Oscilloscope Selection

Since its advent, the oscilloscope has been one of the most important and commonly used electronic test instruments. Due to the development of electronic technology, the capabilities of oscilloscopes are constantly improving, and their performance and prices are also varied, and the market is uneven. Oscilloscopes seem simple, but there are also many problems in how to choose them. Based on years of experience and combined with the selection guide of Beijing Ocean Xingye Technology Co., Ltd., this article will tell you from several aspects what you should pay attention to when choosing an oscilloscope:

1. Understand the signal you need to test

You need to know what to observe with the oscilloscope? What is the typical performance of the signal you want to capture and observe? Does your signal have complex characteristics? Is your signal a repetitive signal or a single signal? What is the bandwidth or rise time of the signal transition process you want to measure? What signal characteristics do you plan to use to trigger short pulses, pulse widths, narrow pulses, etc.? How many signals do you plan to display at the same time? What processing do you do with the test signal?

2. Core technology differences in choosing oscilloscopes: analog (DRT), digital (DSO), or digital-analog hybrid (DPO)

The traditional view is that analog oscilloscopes have familiar control panels and are inexpensive, so they are always considered "easy to use". However, with the increasing speed and decreasing price of A/D converters year by year, as well as the increasing measurement capabilities and virtually unlimited measurement functions of digital oscilloscopes, digital oscilloscopes have become the leader. However, digital oscilloscopes have three-dimensional display defects and slow processing of continuous data, and require oscilloscopes with digital-analog hybrid technology, such as DPO digital phosphor oscilloscopes.

3. Determine the test signal bandwidth

Bandwidth is generally defined as the frequency when the amplitude of the sine wave input signal decays to -3dB, that is, 70.7% of the amplitude. Bandwidth determines the basic measurement capability of the oscilloscope for the signal. If there is not enough bandwidth, the oscilloscope will not be able to measure high-frequency signals, the amplitude will be distorted, the edges will disappear, and the detailed data will be lost; if there is not enough bandwidth, all the characteristics of the obtained signal, including ringing and ringing, are meaningless.

An effective rule of thumb for determining the oscilloscope bandwidth you need - the "5x rule of thumb": multiply the highest frequency component of the signal you want to measure by 5 to obtain a measurement result with an accuracy higher than 2%.

In some applications, you don't know the bandwidth of your signal of interest, but you know its fastest rise time. In this case, the frequency response uses the following formula to calculate the correlation bandwidth and instrument rise time: Bw=0.35/fastest rise time of the signal.

There are two types of digital oscilloscope bandwidth: repetitive (or equivalent time) bandwidth and real-time (or single-shot) bandwidth. Repetitive bandwidth only applies to repetitive signals and displays samples from multiple signal acquisition periods. Real-time bandwidth is the highest frequency that can be captured in a single sampling of the oscilloscope, and it is more important when the captured event is not a frequent or transient signal. Real-time bandwidth is closely related to the sampling rate. The higher

the bandwidth, the better, but higher bandwidth often means a higher price, so you should choose the signal frequency component you want to observe according to your budget.

Fourth, the sampling rate (or sampling speed) of the A/D converter

is measured in samples per second (S/s), which refers to the frequency at which the digital oscilloscope samples the 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 events.

If you need to observe slow-changing signals or low-frequency signals over a longer time range, the minimum sampling rate comes into play. 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 change of the horizontal adjustment knob.

How to calculate the sampling rate? The calculation method depends on the type of waveform measured and the signal reconstruction method adopted by the oscilloscope, such as sine interpolation, vector interpolation, etc. In order to accurately reproduce the signal and avoid confusion, the Nyquist theorem stipulates that the sampling rate of the signal must be no less than twice its highest frequency component. However, the premise of this theorem is based on infinite time and continuous periodic signals. Since the oscilloscope cannot provide an infinite time record length, and by definition, low-frequency interference is discontinuous and not periodic, it is usually not enough to use a sampling rate twice the highest frequency component.

In fact, the accurate reproduction of the signal depends on its sampling rate and the interpolation method used for the gap between signal sampling points, that is, waveform reconstruction. Some oscilloscopes offer the operator the choice of sinusoidal interpolation for measuring sinusoidal signals and linear interpolation for measuring rectangular waves, pulses, and other signal types.

