Introduction to classification of oscilloscopes

Electronic devices can be divided into two categories: analog devices and digital devices. The analog device's voltage changes continuously, while the digital device processes the discrete binary code representing the voltage samples. Traditional record players are analog devices, while CD players are digital devices.

Similarly, oscilloscopes can be divided into analog and digital types. Both analog and digital oscilloscopes are capable of most applications. However, for some specific applications, each type has a suitable and unsuitable place due to the different characteristics of the two. For further division, digital oscilloscopes can be divided into digital storage oscilloscopes (DSOs), digital phosphor oscilloscopes (DPOs), and sampling oscilloscopes.

Analog scope

In essence, analog oscilloscopes work by measuring the signal voltage directly and plotting the voltage in the vertical direction through the electron beam passing through the oscilloscope screen from left to right. The oscilloscope screen is usually a cathode ray tube (CRT). The electron beam is thrown somewhere on the screen, and there is always a bright fluorescent substance behind the screen. When the electron beam is horizontally swept across the display, the voltage of the signal is deflected up and down by the electron beam, and the tracking waveform is directly reflected on the screen. The more frequently the electron beam is projected at the same position on the screen, the brighter the display.

The CRT limits the frequency range displayed by the analog oscilloscope. At very low frequencies, the signal appears bright and slowly moving, making it difficult to distinguish the waveform. At high frequencies, the limitation is the write speed of the CRT. When the signal frequency exceeds the write speed of the CRT, the display is too dim and difficult to observe. The limit frequency of an analog oscilloscope is approximately 1 GHz.

When the oscilloscope probe and circuit are connected together, the voltage signal passes through the probe to the vertical system of the oscilloscope. Figure 13 illustrates how an analog oscilloscope displays the signal under test. By setting the vertical scale (controlling the volts/div), the attenuator can reduce the voltage of the signal, and the amplifier can increase the signal voltage.

The signal then goes directly to the vertical deflection plate of the CRT. A voltage is applied to these vertical deflection plates causing the bright spots to move through the screen. The highlight is produced by an electron beam hitting the fluorescent material inside the CRT. A positive voltage causes the point to move upwards, while a negative voltage causes the point to move downward.

The signal also passes through the trigger system to initiate or trigger a horizontal scan. Horizontal scanning is the act of horizontal system highlights moving across the screen. After the horizontal system is triggered, the bright points are moved from left to right at specific time intervals based on the horizontal time base. Many fast-moving highlights blend together to form solid lines. If the speed is high enough, the highlights sweep the screen up to 500,000 times per second.

The horizontal scan and the vertical deflection work together to form a signal image that is displayed on the screen. The flip-flop stabilizes the repetitive signal, ensuring that the scan always starts at the same point of the repetitive signal, with the goal of making the rendered image clear. Refer to Figure 14.

In addition, the analog oscilloscope has control over focus and brightness to adjust for sharp and clear display results. Analog oscilloscopes are often recommended to display signals that change rapidly under "real-time" conditions or under sudden conditions. The display portion of an analog oscilloscope is based on a chemiluminescent substance that has a brightness level. The more the signal appears, the brighter the trajectory. Through the brightness level, only the brightness of the track can be observed to distinguish the details of the signal.

Digital scope

Unlike analog oscilloscopes, digital oscilloscopes convert analog voltages into digital information through analog-to-digital converters (ADCs). It captures a series of samples of the waveform and stores the samples. The storage limit is to determine whether the accumulated samples can draw the waveform. The digital oscilloscope then reconstructs the waveform. (See Figure 15.)

Digital oscilloscopes are divided into digital storage oscilloscopes (DSO), digital phosphor oscilloscopes (DPO) and sampling oscilloscopes.

The digital means that the waveform of any frequency can be displayed stably, brightly and clearly within the scope of the oscilloscope. For repeated signals, the bandwidth of a digital oscilloscope is the analog bandwidth of the front-end components of the oscilloscope, commonly referred to as the 3dB point. For single pulses and transient events, such as pulses and step waves, the bandwidth is limited to the oscilloscope's sample rate. For more details, please refer to the Performance Terms and Sample Rates section of the Applications section.

