Waveform digital oscillograph in automatic test system

Electronic oscilloscopes are instruments widely used by engineers in laboratories, factories, and on-site. In fact, electronic oscilloscopes are also products that pass the electronic test and measure the sales volume and sales amount of the instrument. In the late 1930s and early 1940s, driven by the rapid development of TV broadcasting and radar ranging, analog electronic oscilloscopes were basically finalized and divided into four parts: vertical amplification, horizontal scanning, trigger synchronization, and oscilloscope (CRT) display. . The real-time bandwidth of analog electronic oscilloscopes reached a peak of 1000MHz in the 1970s. With the advent of digital technology and integrated circuits, analog electronic oscilloscopes dominated by vacuum tubes and broadband amplifier circuits have been gradually replaced by digital electronic oscilloscopes since the 1980s. With the explosive development of the information technology and digital communication market, the real-time bandwidth of Sri Lanka's digital electronic oscilloscope has exceeded 1 GHz after the 1990s. In the 2010s of the 21st century, digital electronic oscilloscopes have also made great strides, with real-time bandwidth exceeding 10 GHz and equivalent sampling bandwidth reaching 100 GHz.

The circuit composition of a digital electronic oscilloscope is simpler than that of an analog electronic oscilloscope. It is mainly composed of four parts: analog/digital converter (ADC), waveform storage/processor, digital/analog converter (DAC) and liquid crystal (LCD) waveform display. The analog electronic oscilloscope needs to have a broadband response from the signal input front end to the waveform display back end, but the digital electronic oscilloscope only needs the front end analog/digital converter to have the same broadband response as the input signal, and then the frequency response of various circuits are reduced accordingly. According to the sampling principle, the sampling frequency is equal to 2 times the input analog signal frequency under the conditions. After the ADC output digital information is filtered and DAC processed, the waveform of the input signal can be reproduced. Obviously, the clock frequency of the DAC can be much lower than the sampling frequency of the ADC. In addition, in order to reduce the aliasing signal caused by signal filtering and processing, the sampling frequency actually used by the ADC of the digital electronic oscilloscope is 4 times the frequency of the analog input signal instead of 2 times.

At present, the horizontal ADC sampling frequency reaches 20GHz and the resolution is 8 bits. If two ADCs with a sampling frequency of 20 GHz are superimposed on the time axis, an equivalent ADC function with a resolution of 8 bits at a sampling frequency of 40 GHz will be obtained. In other words, with the help of an ADC with a sampling frequency of 20 GHz, an implementation bandwidth of 10 GHz can be achieved, but the resolution is only 8 bits. If it is allowed to reduce the sampling rate of the ADC, it is not difficult to increase the resolution of the ADC. For example, an ADC with a sampling rate of 1MHz can achieve 28-bit resolution. Digital electronic oscilloscopes with a real-time bandwidth of more than 100MHz fully adopt 8-bit resolution. In order to improve the resolution, multiple samples can be averaged, but the measurement time is also increased accordingly. Digital electronic oscilloscopes with real-time bandwidth below 100MHz can provide products with resolutions of 8, 10, and 16 bits or more.

Concise waveform digitizer

From the above introduction, it can be seen that digital electronic oscilloscopes are tabletop instruments with complete performance, with strong visibility, interactivity, and signal integrity. The measurement function is defined by hardware, a good user interface, and real-time bandwidth. It is suitable for the development and development of electronic products. Evaluation, measurement, and troubleshooting applications.

However, in the product line of electronic components and equipment, it is often necessary to increase the measurement speed and use a short measurement time to obtain all the data of the electrical indicators to ensure that the products are on the market in time. In this case, the waveform digitizer came into being, in essence, the waveform digitizer is a simplified version of the digital electronic oscilloscope. The waveform digitizer only retains the ADC front-end and data storage/arithmetic unit, omitting the back-end DAC and LCD display. In other words, keeping high speeds, discarding low speeds, and simplifying the complicated ones, makes the waveform digitizer more suitable for the application of automatic test systems.

