A Fully Functional Digital Oscilloscope Oscilloscopes are widely used instruments by engineers in laboratories, factories, and field operations. In fact, they are the best-selling and highest-grossing electronic test and measurement instrument. Driven by the rapid development of television broadcasting and radar ranging in the late 1930s and early 1940s, analog oscilloscopes were largely standardized, divided into four main parts: vertical amplification, horizontal scanning, trigger synchronization, and CRT display. The real-time bandwidth of analog oscilloscopes reached a peak of 1000MHz in the 1970s. With the advent of digital technology and integrated circuits, analog oscilloscopes, dominated by vacuum tubes and wideband amplifier circuits, were gradually replaced by digital oscilloscopes starting in the 1980s. With the explosive growth of information technology and the digital communications market, the real-time bandwidth of digital oscilloscopes exceeded 1GHz in the late 1990s. In the 2010s, digital oscilloscopes also made a leap forward, with real-time bandwidth exceeding 10GHz and equivalent sampling bandwidth reaching 100GHz. Digital oscilloscopes have a simpler circuit structure than analog oscilloscopes, mainly consisting of four parts: an analog-to-digital converter (ADC), a waveform storage/processor, a digital-to-analog converter (DAC), and an LCD waveform display. Analog oscilloscopes require wideband response from the signal input front-end to the waveform display back-end; however, digital oscilloscopes only require the front-end ADC to have the same wideband response as the input signal, with subsequent circuits having correspondingly lower frequency responses. According to the sampling principle, under optimal conditions, the sampling frequency is equal to twice the highest frequency of the input analog signal. After filtering and DAC processing, the waveform of the input signal can be reproduced from the ADC output digital information. Clearly, the DAC clock frequency can be much lower than the ADC sampling frequency. Furthermore, to reduce signal interference from filtering and processing, the actual sampling frequency used by the ADC in a digital oscilloscope is four times, not twice, the highest frequency of the analog input signal. Currently, the highest level of ADC sampling frequency reaches 20 GHz and resolution is 8 bits. If two 20 GHz ADCs are superimposed on the time axis, an equivalent ADC function with a 40 GHz sampling frequency and 8-bit resolution can be obtained. In other words, an ADC with a sampling frequency of 20 GHz can achieve a bandwidth of 10 GHz, but the resolution is only 8 bits. If the sampling rate of the ADC can be reduced, it is not difficult to increase the ADC resolution. For example, an ADC with a sampling rate of 1 MHz can achieve 28-bit resolution. Digital oscilloscopes with a real-time bandwidth of 100 MHz or higher use 8-bit resolution exclusively. To improve resolution, multiple sampling averaging can be used, but the measurement time will also increase accordingly. Digital oscilloscopes with a real-time bandwidth below 100 MHz can provide products with 8-bit, 10-bit, and even higher resolutions. A concise waveform digitizer . As can be seen from the above introduction, digital oscilloscopes are high-performance benchtop instruments with strong visibility, interactivity, signal integrity, hardware-defined measurement functions, a user-friendly interface, and the highest real-time bandwidth, making them suitable for applications in the development, evaluation, measurement, and troubleshooting of electronic products. However, in the product lines of electronic components and equipment, there is often a need to improve measurement speed, obtaining all electrical specification data in the shortest possible measurement time to ensure timely product launch. In this context, waveform digitizers emerged, essentially simplified versions of digital oscilloscopes. They retain only the ADC front-end and data storage/processing unit, omitting the back-end DAC and LCD display. In other words, by retaining high speed and discarding low speed, and simplifying the process, waveform digitizers are better suited for automated test systems. Circuit Structure of a Waveform Digitizer The circuit block diagram of a waveform digitizer is shown in Figure 1. As a module, the waveform digitizer focuses on digitizing the input analog signal. Its circuit structure is very concise: the front end is an ADC chip, followed by a digital memory and an on-board digital signal processor to perform arithmetic operations and waveform analysis. Finally, the signal is transmitted to the data acquisition controller of the automated test system via a high-speed bus. Compared to digital oscilloscopes, waveform digitizers, by retaining only the high-speed front-end ADC circuit, directly utilize the high-speed peripheral buses of the PC to transmit digital data. For example, PCI and PCI Express buses offer higher transmission speeds than general-purpose instrument buses such as GPIB, VXI, and LXI. Furthermore, the on-board