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How to select an oscilloscope?
- Apr 23, 2018 -

Understand the signals you need to test.

What do you want to know with an oscilloscope? What are the typical characteristics that you want to capture and observe? Do your signals have complex features? Is your signal a duplicate or a single signal? What is the bandwidth of the signal transition process that you want to measure, or what is the increase time? What signal characteristics do you intend to use to trigger short pulses, pulse width, and narrow pulses? How many signals are you going to show at the same time? What do you do with test signals?

2. Determine the test signal bandwidth.

Bandwidth is generally defined as the frequency of sinusoidal input signal attenuation to -3db, namely 70.7% of the amplitude. The bandwidth determines the basic measurement capability of the oscilloscope. If there is not enough bandwidth, the oscilloscope will not be able to measure the high frequency signal, the amplitude will be distorted, the edge will disappear, the detail data will be lost; If there is not enough bandwidth, all the characteristics of the signal, including the ringing and the ringing, are meaningless.

A useful experience in determining the scope of the oscilloscope bandwidth you need - "five-fold rule of thumb" : multiply the highest frequency component of the signal you want to measure by 5, making the measurement more than 2 percent accurate.

In some applications, you don't know what you're interested in the signal bandwidth, but do you know it's the fastest rise time, the frequency response with the following formula to calculate the link bandwidth of the instrument and rise time: Bw = 0.35 / signal rise fastest time.

There are two types of digital oscilloscope bandwidths: repeat (or equivalent time) bandwidth and real-time (or single) bandwidth. The repeated bandwidth is only applicable to repeated signals, which show samples from multiple signal acquisition periods. Real-time bandwidth is single sampling of oscilloscope can capture the highest frequency, and when the capture events are not often appear or transient signal is more important, closely linked to the real-time bandwidth and sampling rate.

The higher the bandwidth, the better, but higher bandwidth tends to mean higher prices, so you should choose the frequency component you want to observe according to the budget.

3. Sampling rate (or sampling speed) of A/D converter

The unit is the sampling frequency per second (S/ S), and the frequency of the exponential oscilloscope to the signal sampling. The faster the oscilloscope is sampled, the higher the resolution and clarity of the waveform will be displayed, and the less likely it is that important information and events will be lost.

If you need to slowly varying signal within the scope of observation for a long time or low frequency signal, the minimum sampling rate can play a role, in order to display the waveform of record keeping and fixed number of waveform, need to adjust the level control knob, and shows the sampling rate will also changes with the change of 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 oscilloscope, such as sine interpolation method and vector insertion method. To accurately reproduce the signal and avoid confusion, the Nyquist theorem states that the sampling rate of the signal must be no less than twice the maximum frequency component. However, the premise of this theorem is based on infinite long time and periodic continuous signals. As a result of the oscilloscope cannot provide unlimited time record length, and from the point of view of definition, low frequency interference is discontinuous, nor cycle, so by twice the highest frequency component of the sampling rate are usually not sufficient.

In fact, the exact reproduction of the signal depends on the interpolation method adopted by the sampling rate and the gap of the signal sampling point, namely waveform reconstruction. Some oscilloscopes will provide operators with the following options: the sine interpolation method for measuring sinusoidal signals, and linear interpolation for measuring rectangular waves, pulses, and other signal types.

Have a compare the sampling rate and signal bandwidth useful rule of thumb: if you are observing the oscilloscope with interpolation (through screening to regenerate) between sampling points, the sampling rate/signal (bandwidth) ratio should be at least 4:1; In the absence of sinusoidal interpolation, the ratio of 10 to 1 should be taken.

4. Screen refresh rate is also known as waveform update speed.

All the oscilloscope will be flashing, oscilloscope signal to a specific number of capture every second, will no longer be measured between these points, this is the waveform capture rate, also known as the screen refresh rate, is expressed as wave number (WFMS/s) per second. Be sure to distinguish between the wave capture rate and the A/D sampling rate. The sampling rate indicates the frequency of the oscilloscope in A/D sampling input signal in A waveform or period; The waveform capture rate is the speed of the oscilloscope to collect the waveform. The waveform capture rate depends on the type and performance level of the oscilloscope and has a wide range of changes. Takanami shape capture rate oscilloscope will provide more important signal features, which can greatly increase the oscilloscope to capture fast transient abnormal situation, such as jitter, short pulse, low frequency interference and the instantaneous error probability.

5. Select appropriate storage depth, also known as record length.

The depth of storage is a measure of how many samples the oscilloscope can store. If you need to capture an impulse train continuously, the oscilloscope is required to have enough memory to capture the whole event. The desired storage depth can be calculated by dividing the length of time captured by the sampling rate required for the exact reproduction of the signal.

