How to choose the oscilloscope that works best for you
Время публикации: 2019-07-30
The oscilloscope is the basic tool in all electronic designs to measure signal changes over time. After a digital multimeter, an oscilloscope is often the second most common device on an electronic test bench.
As the saying goes, if you want to do something good, you must first sharpen your tools. You have a suitable basic tool to design and debug more efficiently and avoid accidental stepping on the pit.
There are so many elements of an oscilloscope. What do we need to pay most attention to? In fact, there are four concepts and factors that need to be remembered when choosing an oscilloscope: bandwidth, sampling rate, maximum memory depth, and waveform refresh rate.
How to choose the most cost effective oscilloscope
First, you should know the type of signal you are going to measure. Are you troubleshooting a simulated system with a sine wave or a digital system with a short TTL pulse? Do you need to look for fast intermittent peaks? What is the expected voltage range and frequency of the signal you are planning to measure?
When looking at a manufacturer's specifications for a particular feature, it's important to understand how the specification is determined. If it is a high quality range, there are usually a large number of footnotes on the data sheet indicating how the specification was derived and/or measured. It is also important to know if the specifications are measured and calibrated during the manufacturing process. For key specifications in your application, we recommend that you avoid the word “typical” in the specification sheet, as most of the typical values are made for spelling data and cannot meet special measurement requirements.
What is the bandwidth in the oscilloscope?
Obviously, bandwidth is usually the first specification that people consider. The bandwidth of an oscilloscope is usually defined as the maximum frequency that an oscilloscope can measure, without causing significant distortion by the front-end amplifier. Most oscilloscope manufacturers specify this value at 3 dB, which is equivalent to an amplitude loss of approximately 30%.
This limitation has led many engineers to use the "five-fold principle" when choosing the oscilloscope bandwidth. Essentially, this means that if you are measuring a 100 MHz signal, you need to use a 500 MHz oscilloscope to ensure that the signal is captured and measured correctly.
In particular, the probe is also an important part of the measurement system. If the probe is used in the test, the influence of the probe on the bandwidth must be considered.
The oscilloscope's bandwidth also determines the rising edge time the oscilloscope can measure. The oscilloscope has an attenuation on the amplitude of the measured signal. According to the definition of rise time, the error of the amplitude will inevitably lead to an error in the rise time, thus introducing an error. In daily use, the rise time measured by the oscilloscope must take into account the impact of the oscilloscope on it. In general, you can introduce the concept of an oscilloscope's own rise time based on the bandwidth of the oscilloscope.
Ttro, which is defined as Ttro=0.35/BW, is the bandwidth of the oscilloscope. For a 200MHz oscilloscope, available
For a signal with a rise time of T, according to the empirical formula, the measured rise time can be
Tmea = sqrt(Ttro^2+ T^2)
The difference between T and Tmea is the error of measurement. Based on empirical formulas, the measured rise time can also be used to evaluate whether the oscilloscope bandwidth meets its nominal bandwidth. Input a very fast rising edge signal to the oscilloscope with a rise time of ps. In this way, T is negligible in the formula. The function of the formula is the oscilloscope's own Ttro. At this point, the oscilloscope's measured value should be less than its calculated rise time (because the oscilloscope's bandwidth is margined).
The table below provides typical bandwidth and rise times for different logic systems.
Oscilloscope sampling rate
The sampling rate of the oscilloscope determines the number of data points of the measured waveform. It is recommended that you select at least twice the sample rate of the waveform you are trying to measure. To ensure that you find any transient events in the waveform, it is best to use four to five times the required sample rate.
The sampling rate should be significantly higher than the frequency of any sampled waveform to be measured.
The sample rate is usually specified as Gsample / s or Msample / s. Most oscilloscope specifications offer two different maximum sample rates. The first (usually a larger sample rate) assumes that you are using half of the available channels, and the second assumes that all channels are used, which means that if you only use one channel of a two-channel oscilloscope, you will get higher sampling. rate.
Maximum storage depth
In order to be able to view and/or analyze your signals, you not only need to have the correct bandwidth and sample rate, but also enough storage space to store the signals. The manufacturer specifies the memory as Max Memory Depth. This is usually specified in Mpoints or kpoints. Simple? Choose the largest Max Memory Depth and set it up.
The oscilloscope needs to have the processing power to store the signal and retrieve it for display on the screen. Therefore, if you are trying to debug a digital system with a glitch, you need to dig deeper to ensure that the oscilloscope can capture this failure and allow you to start debugging.
Waveform refresh rate
The oscilloscope waveform refresh rate defines the processing speed of the acquired signal, sent to the screen, and then resumes acquisition. The waveform update rate is specified in Wfms / s (number of waveforms per second). When the oscilloscope does not measure your signal, the processing time is basically "dead time". Therefore, you need to ensure that the oscilloscope waveform update rate matches the signal you expect to measure.
Other factors we have in reviewing and selecting the best oscilloscope include resolution and measurement accuracy. We also considered the ease of use of the oscilloscope and the intuitiveness of the front panel when it comes to finding key features. A powerful set of measurement functions available on the oscilloscope will make it easier for engineers to troubleshoot or test circuits.
In addition to the layout of the front panel, we also considered the screen size. Large, high-resolution screens make it easier to display multiple measurements and find those cumbersome transients.
In addition, upgradeability, the number of attachments is also a project that needs to be considered for oscilloscope selection.
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