digital signal processor of the waveform digitizer analyzes and processes the analog-to-digital converted data in real time, before handing it over to the PC of the data acquisition subsystem for background processing. The data flow of a waveform digitizer is shown in Figure 2. Analog instruments lack physical panels and displays, relying instead on virtual panels. Therefore, the operating status of each stage of the waveform digitizer's circuitry is controlled by commands issued through a graphical interface, including input signal conditioning and signal acquisition methods. The input signal is converted into a data stream by an ADC, which is then temporarily stored in memory and processed by digital signal processing (DSP). The memory continuously acquires the data stream digitally and stores a large number of waveforms, while the DSP performs mathematical operations and parameter analysis on the waveforms. The results are sent to the background computer of the automated test equipment subsystem via a high-speed peripheral bus for measurement result processing. Thus, a waveform digitizer is a software-defined measurement instrument, where software plays a crucial role, providing measurement engineers with a platform for multi-channel signal input, large-scale data processing, short measurement time, small footprint, convenient maintenance, and low measurement costs. Test and measurement instrument companies and automated test system companies all offer waveform digitizers, with real-time bandwidths covering low frequencies to radio frequencies and a wide variety of models. Notably, waveform digitizer suppliers such as National Instruments (NI) utilize their proprietary PXI instrument bus platform, combined with LabVIEW graphical programming software, to create a data acquisition system that plays a vital role in automated test systems and automated measurements. ZTEC Instruments, another company, is a professional supplier of analog digitizers. Its strength lies in supplying both PC industry-standard analog digitizers and modules for standard bus systems in measurement instruments. The highest-performance waveform digitizers utilize ADC chips with a sampling rate of 4 GS/s, 8-bit resolution, and a real-time bandwidth of 1 GHz. Because the circuitry of waveform digitizers is not complex, there are many types of ADC chips available on the market. Information technology and mobile communication products, in particular, require video signal processing, driving the development and production of video ADC chips. Well-known companies such as Texas Instruments (TI) and Analog Devices (ADI) offer a variety of ADC series chips with excellent cost-performance ratios. If a dedicated waveform digitizer is needed when designing an automated test system, in addition to purchasing off-the-shelf products, designing one in-house can also be considered. In particular, ADC chips with a sampling rate of 200 MS/s, 16-bit resolution, and 100 MHz real-time bandwidth are widely used in wireless communication, digital cameras, mobile phones, radar, medical imaging, data acquisition, and test and measurement fields, where they are in high demand. Taking Texas Instruments' ADS548X series ADC chip as an example, it has the following characteristics: • Maximum sampling rate of 200 MS/s, bandwidth of 100 MHz • 16-bit resolution, full-scale background noise of 78 dB • Spurious-free dynamic range of 95 dBc • On-chip high-impedance differential buffered input amplifier • High-efficiency low-voltage differential signal (LVDS) digital output • Supply voltage of +5V and +3V, power dissipation of 70 mW • 64-pin QFN-64 package, footprint of 9 × 9 mm • Temperature range of -0 to 85°C. The block diagram of the ADS548X series ADC chip is shown in Figure 3. As can be seen from the figure, the input signals INP and INM are amplified by the buffered input stage and then sampled time-division by the sampling clock of the 16-bit ADC analog-to-digital converter. After digital correction and shaping, the sample values ​​are sent to eight groups of low-voltage differential amplifiers and output to the subsequent memory via data lines D0, D2, and D14. The ADS548X chip also includes auxiliary function blocks such as power regulation circuitry, reference voltage, timing control, and operating mode control. As the front-end chip of a waveform digitizer, the ADS548X is a highly integrated and high-performance parallel analog-to-digital converter (ADC). Combined with a back-end buffer memory and digital signal processor, and by selecting the interface bus of the signal acquisition system, a complete waveform digitizer module can be constructed. Texas Instruments also provides evaluation boards for the ADS548X chip. As a leading digital signal processor supplier, Texas Instruments offers a variety of DSP chips for engineers to choose from, or utilizes corresponding DSP development kits to simplify the software design process of the module and achieve rapid design and evaluation of waveform digitizers. The appearance of several waveform digitizers is shown in Figure 4. Edited by: He Shiping