A useful rule of thumb for comparing sampling rate and signal bandwidth is that if the oscilloscope you are looking at has interpolation (filtering to regenerate between sampling points), the ratio of (sampling rate/signal bandwidth) should be at least 4:1; without sinusoidal interpolation, the ratio should be 10:1.

5. Screen refresh rate, also known as waveform update rate

All oscilloscopes flash. The oscilloscope captures the signal a certain number of times per second. No measurements are made between these measurement points. This is the waveform capture rate, also known as the screen refresh rate, expressed as waveforms per second (wfms/s). It is important to distinguish between the waveform capture rate and the A/D sampling rate. The sampling rate indicates how often the oscilloscope A/D samples the input signal in one waveform or cycle; the waveform capture rate refers to how fast the oscilloscope acquires waveforms. The waveform capture rate depends on the type and performance level of the oscilloscope and varies widely. An oscilloscope with a high waveform capture rate will provide more important signal characteristics and greatly increase the probability of the oscilloscope quickly capturing transient abnormal conditions, such as jitter, short pulses, low-frequency interference, and transient errors.

Generally speaking, analog oscilloscopes have a high screen refresh rate due to their simple circuits, while digital storage oscilloscopes (DSOs) can capture 10 to 5,000 waveforms per second using a serial processing structure. In order to change the problem of low screen refresh rates of digital oscilloscopes, digital phosphor oscilloscopes use a parallel processing structure, which can provide a higher waveform capture rate, some up to millions of waveforms per second, greatly improving the possibility of capturing intermittent and difficult-to-capture events, and allowing you to find problems with signals more quickly.

6. Choose the appropriate memory depth, also known as record length

. Memory depth is a measure of how many sampling points an oscilloscope can store. If you need to capture a pulse train continuously, the oscilloscope is required to have enough memory to capture the entire event. The required memory depth can be calculated by dividing the length of time to be captured by the sampling rate required to accurately reproduce the signal.

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

Modern oscilloscopes allow users to select record lengths to optimize the details of some operations. To analyze a very stable sinusoidal signal, only a record length of 500 points is required; but if you want to analyze a complex digital data stream, a record length of one million points or more is required.

Effective triggering that captures the signal at the right position can usually reduce the amount of memory actually required by the oscilloscope.

7. Select different trigger functions according to needs

The trigger of the oscilloscope can synchronize the horizontal scanning of the signal at the right position to make the signal characteristics clear. The trigger control button can stabilize the repetitive waveform and capture a single waveform.

Most users of oscilloscopes only use edge triggering. If you have other triggering capabilities, it is very useful in some applications, especially for troubleshooting of new design products. Advanced triggering methods can separate the events of concern and find out the abnormal problems you are concerned about, so as to make the most effective use of sampling rate and memory depth.

There are many oscilloscopes with advanced triggering capabilities. The triggering capabilities mainly focus on three aspects: ① vertical amplitude, such as transient spike trigger, over-pulse or short pulse trigger, etc.; ② horizontal time-related trigger, such as pulse width, narrow pulse, setup/hold time and other trigger forms with set time width; ③ combination of extended and conventional triggering functions, such as triggering video signals or other difficult-to-capture signals by setting trigger conditions through time and amplitude combination. The improvement of triggering capabilities can greatly improve the flexibility of the test process and simplify the work, especially the triggering capabilities of today's oscilloscopes for data buses, such as CAN, I2C, etc. [page]

8. Channel capability, including the number of channels, the ability of channels to float to ground, and the isolation between channels

The number of channels you need depends on your application. For common economical fault-finding applications, a dual-channel oscilloscope is required, but if you want to observe the relationship between several analog signals, you will need a 4-channel oscilloscope. Many system engineers working with both analog and digital signals can choose a mixed signal oscilloscope (MSO), which combines the channel count and triggering capabilities of a logic analyzer with the higher resolution of an oscilloscope into a single instrument with a time-correlated display. If you are measuring three-phase electricity, active devices or lines such as thyristors, there is no absolute zero point between the two ends, which is the so-called floating signal. At this time, from the perspective of operational safety and accuracy, an isolated channel oscilloscope should be selected; if you compare the timing and phase shift of multiple channels, you should choose an oscilloscope with more than two channels, and the isolation between channels is even more important.

