15 Key Considerations for Choosing a Basic Oscilloscope

The main function of an oscilloscope is to observe the curve of voltage changing over time. Through different sensor probes, the oscilloscope can also measure current, pressure, etc. An oscilloscope is a must-have instrument for every electronics enthusiast and electronic engineer. Today we will look at what factors we should consider when choosing an entry-level basic oscilloscope.

bandwidth

Bandwidth is the primary parameter of an oscilloscope. When the bandwidth is insufficient, the waveform will be severely distorted, and the square wave may even become a sine wave. We use oscilloscopes with 20M bandwidth and 100M bandwidth to observe a 20M square wave signal, and the results are shown in the figure below:

From this figure, we can see that the 20M oscilloscope can hardly observe the square wave shape, and the observation effect of the 100M oscilloscope is better than that of the 60M oscilloscope. What causes this? The figure below is a spectrum diagram of a 20M square wave after FFT. We can see that the square wave is composed of the fundamental wave and the 3rd, 5th, 7th, 9th... harmonic components. Therefore, the 20M square wave contains the 20M fundamental wave, the 60M third harmonic, the 100M fifth harmonic, the 140M seventh harmonic...

If you want to measure the waveform accurately, the bandwidth of the oscilloscope should be greater than the main harmonic component of the waveform. Therefore, for a sine wave, the bandwidth of the oscilloscope can be required to be greater than the frequency of the waveform, but for a non-sinusoidal wave, the bandwidth of the oscilloscope is required to be greater than the maximum main harmonic frequency of the waveform.

In terms of bandwidth selection, generally 5 times the maximum frequency of the measured signal is the most appropriate bandwidth. For a basic oscilloscope, a 100MHz bandwidth oscilloscope can be used in many measurement scenarios.

Sampling rate and memory depth

Sampling rate and memory depth are two other important indicators of an oscilloscope. Why do we talk about them together? Because they have an inseparable symbiotic relationship with the waveform recording time: Sampling rate = memory depth ÷ waveform recording time

The waveform displayed by a digital oscilloscope is actually a sampling point, just like a photo. The more sampling points, the closer the signal is to reality. The sampling rate expresses the ability of the oscilloscope to capture these sampling points per second. For example, a sampling rate of 1GSa/s means that the oscilloscope can collect 1 billion sampling points per second. The storage depth expresses how many sampling points an oscilloscope screen can contain at most. For example, a storage depth of 28Mpts means that an oscilloscope screen can have a maximum of 28 million sampling points. We must ensure that the sampling rate is at least twice the bandwidth of the signal being measured. In fact, we recommend 3-5 times or more, so that it is easier to capture abnormal information of the waveform.

One thing to note here is that the sampling rate is divided into equivalent sampling and real-time sampling. The current oscilloscopes are basically marked with real-time sampling rates, because equivalent sampling is only applicable to periodically changing signals. If you find that the sampling rate is different from the public price when buying an oscilloscope, be sure to ask the seller whether it is marked with real-time sampling or equivalent sampling.

Number of channels

The most common oscilloscopes are 2-channel and 4-channel. When considering which channel oscilloscope is suitable for you, you should mainly consider whether you need to measure 3-4 signals simultaneously. If not, you can choose a 2-channel oscilloscope.

Waveform capture rate

Digital oscilloscopes cannot simultaneously collect, store, process, and display signals. Therefore, when the oscilloscope is processing and displaying, some information will be missed, which is also called the dead time of the oscilloscope. The waveform capture rate expresses the ability of the oscilloscope to capture waveforms many times per second. The greater this ability, the smaller the dead time, and the easier it is to capture abnormal waveforms.

It should be noted here that the waveform capture rate is not fixed. It will change with the various settings of the oscilloscope. Generally, the waveform capture rate marked by the manufacturer is the maximum value that can be achieved.

Trigger method

If the oscilloscope captures the waveform like taking a photo, the trigger determines when the oscilloscope presses the shutter. The trigger determines how the waveform sampling points we want to capture are displayed on the screen.

