10 factors to consider when choosing an oscilloscope-EEWORLD

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Overview

Compared to the cathode ray oscilloscope invented by German scientist Karl Ferdinand Braun in 1897, the current digital storage oscilloscope is very different. Advances in technology continue to improve the new features of oscilloscopes, making them more suitable for engineers to use. One of the most significant improvements is the entry of oscilloscopes into the digital field, introducing powerful functions such as digital signal processing and waveform analysis. Today's digital oscilloscopes include high-speed, low-resolution (usually 8-bit) analog-to-digital converters (ADCs), defined control elements and display functions, and built-in processors that can execute software algorithms for common measurement operations.

In addition, oscilloscopes can take advantage of the latest processing capabilities and high-resolution displays of computers while retaining other features of high-speed oscilloscopes. Since oscilloscopes are computer-based, users can define instrument functions through software. Therefore, oscilloscopes can be used not only for oscilloscope measurements, but also for customized measurements, and even for spectrum analyzers, frequency counters, ultrasonic receivers, or more instruments. Compared to traditional stand-alone oscilloscopes, open architecture and flexible software give oscilloscopes more advantages. However, digitizers and high-speed oscilloscopes have many similarities, so there are many key points to consider when choosing.

This article will discuss the top 10 factors to consider when choosing an oscilloscope.

1. Bandwidth

Bandwidth represents the frequency range over which the input signal passes through the analog front end with minimal amplitude loss, from the probe tip or test equipment to the input of the ADC. The bandwidth should be the frequency at which the amplitude of the sinusoidal input signal is attenuated to 70.7% of the original amplitude, also known as the -3dB point. In general, the frequency of the oscilloscope should be able to exceed the highest frequency of the signal by more than 2 times.

Oscilloscopes are often used to measure the rise time of signals, such as digital pulses or other signals with sharp edges. Such signals are composed of high-frequency signals. In order to capture the exact shape of the signal, a high-bandwidth oscilloscope is required. For example, a 10 MHz square wave is composed of a 10 MHz sine wave and countless harmonics. To obtain the actual shape of the signal, the bandwidth of the selected oscilloscope must be able to capture several harmonics. Otherwise, signal distortion and erroneous measurements will result.

Figure 1. When acquiring high-frequency waveforms, a high-bandwidth oscilloscope is necessary.

The following formula calculates the signal bandwidth based on the rise time, which is the time required for the signal amplitude to rise from 10% to 90%.

Figure 2. Rise time is the time it takes for a signal to rise from 10% to 90% of its full value.
Rise time is directly related to bandwidth, so the above formula can be used to convert these two sets of values.

Ideally, the oscilloscope bandwidth should be 3 to 5 times the signal bandwidth obtained by the above formula. In other words, in order to minimize the error of signal acquisition, the rise time of the oscilloscope should be 1/5 to 1/3 of the signal rise time. The following formula can be used to invert the actual signal bandwidth.

= measured rise time; = actual signal rise time; = oscilloscope rise time

2. Sampling rate

Bandwidth is one of the most important specifications for an oscilloscope. But if the sampling rate is not sufficient, the bandwidth is useless.
Bandwidth represents the highest frequency sine wave that can be digitized with minimal attenuation, while the sampling rate is the clocking rate at which the ADC in the oscilloscope digitizes the input signal. Note that there is no direct correlation between sampling rate and bandwidth. However, there is a necessary relationship between these two important specifications:

Oscilloscope's real-time sampling rate = 3 to 4 times the oscilloscope's bandwidth

The Nyquist theorem states that in order to avoid distortion, the oscilloscope sampling rate should be at least twice the highest frequency element of the measured signal. However, a sampling rate of only twice the highest frequency is still not enough to accurately reproduce the time domain signal. In order to accurately digitize the input signal, the oscilloscope's real-time sampling rate should be at least 3 to 4 times the oscilloscope's bandwidth. The following figure illustrates the digitized signal that the user hopes to see through the oscilloscope.