Digital storage scope

A conventional digital oscilloscope is a digital storage oscilloscope (DSO). Its display portion is based more on raster screens than on fluorescence.

Digital Storage Oscilloscopes (DSOs) make it easy for you to capture and display events that may only occur once, often called transients. The waveform information is represented in digital form, and the actual stored binary sequence. In this way, analysis, archiving, printing, and other processing are facilitated by the oscilloscope itself or an external computer. The waveform does not have to be continuous; even if the signal has disappeared, it can still be displayed. Unlike analog oscilloscopes, digital storage oscilloscopes preserve signals for long periods of time and extend the way waveforms are processed. However, DSOs do not have real-time brightness levels; therefore, they cannot represent different brightness levels in the actual signal. Some of the subsystems that make up the DSO are similar to some parts of an analog oscilloscope. However, the DSO contains more data processing subsystems, so it can collect data that shows the entire waveform. From capturing signals to displaying waveforms on the screen, DSO uses a serial processing architecture, as shown in Figure 16. The serial processing system will be explained later.

Serial processing architecture

Like an analog oscilloscope, the first part of the DSO (input) is a vertical amplifier. At this stage, the vertical control system allows you to adjust the amplitude and position range. Then, in the analog-to-digital converter (ADC) section of the horizontal system, the signal is sampled at discrete points in real time, and the signal voltage at the sampling position is converted to a digital value, which is called a sampling point. This process is called signal digitization. The sampling clock of the horizontal system determines the frequency of ADC sampling. This rate is called the sampling rate and is expressed as samples per second (S/s).

The sample points from the ADC are stored in the capture memory area, called waveform points. Several sample points can form a waveform point. The waveform points together form a waveform record. The number of waveform points that create a waveform record is called the record length. The trigger system determines the start and end points of the record. The DSO signal path includes a microprocessor, and the signal under test is processed by the microprocessor before being displayed. The microprocessor processes the signals, adjusts the display operation, manages the front panel adjustments, and so on. The signal passes through the memory and is finally displayed on the oscilloscope screen.

Within the capabilities of the oscilloscope, the sample points are supplemented and the display is enhanced. Pre-trigger can be added so that the result can be observed before the trigger point. Most digital oscilloscopes also offer automatic parameter measurements to simplify the measurement process.

The DSO provides the ability to handle single-pulse signals and multiple channels with high performance (see Figure 17). The DSO is the perfect tool for low repetition rate or single pulse, high speed, multi-channel design applications. In digital design practice, engineers often check four or more signals simultaneously, and DSO becomes a standard partner.

Digital phosphoroscope

Digital Phosphor Oscilloscopes (DPOs) add a new type to the oscilloscope family. The DPO architecture enables unique capture and display capabilities to accelerate signal reconstruction. DSO uses serial-processed fabrics to capture, display, and analyze signals; relatively speaking, DPO adopts a parallel architecture for accomplishing these functions, as shown in Figure 18. The DPO uses an ASIC hardware architecture to capture waveform images, providing a high rate of waveform acquisition and a high degree of signal visibility. It increases the likelihood of proving transient events in digital systems. This parallel processing architecture will then be explained.

Serial processing architecture

The first stage (input) of the DPO is similar to an analog oscilloscope (vertical amplifier), and the second stage is similar to DSO (ADC). However, after analog-to-digital conversion, the DPO has significant differences compared to the original oscilloscope.

For all oscilloscopes, including analog, DSO, and DPO oscilloscopes, there is a hold-off time. During this time, the instrument processes the most recently captured data, resets the system, and waits for the next trigger event to occur. During this time, the oscilloscope is blind to all signals. As the holdoff time increases, the likelihood of viewing low frequency and low repetition events decreases.

Note that it is not possible to simply infer the probability of collecting an event from the displayed update rate. If you only rely on the display update rate, you can confirm that the oscilloscope can collect all the relevant information of the waveform, so it is easy to make mistakes, because the oscilloscope does not actually do it. The digital storage oscilloscope serially processes the acquired waveform. Since the microprocessor limits the acquisition rate of the waveform, the microprocessor is the bottleneck of serial processing.

