DC component and AC component of the same signal, it is very convenient to use the two channels of a dual-channel oscilloscope, one channel to measure the DC component and the other channel to measure the AC component.

When repairing household electrical appliances such as televisions, DVD players, and induction cookers, a 40MHz dual-channel oscilloscope is usually selected to meet the repair requirements, because in repair work, it is often not necessary to accurately measure various parameters of the signal, but only to roughly observe the signal waveform and estimate the frequency and period. In addition, the frequency characteristics of many signals in household electrical appliances are within the measurement range of the oscilloscope, such as audio signals, video signals, line synchronization, field synchronization, control signals, etc., which are all within the measurement range of the oscilloscope, and some clock signals greater than 40MHz can also be measured.

If you have good financial conditions, you should buy a 100MHz oscilloscope, which can measure high-frequency signals with higher accuracy.

The oscilloscope used in home appliance maintenance work should preferably have line selection (line signal of TV signal) and field selection (field synchronization) functions, which facilitates synchronization when observing video signals and makes the observed waveform stable.

philips_lu

I haven't used an oscilloscope for a long time. It's been gathering dust in the firewood room. However, the author focuses on digital oscilloscopes. Each has its own advantages.

bigbat

The most common use of an oscilloscope is to test digital signals. A very low-end application is to see if there is a signal or if the crystal oscillator is oscillating. What I want most is an oscilloscope that can decode digital signals.