9. Capture of abnormal phenomena

Three main factors affect the ability of an oscilloscope to display unknown and complex signals encountered in daily testing and debugging: screen refresh rate, waveform capture method, and trigger capability. The waveform capture modes include: sampling mode, peak detection mode, high resolution mode, envelope mode, average mode, etc. The screen refresh rate tells you how quickly the oscilloscope reacts to changes in signals and controls. Using peak detection helps capture the peak of a fast signal in a slower signal.

10. Oscilloscope performance and indicators

There are many indicators of an oscilloscope: such as vertical sensitivity, sweep speed, vertical accuracy, time base, vertical resolution, etc. The performance of an oscilloscope depends on the quality of the brand, and the key lies in quality, stability and calibration services.

11. Analysis functions help you get more results with less effort

The biggest advantage of digital oscilloscopes is that they can measure the data they get, and various analysis functions can be implemented at the touch of a button. Although the available functions vary by manufacturer and model, they generally include measurements such as frequency, rise time, pulse width, etc. Some oscilloscopes also provide many analysis modules, such as FFT, power analysis, advanced mathematical operations and other extraordinary functions.

12. Corresponding accessories and probes

It is easy to forget that when a probe is installed, it becomes part of the entire test circuit. As a result, the probe will cause resistive, capacitive and inductive loads, causing the oscilloscope to present different measurement results than the object being measured. Therefore, it is necessary to equip the corresponding probes for different applications, and then choose one of them to minimize the loading effect and make the signal reproduced most accurately. Due to the development of SMT components, connections are more difficult, and different accessories are used to meet special needs.

13. Oscilloscope operation performance

Obviously, if you cannot access various functions or spend a lot of time learning them, then your oscilloscope will be of little value. Proper training and Chinese operating interface will enable you to break through the barriers to use.

14. Oscilloscope data management and communication capabilities

Analysis of measurement results is very important. It is becoming increasingly important to easily save and share information and measurement results in high-speed communication networks.

The connectivity of oscilloscopes provides advanced analysis capabilities for results and simplifies the archiving and sharing of results. Oscilloscopes provide a range of functions and control methods through various interfaces (GPIB, RS-232, USB or Ethernet) and network communication modes.

15. Expansibility of Oscilloscope Functions

In order to adapt to changing needs, the oscilloscope functions should be able to expand randomly:

○ Increase channel memory to analyze longer record lengths
○ Increase measurement functions for specific applications
○ Have a complete set of compatible probes and modules to enhance the capabilities of the oscilloscope
○ Work with common third-party Windows-compatible analysis software, such as OIscope oscilloscope software.
○ Add accessories, such as battery packs and rack mounts.

In short, the choice of oscilloscope is a seemingly simple but difficult problem for you to deal with. There are many products on the market, and the technologies are different. Sometimes it is difficult for you to make a decision. The above description may give you some suggestions. It will be more beneficial for you to adopt the selection process in the above figure. According to many years of experience, there are the following "rules of thumb" for choosing oscilloscopes:

ART analog oscilloscopes, choose four factors: cost performance (comparative advantage of price and product quality brand), test bandwidth (5 times rule of thumb), number of channels (2 or 4), supplier capabilities (whether after-sales service is guaranteed).

DSO digital storage oscilloscopes, find a balance between test signal bandwidth, oscilloscope bandwidth, oscilloscope real-time sampling rate, and oscilloscope storage depth. There are the following experience to follow: the oscilloscope bandwidth is preferably 5 times the signal bandwidth; the oscilloscope real-time sampling rate ≥ 4 times the oscilloscope bandwidth; the storage depth ≥ sampling rate × the maximum required storage time.

DPO digital-analog hybrid oscilloscopes are consistent with DSOs in basic indicator requirements, but need to introduce two capabilities: screen refresh rate, waveform triggering and analysis capabilities.
Special functional requirements.

① If you need to work on-site and need battery power, have strict requirements on the size of the instrument, and need other tests for the instrument's functions in addition to oscilloscope measurement (such as multimeter functions), you'd better use a handheld oscilloscope (HSO).

② If your safety cannot be guaranteed when you are isolated or suspended, and you need to analyze power and phase shift, please use an isolated oscilloscope (DIO), especially a multi-channel DIO.

③ If you need a multi-channel mixed test of analog and digital signals, in addition to an oscilloscope with serial bus triggering function, you'd better use an MSO hybrid oscilloscope. (end)