The depth of storage is closely related to the sampling rate. The depth of storage you need depends on the total time span you want to measure and the time resolution required. Modern oscilloscopes allow users to select record lengths to optimize the details of some operations. To analyze a very stable sinusoidal signal, it only needs a record length of 500 points; But if you want to parse a complex numeric data stream, you need to have a million points or more. The effective triggering of the signal in the correct position can usually reduce the actual storage capacity of the oscilloscope.

6. Select different trigger functions as needed.

The trigger of the oscilloscope can enable the signal to be scanned at the correct position point to make the signal characteristic clear. The trigger control button can stabilize the repeated waveform and capture the single waveform.

Most of the oscilloscope for users to use only edge-triggered way, if you have other triggered ability is very useful in some applications, especially for the new design product fault search, advanced triggered concerns events can be separated and find out you are concerned about abnormal problem, thus the most efficient use of sampling rate and depth of the storage.

Nowadays there are many oscilloscopes with advanced triggering ability. The trigger ability mainly consists of three aspects: the magnitude of the vertical direction, the transient spike triggering, overpulse or short pulse triggering, etc. The triggering form of time width, such as pulse width, narrow pulse, establishment/retention time, etc. The combination of the extension and the normal trigger function, for example the video signal or other hard-to-catch signals, triggered by the combination of time and amplitude combination. The increase of trigger capability CAN greatly improve the flexibility of the test process and simplify the work, especially the current oscilloscope has greatly improved the triggering ability of the data bus, such as CAN, I2C, etc.

7. Channel capacity, including the number of channels and the suspension capacity of the channel to the ground and the inter-channel isolation ability.

Number of channels you need depends on your application, for economical fault search application, usually need is double-channel oscillograph, however, the relationship of the requirements to observe several analog signals, will need a 4 channel oscilloscope, a lot of work in analog and digital two kinds of signal systems engineers can choose mixed signal oscilloscope (the oledata.mso), it will be logic analyzer channel count and triggered abilities associated with the comprehensive to have higher resolution time of oscilloscope display in a single instrument. If you measure three-phase electricity, silicon controlled rectifier active device or circuit, there is no absolute zero point between two ends, known as the floating signal, this time from the operating safety and precision, should choose the isolation channel oscilloscope; If the time sequence and phase shift of multi-channel are compared, two channels and above oscilloscope should be selected, and the isolation between channels is more important.

8. Capture of abnormal phenomena.

The three main factors affect the oscilloscope's ability to display the unknown and complex signals encountered in daily testing and debugging: screen refresh rate, waveform capture mode and trigger capability. Waveform capture modes include: sampling mode, peak detection mode, high resolution mode, envelope mode, average mode, etc. Screen refresh rate refers to how fast the oscilloscope reacts to signal and control changes, and peak detection helps to capture the peak of fast signals in slower signals.

9. Performance and index of oscilloscope.

There are many indicators of oscilloscope: vertical sensitivity, scanning speed, vertical precision, time base, vertical resolution, etc. The performance of oscilloscope depends on quality, stability and calibration service.

10. The analysis function will help you get twice the result.

The greatest advantage of digital oscilloscopes is that they can be measured by the data they can get, and the various analysis functions can be realized by pressing the button. Although the function of the available varies between manufacturer and model, but they typically include frequency, rise time, pulse width measurement, such as some oscilloscope also provides many analysis module, cases of FFT, power analysis, advanced mathematics, and exceptional functions.

11. Corresponding accessory and probe.

Easy to forget that when fitted with a probe, it will become a part of the whole test circuit, the probe will cause resistive, capacitive and inductive loads, oscilloscope brings out the measured object with different measurement results. Therefore, a corresponding probe is provided for different applications, and then one of them is selected to minimize the load effect, so that the signal can get the most accurate recurrence. Because of the development of SMT components, connections are more difficult, and different attachments are used to meet special needs.

12. Operating performance of oscilloscope.

Obviously, if you can't access to a variety of functions, or to spend a lot of time to learn them, so your oscilloscope will be of little value, proper training and Chinese operating interface makes you break through barriers to use.

13. Data management and communication capacity of oscilloscope.

The analysis of the measurement results is very important. It is increasingly important to conveniently save and share information and measurement results in high-speed communication networks.

The interconnectedness of the oscilloscope provides advanced analytical capability for results and simplifies the archiving and sharing of results. The oscilloscope provides a series of functions and control modes through various interfaces (Pass/Fail, rs-232, USB or Ethernet) and network communication mode.