Waveform digitizer circuit configuration

The circuit block diagram of the waveform digitizer is shown in Figure 1. It can be seen that the waveform digitizer as a module focuses on the digitization of the input analog signal. The circuit structure is very concise. The front end is the ADC chip, followed by the digital memory and the on-board digital signal processor to perform arithmetic operations and waveform analysis on the signal, and then transmit it via the high-speed bus. The data acquisition controller of the automatic test system. Compared with digital electronic oscilloscopes, because the waveform digitizer only retains the high-speed front-end ADC circuit, it directly uses the PC's high-speed peripheral bus to transmit digital data. For example, PCI and PCI Express buses are better than GPIB, VXI, LXI and other instrument general buses. Higher transmission speed. In addition, the on-board digital signal processor of the waveform digitizer analyzes and processes the data after the analog/digital conversion in time, and then transfers it to the PC of the data acquisition subsystem for background operation.

The data flow of the waveform digitizer is shown in Figure 2. The analog instrument has no physical panel and display screen, only a virtual panel. Therefore, the working status of the various circuits of the waveform digitizer is issued through a graphical interface, including the input signal conditioning and signal acquisition methods of the front end. The input signal is converted into a data stream by the ADC. That is, the operation is performed by the memory temporary storage and digital signal processing. The memory performs continuous digital acquisition of the data stream and storage of a large number of waveforms, while the digital signal processor performs mathematical operations and parameter analysis on the waveforms. The result obtained is sent to the background computer of the automatic test equipment subsystem through the high-speed peripheral bus for measurement result processing. It can be seen that the waveform digitizer is a software-defined measurement instrument, and the software plays a key role. It provides measurement engineering technicians with multiple signal inputs, large amounts of data processing, short measurement time, small footprint, convenient maintenance, and low measurement cost. platform.

Test and measurement instrument company and automatic test system company both have waveform digitizers. The real-time bandwidth of the products covers low frequency to radio frequency, and there are various models. It is worth noting that the waveform digitizer supplier such as National Instruments (NI), which cooperates with the Lab VIEW graphical programming software on the PXI instrument bus platform developed by itself, constitutes a data acquisition system that occupies an important position in the automatic test system and automatic measurement. . Another ZTEC instrument company is a supplier of analog digitizers, which is characterized by not only being able to supply PC industry standard simulation, but also measuring instrument standard bus modules. The indicator's waveform digitizer uses an ADC chip with a sampling rate of 4GS/S, a resolution of 8 bits, and a real-time bandwidth of 1GHz.

Because the circuit structure of the waveform digitizer simulation is not complicated, there are many types of ADC chips available on the market, especially information technology and mobile communication products, which need to process video signals, thereby promoting the development and production of video ADC chips. Well-known Texas Instruments (TI), Modular Devices (ADI), etc. have a variety of ADC series chips available, and the price/performance ratio is quite high. If we need a dedicated waveform digitizer simulation when designing an automatic test system. In addition to purchasing ready-made products, you can also consider your own design. In particular, ADC chips with a sampling rate of 200MS/S, a resolution of 16 bits, and a real-time bandwidth of 100MHz are required in wireless communications, digital cameras, mobile phones, radar, medical images, data acquisition, test and measurement and other fields due to their wide range of applications. Lots of products. Take Texas Instruments' ADS548X series ADC chip as an example, it has the following characteristics:

sampling rate 200MS/S, bandwidth 100MHz

Resolution 16 bits, full-scale background noise 78dB

Spurious frequency free dynamic range 95dBc

On-chip high impedance differential buffer input amplifier

High-efficiency low-voltage differential signal (LVDS) digital output

Power supply voltage +5V and +3V, power dissipation 70mW

64-pin QFN-64 package with 99mm footprint

Temperature range -40 to 85℃

The block diagram of the ADS548X series ADC chip is shown in Figure 3. It can be seen from the figure that the input signals INP and INM are amplified by the buffer input stage and then time-division sampled by the sampling clock of the 16-bit ADC analog-to-digital converter. After digital correction and shaping, the sample value is sent to 8 groups of low-voltage differential amplification, and output to the subsequent stage memory by the data lines D0, D2 and D14.

ADS548X chip also has auxiliary function blocks such as power regulation circuit, reference voltage, timing control, and working mode control. As the front-end chip of the waveform digitizer, ADS548X is an ADC parallel analog-to-digital converter with high integration and performance. With the back-end buffer memory and digital signal processor, select the interface bus of the signal acquisition system to form a complete waveform. Digitizer module. Texas Instruments also provides an evaluation experiment board for the ADS548X chip. As a digital signal processor supplier in the industry, there are a variety of series of DSP chips for engineering designers to choose, or with the help of the corresponding DSP development kit, the software design process of the module can be simplified and realized Rapid design and evaluation of waveform digitizers.