How to use oscilloscope correctly?
How to use oscilloscope correctly? At the beginning of choosing an oscilloscope, you already have a price range in mind. The price of an oscilloscope depends on a number of factors, including bandwidth, sampling rate, number of channels, and storage depth. If you only buy it based on price, you may not end up getting the performance you need.
Methods to choose one oscilloscope
(1) Confirm whether you need one analog or digital oscilloscope?
Both digital and analog oscilloscopes have its advantages and disadvantages.
With the development of modern technology, the function of digital oscilloscope is stronger, the response is faster and the price is lower.These advantages make it difficult for analog oscilloscopes to compete with advanced digital oscilloscopes. At present, almost all customers choose the digital oscilloscope, analog oscilloscope has been OUT of the basic.
(2) Confirm your demand on bandwidth
Instruments that measure ac waveforms usually have a frequency upper limit, and if the frequency of the waveform is above that the accuracy of the measurement will deteriorate.The upper limit of the frequency is the bandwidth of the instrument. Usually defined as the response of the instrument to lower the frequency by 3bB, the value of the bandwidth of the instrument you need depends on the characteristics of the signal being measured and the accuracy of the measurement you wish to obtain. An oscilloscope has two types of widths, namely, repeat (or analog) bandwidth and real time bandwidth. Many digital oscilloscopes provide analog bandwidths that are higher than their basic sampling rates. This is possible. If a signal repeats, the oscilloscope does not have to complete all the collection at one time. Instead, it can obtain part of the waveform when each trigger occurs, and constitute the display waveform after several cycles of triggering. (This process is usually so fast that you won't notice it happening.) the repeat-bandwidth metric is independent of the oscilloscope's sampling rate.In fact, this indicator is usually used to measure the bandwidth of the oscilloscope analog amplifier portion.Real - time bandwidth is suitable for non - repeating or single signal. The oscilloscope completes digitization in a trigger process, so the real-time bandwidth depends on the sampling rate of the oscilloscope, and the ratio between the sampling rate and the bandwidth is not fixed.If the oscilloscope is digitally reconfigurable, the ratio is closer to 4:1. If it is not refactored, the ratio is usually 10:1.
(3) Confirm numbers of channels you need
In general, the number of channels you need depends on the subject to be measured. At present, dual channel oscilloscopes are the most popular. For most engineers, however, a four-channel oscilloscope is more useful for some applications. The following points should be considered: do you need to capture multi-channel signals at the same triggering event?If so, choose an oscilloscope with each channel available for simultaneous sampling or independent A/D conversion. If the signal you are observing is a repeated signal, then it is not necessary to collect at the same time. Some oscilloscopes are in 2+2 form, that is, two channels are fully functional, and the other two channels are auxiliary channels with limited attenuation range. In this case, two A/D converters are Shared by four channels. Auxiliary channels provide additional flexibility when you are observing digital signals. For dual channel oscilloscopes, external triggers may be useful. It can use a signal without observation as an external trigger source without occupying the input channel of an oscilloscope. If you want to make digital timing measurements that require more than four channels of oscilloscopes, you may wish to consider using a logic analyzer. Although at this point you give up the vertical resolution of the measurement, you gain multiple channels and additional triggering and analysis capabilities.
(4) Confirm sampling rate you need
For a single signal measurement, the most critical performance indicator is the sampling rate, that is, the rate at which the oscilloscope takes a "quick picture" of the input signal. High sampling rate can produce high real - time bandwidth and high real - time resolution. Most oscilloscope manufacturers use a ratio of sampling rate to real time bandwidth of 4:l (if digital reconfiguration is used) or 10:1 (no digital reconfiguration) to prevent false waves. Some oscilloscopes provide the ability to independently control the sampling rate so that you can adjust both the sampling rate and the amount of data (time base) displayed on the screen so that the two Settings do not have to be tied down. This feature allows you to observe the waveform at the desired temporal resolution.
(5) Confirm storage depth you need
The storage depth of your oscilloscope depends on the total time range required and the time resolution required. If you want to store long periods of time at high resolution, you need to choose a deep storage oscilloscope. In this way, you can use a high sampling rate when the horizontal scanning speed is low. This will greatly reduce the chance of false waves and obtain more detailed information about the waveform.
