Ten steps to choosing an oscilloscope

How to choose such an important oscilloscope? Let’s look at several important indicators.

1: Understand the signal you need to test

What is the typical performance of the signal you want to capture and observe?

Ÿ Does your signal have complex characteristics?

Ÿ Is your signal a repetitive signal or a one-shot signal?

What is the bandwidth, or rise time, of the signal transition you want to measure?

What signal characteristics do you intend to use to trigger on short pulses, pulse widths, narrow pulses, etc.?

How many signals do you plan to display simultaneously?

· What do you do with the test signal?

2: Bandwidth > five times the highest signal frequency

The bandwidth of an oscilloscope is the frequency at which the amplitude of a sine wave input signal decays to -3dB, which is 70.7% of the true amplitude of the signal. Bandwidth determines the basic measurement capability of an oscilloscope for a signal and is also a key determinant of price. When selecting bandwidth, the five-fold rule can be applied, that is, the bandwidth of the oscilloscope ≥ 5 X the maximum frequency of the input signal. If the oscilloscope does not have sufficient bandwidth, it will not be able to test high-frequency signals, the amplitude will be distorted, the edges will disappear, and the detailed data will be lost.

3: Appropriate number of channels

Digital oscilloscopes sample, store, and display data from analog channels. Generally speaking, the more channels, the better, but adding channels also increases the price. The more time-correlated analog and digital channels an oscilloscope has, the more points in the circuit can be measured simultaneously, and the easier it is to decode parallel buses.

4: Sampling rate > 5 × (highest frequency component)

The unit is samples per second (S/s), which refers to the frequency at which a digital oscilloscope samples a signal. The faster the oscilloscope samples, the higher the resolution, the better the waveform details displayed, and the less likely it is that key information or events will be lost. The sampling rate of an oscilloscope should be at least four times the bandwidth of the oscilloscope, or a minimum of 5 times oversampling to ensure that signal details are captured and false signals are avoided.

5: Storage depth = sampling rate × display time

Memory depth is the number of samples an oscilloscope can digitize and store in one acquisition. The deeper the oscilloscope's memory, the more time you can capture at the full sample rate. The memory depth you need depends on how much you want to view on the display and the sample rate you want to maintain. If you want to view a longer period of time with higher resolution between different sample points, you need to use deep memory: memory depth = sample rate × display time. After determining the memory depth, it is also important to examine how the oscilloscope operates when using the deepest memory setting.

6: Display function

The display performance of an oscilloscope depends largely on the digital processing algorithms rather than the physical characteristics of the display device. There is no good way to determine which oscilloscope is best suited for your lab environment by studying the oscilloscope's technical specifications. Only when your waveform is demonstrated and used in real time on your workbench can you determine which oscilloscope is best suited to meet your needs. Current digital oscilloscopes fall into two categories: waveform viewing instruments and waveform analyzers. Oscilloscopes designed for viewing waveforms are typically used in test and problem diagnosis applications, where the waveform image will provide all the information the user needs.

7: Trigger function

Most oscilloscope users use only edge triggering, but in some applications you may need to use other triggering functions. Advanced triggering functions can isolate the event you want to see. At the same time, advanced triggering options can also save a lot of time in daily debugging tasks. What if you need to capture a rare event? Glitch triggering allows you to trigger on positive or negative glitches, or trigger on pulses greater than or less than a specified width. When diagnosing a problem, you can trigger on the problem and go back to see the root cause of the problem.

8: Probe

When the 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 show different measurement results from the object being measured. Therefore, corresponding probes are equipped for different applications, and then one of them is selected to minimize the loading effect and obtain the most accurate reproduction of the signal. Choose the right probe:

1) Passive probe: economical, easy to use, and provides a wide range of measurement functions;

2) Active high-frequency probes: Active probes provide perfect versatility and accuracy when measuring high-frequency signals in today's complex circuits;

3) Differential probe: provides the highest CMRR, wide frequency range and minimum time offset between inputs, making it the best choice for accurately measuring differential signals;

4) Single-ended high voltage probes: High voltage measurement solutions extend the oscilloscope's ability to safely and accurately capture real-time signal information from boosted or floating voltage systems.

9: Analysis function

Automatic testing and built-in analysis functions can save users time and make work easier. Digital oscilloscopes usually come with a series of measurement functions and analysis options that are not available on analog oscilloscopes. Mathematical operation functions include addition, subtraction, multiplication, division, integration, and differentiation. Measurement statistics (minimum, maximum, and average) can verify measurement uncertainty, which is an important resource when verifying noise and timing margins. Many digital oscilloscopes also provide FFT functions.

10: Demonstration

Last but not least: Demo, demo, demo! If you have considered the previous nine factors, you have probably narrowed down the field to a small number of oscilloscopes that meet your criteria. Now it is time to use the oscilloscopes for demonstrations and compare them, including ease of use and display responsiveness.