Figure 3. The oscilloscope on the right has an effectively high sampling rate, which allows for accurate signal reconstruction and more precise measurements.

Although both sets of actual signals above have passed through the front-end analog circuit, the sampling rate of the left figure is insufficient, resulting in distortion of the digitized signal. The right figure has enough sampling points to accurately reconstruct the signal, thereby achieving more accurate measurement operations. For time domain applications (such as rise time, overshoot, or other pulse measurements), it is extremely important to clearly present the signal, so an oscilloscope with a high sampling rate can provide better advantages in such applications.

3. Sampling mode

There are two main sampling modes: real-time sampling and equivalent time sampling (ETS).
As mentioned above, the real-time sampling rate represents the ADC frequency and the maximum rate at which a signal can be acquired in a single acquisition. ETS is a signal reconstruction method based on the trigger waveform acquired in the single acquisition mode. The advantage of ETS is that it has a higher effective sampling rate, but the disadvantage is that it takes longer and is only applicable to repetitive signals. Please note that ETS does not increase the analog bandwidth of the oscilloscope and is only applicable to reproducing signals at a higher sampling rate. The most common ETS is random interval sampling (RIS), which is available on most NI oscilloscopes.

Oscilloscope Model	Number of channels	Real-time sampling rate	Equi-temporal sampling rate	bandwidth	Resolution
NI 5152	2	2 GS/s	20 GS/s	300 MHz	8-bit
NI 5114	2	250 MS/s	5 GS/s	125 MHz	8-bit
NI 5124	2	200 MS/s	4 GS/s	150 MHz	12 bits
NI 5122	2	100 MS/s	2 GS/s	100 MHz	14-bit
NI 5105	8	60 MS/s	–	60 MHz	12 bits
NI 5922	2	500 kS/s to 15 MS/s	–	6 MHz	16 ~ 24 user defined

4. Resolution and dynamic range

As mentioned above, the ADC in an oscilloscope can convert analog signals into digital signals. The number of bits returned by the ADC is the oscilloscope resolution. For any given input range, the possible discreteness of the signal digitization is often expressed as "2b", where "b" is the oscilloscope resolution. The input range is divided into 2b steps, and "input range/2b" is the minimum voltage that the oscilloscope can detect. For example, an 8-bit oscilloscope can cut a 10Vpp input range into 28 = 256 levels, each level is 39 mV; a 24-bit oscilloscope can cut a 10 Vpp input range into 224 = 16,777,216 levels, each level is 596 nV (about 1/65,000 of 8 bits).

One of the reasons for choosing a high-resolution oscilloscope is to measure smaller signals. Sometimes we can't help but ask: Why not use a low-resolution instrument and a smaller range of signals to "zoom" the signal and measure low voltages? The problem is that many signals have both small and large signals. Although a large range can measure large signals, the small signal will be hidden in the noise of the large signal. In other words, using a small range must compress the large signal, resulting in measurement distortion and error. Therefore, for dynamic signal applications (signals with both large and small voltages), a high-resolution instrument with a larger dynamic range is required to measure small signals in large signals.

Traditional oscilloscopes usually use 8-bit resolution ADCs, but they are difficult to meet the needs of spectrum analysis or dynamic signal applications (such as modulated waveforms). For such applications, you can choose the high-resolution oscilloscopes in the table below, including the NI PXI-5922 flexible resolution oscilloscope, which was awarded the 2006 Test Product of the Year by Test and Measurement World. This module achieves the industry's highest oscilloscope dynamic range through linearization technology.

Oscilloscope Model	Resolution	Number of channels	Real-time sampling rate	bandwidth
NI 5922	16 ~ 24 bits (user defined)	2	500 kS/s to 15 MS/s	6 MHz
NI 5122	14-bit	2	100 MS/s	100 MHz
NI 5124	12 bits	2	200 MS/s	150 MHz
NI 5105	12 bits	8	60 MS/s	60 MHz

5. Trigger

Generally speaking, oscilloscopes are used to acquire signals for specific events. The trigger function of the instrument can isolate specific events to acquire signals before and after the event occurs. Many oscilloscopes have analog edge, digital, and software triggering functions. Other triggering options include window, hysteresis, and video triggering (NI 5122, NI 5124, and NI 5114 have this function).

