Which Oscilloscope is Right for You?

Since its introduction, the oscilloscope has been one of the most important and commonly used electronic test instruments. With the development of electronic technology, the capabilities of oscilloscopes are constantly improving, and their performance and prices are also varied, with uneven market conditions. Oscilloscopes may seem simple, but there are many problems in choosing one. Based on years of experience and 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

What do you want to observe with your oscilloscope? What is the typical performance of the signal you want to capture and observe? Does your signal have complex characteristics? Is your signal repetitive or a single-shot 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 on short pulses, pulse widths, narrow pulses, etc.? How many signals do you plan to display simultaneously? What processing do you do to the test signal?

2. The core technology differences in choosing an oscilloscope: 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, 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 the disadvantages of three-dimensional display 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 at which the amplitude of a sine wave input signal decays to -3dB, or 70.7% of the amplitude. Bandwidth determines the basic measurement capability of an oscilloscope. Without sufficient 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; without sufficient bandwidth, all the characteristics of the obtained signal, including ringing and humming, are meaningless.

An effective rule of thumb for determining the oscilloscope bandwidth you need is the "5x rule of thumb": multiply the highest frequency component of the signal you want to measure by 5 to achieve a measurement accuracy better than 2%.

In some applications, you do not know the bandwidth of your signal of interest, but you do know its fastest rise time. In this case, the frequency response is calculated using the following formula to relate the bandwidth and rise time of the instrument: 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 is more important when the events captured are not frequently occurring or transient signals. 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 components you want to observe according to your budget.

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

The unit is the number of samples per second (S/s), which refers to the frequency at which a digital oscilloscope samples a 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 frame, 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 being measured and the signal reconstruction method used 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 a signal with infinite time and continuous period. Since an oscilloscope cannot provide an infinite time record length, and low-frequency interference is by definition discontinuous and not periodic, it is usually not enough to use a sampling rate twice that of 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 the signal sampling points, that is, waveform reconstruction. Some oscilloscopes provide operators with the following choices: sinusoidal interpolation for measuring sinusoidal signals and linear interpolation for measuring rectangular waves, pulses and other signal types.

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

5. Screen refresh rate is also called waveform update speed

All oscilloscopes flicker. An oscilloscope captures a signal a certain number of times per second. No measurements are taken between these measurement points. This is the waveform capture rate, also called 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 within a waveform or cycle; the waveform capture rate refers to how quickly the oscilloscope acquires waveforms. The waveform capture rate depends on the type and performance level of the oscilloscope and varies greatly. An oscilloscope with a high waveform capture rate will provide more important signal characteristics and greatly increase the probability that the oscilloscope will quickly capture transient anomalies such as jitter, runt 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 as high as millions of waveforms per second, greatly improving the possibility of capturing intermittent and difficult-to-capture events, and allowing you to find signal problems more quickly.

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

Memory depth is a measure of how many sample points an oscilloscope can store. If you need to capture a pulse train without interruption, the oscilloscope must 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 the user to select the record length to optimize for detail in some operations. Analyzing a very stable sinusoidal signal may require only a 500-point record length, but resolving a complex digital data stream may require a record length of one million points or more.

Effective triggering at the right location to capture a signal can often reduce the amount of memory actually required by the oscilloscope.

7. Choose different trigger functions according to your needs

The trigger of the oscilloscope can synchronize the horizontal scanning of the signal at the correct position point, making the signal characteristics clear. The trigger control button can stabilize the repetitive waveform and capture the single waveform.

Most oscilloscope users only use edge triggering. Having other triggering capabilities is very useful in some applications, especially for troubleshooting of newly designed products. Advanced triggering methods can separate the events of interest and find the abnormal problems you are concerned about, thereby making the most effective use of sampling rate and storage 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.