(6) Examine and evaluate the triggering capability
Many general oscilloscope users are used to using edge trigger. In some applications, if an oscilloscope has other trigger capabilities, you will find it useful for your measurements. Advanced triggering features can isolate the events you wish to observe. In digital applications, an oscilloscope that triggers a specific pattern between multiple channels is useful for problem solving. In addition, the state trigger can be used to synchronize the mode trigger with the outer clock edge. Burr trigger occurs when positive or negative burr occurs or when a pulse is wider or narrower than the set width. These characteristics are especially important for fault detection. When the fault occurs, the forward event (using delay or horizontal position knob) is observed to determine the cause of the problem. If you need more advanced logic triggering capabilities, you can still consider using a logic analyzer. The TV trigger can trigger the presence as well as the specific line you need to observe.On some oscilloscopes, this feature is an option function.
(7) Evaluate burr catching ability
Three important factors affect the oscilloscope's burr capture ability: update rate: the digital oscilloscope must first capture data, then process it, and finally display it. The number of times an oscilloscope can complete these three processes in one second is called update rate. An oscilloscope with a fast update rate has a higher chance of capturing incidental burrs. The multi-processor structure oscilloscope has a faster update rate than the traditional single-processor structure oscilloscope, which makes it more suitable for capturing the occasional events. A multiprocessor architecture can produce display throughput and response times similar to analog oscilloscopes.
Peak detection capability: most digital oscilloscopes lose sampling points at low sweep speeds, thus reducing the effective sampling rate.This leads to the problem that narrow pulses, which are easily observed when set to a fast time base, disappear at low sweep speeds.However, for the special sampling mode of peak detection or burr detection, the maximum sampling rate is maintained at all scanning speeds, and the maximum and minimum values obtained in each sampling period are recorded.The smallest burrs that can be detected are related only to the sampling rate of the oscilloscope.
Burr trigger: an oscilloscope with burr trigger enables you to isolate burr that is difficult to detect and trigger when burr occurs. This feature can help you find the cause of abnormal conditions during circuit operation.
(8) Confirm the analysis function you need
With automatic measurements and the analytical capabilities built into an oscilloscope, you can get your work done easily and in less time. Digital oscilloscopes usually have sequential measurement functions and analysis options that analog oscilloscopes cannot have. Arithmetic functions include addition, subtraction, multiplication, division, integration and differentiation. Statistical measurements (minimum, maximum, and average) can quantitatively describe the uncertainty of a measurement, which is valuable in measuring noise characteristics and timing limits. Some digital oscilloscopes can also provide FFT function. An oscilloscope with all of the above advanced features might cost more, so you should decide for yourself whether the extra money is worth it. You'd better choose an oscilloscope with these characteristics according to the actual application.
(9) Evaluation of archiving capability
Most digital oscilloscopes can be connected to a PC, printer, or plotter via GPIB, rs-232, or a parallel port. But you should find out what kind of interface you can provide and what type of printer it will match. You should know that the output from laser and inkjet printers is of much higher quality than that from thermal printers. Using a digital oscilloscope with a floppy disk drive or software package, you can easily transfer waveform images and waveform data to a PC. These features will save time if you want to include a captured screen image in a report or if you want to convert waveform data into a table.
How to choose digital oscilloscope?
How to choose oscilloscope reasonably as the price difference of oscilloscope nearly 50 times between low-grade with high-grade.
The relationship between the bandwidth selection and the highest signal frequency to be measured is needed if accurate measurements are required. Let’s check below example, there is one pulse signal with 50 MHZ, to ensure scope of signal and accuracy of rising delay, the bandwidth of oscilloscope should be 3-5 times of measured signal’s frequency and the accurate measurement should be 8-10 times or more.
Sine wave: as usual, need more than 5 sampling sites during one period as the waveform will be more close to the actual one with more sampling sites.
Pulse wave: rising delay should be more than 5 sampling sites.
Accurate measurement rising delay should be more than 10 sampling sites.
3. Record length: record length=sample*scanning speed*10, it could also be called waveform observation time.
4. Trigger function: ensure that the measured signal can be captured and synchronized to facilitate the observation and analysis of the measured waveform
Trigger methods: automatic trigger, normal trigger and single trigger
Two kinds of trigger function
1) Edge trigger: all digital oscilloscopes have this function, It refers to positive edge, negative edge trigger, window trigger, before trigger and after trigger.
2) Smart trigger：This is considered perfectly in high-grade oscilloscopes. currently there are: delay trigger, sequence trigger, burr trigger, interval trigger, leakage logic surface trigger, TV trigger, this trigger......
5. Analysis function: Should have very strong automatic processing, computation, test and analysis ability of signal
1) Shape and parameters pass/fail automatic test function
2) Advanced function processing: average, differential, polar, exponential, logarithm, power, square root, envelope, high score system, etc.