Advanced oscilloscopes can quickly restart (re-arm) between 2 triggers and enter the multi-record acquisition mode. The oscilloscope will acquire a specific number of points based on the given trigger and quickly restart to wait for the next trigger. The fast restart function ensures that the oscilloscope will not miss an event or trigger. If only specific data needs to be acquired and stored, the multi-record mode can achieve extremely high performance; in addition to optimizing the use of built-in memory, it can also limit the activity of the computer bus.

6. Built-in Memory

Normally, data is transferred from an oscilloscope to a computer for measurement and analysis. Although these instruments can achieve maximum sampling rates (up to several GS/s), the transfer rate to the computer is limited by the bandwidth of the bus (such as PCI, LAN, and GPIB). Currently, these buses have difficulty reaching rates of several GS/s, but PCI Express and PXI Express can easily achieve this.

If the bus interface cannot achieve continuous data transmission equal to the sampling rate, the instrument's built-in memory will collect data at the highest rate and wait for the computer to perform subsequent processing.

Large memory not only extends acquisition time, but also has related advantages in the frequency domain. The most common frequency domain measurement is the fast Fourier transform (FFT), which can display the frequency content of the signal. If the FFT can achieve higher frequency resolution, discrete frequencies can also be easily detected.

From the above equation, there are two ways to improve the frequency resolution: reduce the sampling rate or increase the sampling points in the FFT. Since reducing the sampling rate will also reduce the frequency bandwidth, it is not an ideal solution. Therefore, it is better to collect more data points for FFT, which will require a larger memory capacity.

Figure 4. More built-in memory allows higher sampling rates and the acquisition of more points over a longer period of time, allowing FFT results to be calculated with higher frequency resolution.

Oscilloscope Model	Number of channels	Real-time sampling rate	Equi-temporal sampling rate	bandwidth	Memory options
NI 5152	2	2 GS/s	20 GS/s	300 MHz	16 MB, 128 MB, 512 MB, 1 GB
NI 5114	2	250 MS/s	5 GS/s	125 MHz	16 MB, 128 MB, 512 MB
NI 5124	2	200 MS/s	4 GS/s	150 MHz	16 MB, 64 MB, 512 MB, 1 GB
NI 5122	2	100 MS/s	2 GS/s	100 MHz	16 MB, 64 MB, 512 MB, 1 GB
NI 5105	8	60 MS/s	–	60 MHz	16 MB, 128 MB, 512 MB
NI 5922	2	500 kS/s ~ 15 MS/s	–	6 MHz	16 MB, 64 MB, 512 MB, 1 GB

7. Channel Density

One of the factors to consider when buying an oscilloscope is the number of channels on the instrument, or whether multiple instruments can be synchronized to increase the number of channels. Many oscilloscopes have 2 or 4 channels, and can also perform synchronous sampling at a specific sampling rate. When using all channels of the oscilloscope, you must pay attention to the extent to which the sampling rate is affected. The so-called time-interleaved sampling, which is common at present, is to cross multiple channels to achieve a higher sampling rate. If the oscilloscope uses this technology and matches all channels, it is more difficult to achieve the maximum acquisition rate.

The number of channels required is completely dependent on the specific application. Traditional 2 or 4 channel products are no longer sufficient for today’s applications and there are two solutions. The first is to choose a high channel density product such as the NI 5105 – a 12-bit, 60 MS/s, 60 MHz oscilloscope with 8 simultaneous channels. If you cannot find an instrument that meets your resolution, speed, and bandwidth requirements, then choose a platform that allows for precise synchronization and the ability to share triggers and frequencies to adjust the test system at any time. Synchronizing multiple box oscilloscopes is not practical due to the high latency, limited throughput, and extra wiring required of GPIB or LAN; the PXI bus is the better solution. PXI has become an industry standard and adds world-class synchronization technology to existing high-speed buses such as PCI and PCI Express.