The DPO further rasterizes the digitized waveform data and stores it in a fluorescence database. Every 1/30th of a second, this is about the fastest speed that human eyes can perceive, and the signal images stored in the database are sent directly to the display system. The waveform data is directly rasterized, and the database data is directly copied into the video memory. The two work together to change the bottleneck of other systems in data processing. The result is an increase in "time of use" to enhance display update capabilities. Signal details, intermittent events, and dynamics of the signal can be acquired in real time. The DPO microprocessor works in parallel with the integrated capture system to perform display management, automatic measurement, and device adjustment control without compromising the oscilloscope's capture speed.

DPO faithfully simulates the best display properties of an analog oscilloscope and displays the signals in three dimensions: time, amplitude, and amplitude variation with time as the argument, all of which are real-time. Unlike analog oscilloscopes, which rely on chemiluminescent materials, DPO uses full electronic digital fluorescence, which is essentially an up-to-date database. For each point on the oscilloscope display screen, there are separate "cells" in the database. Once the waveform is acquired (ie, the oscilloscope is triggered), the waveform is mapped into the unit group of the digital fluorescence database. Each unit represents a location on the screen. When the waveform is related to the unit, the brightness information is added inside the unit; if it is not involved, it is not added. Therefore, if the waveform is frequently swept, the brightness information will gradually accumulate in the unit.

When the digital fluorescence database is transferred to the oscilloscope's display screen, the display shows the area of ​​the waveform in which the brightness is added, based on the ratio of the frequency of the signal occurring at each point, which is very similar to the brightness level characteristics of an analog oscilloscope. DPO can also display information about the frequency of changes, and the display presents different colors for different information, unlike analog oscilloscopes. With DPO, you can compare the similarities and differences between waveforms generated by different triggers, for example, comparing the difference between a waveform and the 100th trigger.

Digital phosphor oscilloscopes (DPOs) break through the barriers between analog and digital oscilloscope technologies. It is also suitable for viewing high and low frequency signals, repetitive waveforms, and real-time signal changes. Only the DPO provides the Z (brightness) axis in real time, and the conventional DSO has lost this function.

DPO is an ideal tool for those who need the best general-purpose design and fault detection tools for a wide range of applications. Typical applications for DPO are: communication mask testing, digital debugging of interrupt signals, repetitive digital design and timing applications.

Digital sampling scope

When measuring high frequency signals, the oscilloscope may not be able to acquire enough samples in one scan. A digital sampling oscilloscope is a good choice if you need to properly acquire a signal with a frequency much higher than the oscilloscope's sampling frequency (see Figure 21). The ability of such an oscilloscope to acquire measurement signals is an order of magnitude higher than other types of oscilloscopes. When measuring repetitive signals, the bandwidth it can achieve and the high-speed timing are ten times that of other oscilloscopes. The continuous equivalent time sampling oscilloscope can achieve a bandwidth of 50 GHz.

Unlike digital storage and digital phosphor oscilloscope architectures, in the architecture of a digital sampling oscilloscope, the position of the attenuator/amplifier on the sampling bridge is replaced, see Figure 20. The input signal is sampled before attenuation or amplification. Due to the role of the sampling gate circuit, the frequency of the signal after passing through the sampling bridge has become lower, so a low bandwidth amplifier can be used, and as a result, the bandwidth of the entire instrument is increased.

However, the negative impact of the increased bandwidth of the sampling oscilloscope is the dynamic range limitation. Since there is no attenuator/amplifier before the sampling gate, the input signal cannot be scaled. The input signal at all times cannot exceed the full dynamic range of the sampling bridge. Therefore, the dynamic range of most sampling oscilloscopes is limited to a peak-to-peak value of 1V. Digital storage and digital phosphor oscilloscopes, on the other hand, are capable of handling inputs from 50 to 100 volts.

In addition, the protection diode cannot be added to the front of the sampling bridge, otherwise the bandwidth will be limited. Therefore, the safe input voltage of the sampling oscilloscope is only about 3V, and other oscilloscopes can be as high as 500V.

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