Common triggering modes of oscilloscopes are edge triggering and pulse width triggering. Some oscilloscopes support more triggering modes in addition to these. You can learn about them and choose according to your needs.

Portability and operability

If you need to carry the oscilloscope with you, then the portability of the oscilloscope is also a factor worth considering, as well as whether the oscilloscope supports battery power, because you may need to work outdoors. Some modern oscilloscopes also support touch screen functions, which will be faster and easier to use.

Screen size and display mode

The size of the oscilloscope's display screen and whether it supports color are also factors that should be considered. Some oscilloscopes support fluorescent display and color temperature display, which is equivalent to adding an additional Z-axis. For some complex signals, such as video signals, modulated signals, and jitter signal testing, oscilloscopes with fluorescent display and color temperature display can help reveal signal details more quickly.

Accuracy

The vertical resolution of the oscilloscope's ADC analog-to-digital converter is the vertical resolution of a digital oscilloscope. The number of bits represents the accuracy with which the oscilloscope converts the input voltage into a digital value.

It should be noted here that the vertical resolution of the oscilloscope display is limited. In addition, when measuring high-frequency signals, the amplitude itself is inaccurate, and there is even a 30% error at the upper frequency limit. Moreover, too high a vertical resolution will increase the analog-to-digital conversion time, affect the sampling rate, and then affect the bandwidth, which is not worth the loss. The vertical resolution of a general oscilloscope is 8 bits, and a high-resolution oscilloscope can reach 12 bits. However, if the accuracy of the oscilloscope's analog circuit itself is not improved, it is meaningless to simply pursue the resolution of the ADC. If you pursue voltage accuracy, you should use a multimeter. The main function of an oscilloscope is to observe the shape of the waveform. The measurement accuracy is generally within 2-3%, which is completely sufficient for most applications.

Measurement and analysis

Oscilloscopes generally support the display of common measurement items, such as the frequency, period, amplitude, etc. of the signal. Some oscilloscopes also support additional mathematical operations, FFT and advanced mathematical functions, which should also be considered if there are calculation requirements in this regard.

Auto function

The auto function of the oscilloscope can replace the multimeter to measure voltage. It can measure the DC component, AC component, frequency, peak-to-peak value, effective value and other information of the signal at one time, and even the waveform, so that you can fully understand the signal at a glance, which is much better than the multimeter only measuring an average voltage, reducing misjudgment and improving efficiency. However, the speed of auto is crucial. The auto speed of the Macosin oscilloscope is 1 second, which is faster than the automatic transmission of ordinary multimeters.

Serial Bus Analysis

Serial buses are widely used in today's data communication designs, so which serial bus analysis the oscilloscope supports is also a factor to consider. We need to clearly understand whether the oscilloscope's decoding method is hardware decoding or software decoding, whether it supports decoding triggering and decoding text display, etc.

Interoperability and Waveform Data

Some oscilloscopes can be remotely controlled via a mobile phone or computer, and waveform data can be saved on the computer and analyzed using computer software. I once met a friend who needed to perform measurements in a room with a temperature of 36 degrees for a long time. The remote control function of the newly purchased oscilloscope with a mobile phone helped him a lot. In the past, he had to stay in the room with the oscilloscope to operate it, but this time he could operate it directly outdoors.

Probe selection

The probe will introduce resistance load, capacitance load and inductance load, which will affect the accuracy of the measured data. To minimize this effect, it is best to use a probe that is the same brand as the oscilloscope. When purchasing a probe, make sure that the bandwidth of the probe is greater than or equal to the bandwidth of the oscilloscope. It is also necessary to clarify whether the voltage or current is being measured, the signal amplitude, whether the signal needs to be measured differentially, and other issues that will affect the choice of probe.

Price and cost performance

Except for the probe, which is a consumable part, the mainframe itself is very stable. It is completely possible to use an oscilloscope for 10 years. The elimination of oscilloscopes is often caused by technological iteration and update. Therefore, it is worthwhile to buy an oscilloscope that is easy to use, convenient and has excellent performance. At present, in the low-end field of oscilloscopes, domestic oscilloscopes are not worse than imported ones, and some are even better.