Figure 5. By using synchronization techniques, a high-channel-count oscilloscope can be created.
The system in the figure above can acquire up to 136 phase-coherent channels. Multiple hosts can also synchronize more channels.

NI oscilloscopes (including NI PXI-5105 and NI PXI-5152) feature T-Clock technology, which can achieve synchronization accuracy of one tenth of a picosecond. For example, a single 18-slot chassis with multiple NI PXI-5152 oscilloscopes can achieve up to 34 synchronized channels and a sampling rate of 1 GS/s. Similarly, multiple NI PXI-5105 oscilloscopes can be used to simultaneously build a system with 136 synchronized channels, and each channel has a sampling rate of 60 MS/s and 12-bit resolution (above). For higher channel counts, PXI can also expand multiple chassis through timing modules to reach a system with up to 5,000 channels.

8. Multiple Instrument Synchronization

Most automated test and multi-bench applications require multiple types of instruments, such as oscilloscopes, signal generators, digital waveform analyzers, digital waveform generators, and switchers.

The existing timing and synchronization capabilities of PXI and NI modular instruments can synchronize all of the above instruments without the need for additional wiring. For example, an oscilloscope (such as the NI PXI-5122) can be integrated with an arbitrary waveform generator (such as the NI PXI-5421) to perform parameter sweeps to characterize the frequency and phase response of the device under test (DUT). The entire sweep process is fully automatic, and there is no need to manually set the scale and generator parameters for subsequent offline analysis. The modular nature of PXI greatly improves speed and reduces the need for time-consuming operations by users, thereby improving efficiency.

9. Mixed signal capabilities

T-Clock technology only requires a single PXI chassis to achieve 136 synchronized channels, or multiple chassis to achieve up to 5,000 channels; of course, the technology can also synchronize different types of instruments. For example, NI oscilloscopes can integrate signal generators, digital waveform generators, and digital waveform analyzers through T-Clock technology to build mixed signal systems.

Figure 6. The VI above shows a mixed signal (analog and digital input) oscilloscope application.
Additionally, digital or analog output capabilities can be added to the application to synchronize all instruments.

Alternatively, you can use a modular PXI oscilloscope with an arbitrary waveform generator or a digital waveform generator/analyzer to build a complete mixed signal application that achieves the benefits of an oscilloscope and a logic analyzer without being limited to an oscilloscope with digital capabilities.

10. Software, analysis functions, and customizable features

When choosing between a modular or standalone oscilloscope, software and analysis capabilities are also important considerations.
Standalone oscilloscopes are defined by the manufacturer, while standard oscilloscopes can be flexibly defined by the user in the application. Box oscilloscopes have many standard features that can meet the common needs of engineers. However, these standard features may not be suitable for all applications, especially for automated test applications. If the user must define the measurements that the oscilloscope will perform, a modular oscilloscope should be selected, which can be customized through the computer architecture to meet special needs without being limited to the fixed functions of a standalone oscilloscope.

NI oscilloscopes can be programmed with the free NI-SCOPE driver software. The driver comes with more than 50 pre-written example programs and specifically enhances the full functionality of NI oscilloscopes. The NI-SCOPE Soft Front Panel also has an interface similar to that of an oscilloscope. For a variety of applications, the same hardware can also be programmed for common/customized measurement operations through NI LabVIEW, LabWindows/CVI, Visual Basic, and .NET. NI-SCOPE also supports functions for setting architectures in LabVIEW.

Figure 7. Using preconfigured Express blocks, you can quickly configure an oscilloscope to acquire data.
NI LabVIEW SignalExpress is an interactive environment that allows you to acquire, analyze, and log data without programming.

11. Conclusion

Although both modular and standalone oscilloscopes can acquire voltage, different instruments have different advantages. The above considerations are extremely important. Users should first consider application requirements, cost constraints, performance, and future expandability, and then select an instrument that can meet their needs.

Keywords：Oscilloscope Reference address：10 factors to consider when choosing an oscilloscope

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