3）FFT spectrum operation function from 10k-4m point, with power spectrum, power density, phase vector, imaginary part, real part and other measurements;
4）The direct square analysis can be used for the stability calculation of the direct square test signal from 500 points to 8M points according to various parameters
5）Waveform parameter Trend analysis function, Jitter and time analysis;
6) Could open 2-8 windows to observe original waveform and processed waveform simultaneously.
X-Y display and X-Y+X-T and Y-T display are provided, can also carry on the vernier measurement makes it specially suitable for vector diagram analysis of digital communication signals.
6. Record and print signal
1) Can be stored in a test line on a floppy and hard disk, and can be read on a PC. Some digital oscilloscopes are equipped with built-in, convenient to print and analyze long time signal;
2) Some oscilloscopes also provide VGA interface.
The response characteristic of digital oscilloscope and its choose and buy skill
Another reason why high speed digital oscilloscope chooses brick wall reaction type is to avoid or minimize Aliasing phenomenon. When using digital oscilloscopes to measure high speed signals, graphics confusion can occur, mainly because some signals are mixed with unnecessary waveforms when reproducing the sampled high speed signals. These mixed signal frequency components can distort the original signal waveform and, in severe cases, cause measurement errors.
Most of the pattern confusion occurs in the analog to digital converter (adc) continuous signals, which contain components that exceed the Nyquist frequency, or half the sampling frequency.This component retraces in the Nyquist frequency domain and appears in the oscilloscope measurement bandwidth. It can be clearly seen from the frequency characteristic diagram that the figure confusion effect of the brick wall reactive oscilloscope is negligible.
Under the same conditions, it can be clearly seen that in the field beyond the Nyquist frequency of 2GHz, there is almost no signal, which can inhibit the occurrence of confusion.
In addition, if 20GHz, 10GHz and 5GHz sampling frequencies are used to measure waveforms with a period of 2.2ns and a rise time of about 90ps, different results will be obtained.The lower the sampling frequency is, the longer the actual measurement value of rising time is, and the waveform cannot be faithfully presented.
At present, the high-speed serial interface measurement USES the real-time sampling broadband digital oscilloscope, and the sampling frequency of the high performance machine equipped with the analog to digital converter is up to about 20GHz. Generally, in order to reduce the occurrence of graphics confusion, the sampling frequency of a gaussian reactive oscilloscope should be 4-6 times of the input signal, while that of a brick wall reactive oscilloscope only needs 2.5 times.
Generally, the frequency band is lower than 1GHz, so gaussian reaction system is mostly adopted, while the instruments higher than 1GHz are mostly brick wall reaction system.Table 2 shows the advantages and disadvantages of the two types of reactive oscilloscopes.
Choose oscilloscope according to performance requirement
So, how to choose the most suitable oscilloscope? There are four simple steps:
Calculate the highest frequency component fmax of the measured signal.The upper limit of the signal frequency component can be calculated by measuring the rising time of the signal.Assuming that the rise time is migrated from 20% to 80%, the approximate value can be estimated using the mathematical formula (0.4/ signal rise time) rather than directly from the data transmission rate.Take the popular third-generation bus PCIExpress, which in most cases has a rise time of about 100ps.
Select the response characteristics of the oscilloscope. That is to choose a suitable one between the gaussian reaction system and the brick wall reaction system.
The necessary input bandwidth must be secured. It is related to the measurement error of rise time. An instrument company has done simulation experiments: if the brick wall reactive system allows 3% error, the bandwidth can be calculated with (1.4 fmax); If the error is limited to 10%, use (1.2 fmax) to calculate. When the tolerance error is 20%, it is calculated by (1.0 fmax).
Estimate the lowest sampling frequency. This value takes advantage of the above bandwidth value, which is a minimum (2.5 bandwidth) for a brick wall reactive oscilloscope.
The above four points can be used to illustrate a case: the rise time of 100ps digital signal, its fmax is 4GHz, select the brick wall reaction oscilloscope, assuming that the error of the rise time is limited to 3%, then the bandwidth of the input signal is 5.6ghz, therefore, the minimum sampling frequency also needs 14GHz.
If the sampling frequency of 14GHz is applied to the gaussian reaction system, the input bandwidth becomes 3.5ghz, and the rising time of the measured signal is 220ps, which is half of the difference with the brick wall reaction system.Some wide-band real-time oscilloscopes rely on the active application of digital signal processing to realize the characteristics of brick wall reactive system.After all, circuit technology alone is unlikely to achieve desirable characteristics.
In a word, whether the bandwidth and sampling frequency are suitable or not is an important pointer when choosing expensive oscilloscope.In addition, understanding the characteristics of the test instrument is also the key to mastering the correct measurement.