Detailed explanation: FPGA-based virtual DPO design

Project Overview

1.1 Project Background

An oscilloscope (Oscilloscope) is an electronic measuring instrument that displays the dynamic waveform of a voltage signal. It converts a time-varying voltage signal into a curve in the time domain. The originally invisible electrical signal is converted into an intuitive visible light signal on a two-dimensional plane, thereby enabling analysis of the time domain properties of the electrical signal.

At present, the world's major oscilloscope manufacturers are concentrated in the United States, and high-end oscilloscopes are monopolized by Tektronix, Agilent and LeCory. For example, Agilent's high-performance 90000 Series Infiniium oscilloscopes achieve 40-GSPS sampling rates on all four channels, while providing ultra-low noise 13 GHz full real-time oscilloscope bandwidth with a memory depth of 1 Gpts.

On the domestic front, due to the gap between high-speed analog-to-digital converters and ASICs in developed countries, the main indicators such as the highest sampling rate, analog bandwidth and memory depth of similar oscilloscopes on the market are still far behind. This topic is to develop a digital fluorescence oscilloscope acquisition and storage system with independent intellectual property rights. Accumulate development experience for high performance acquisition storage technology. At the same time, it fills the gap of digital fluorescent oscilloscope in China and narrows the gap with the development level of similar oscilloscopes abroad

1.2 digital phosphor oscilloscope

The Digital Phosphor Oscilloscope (DPO) is an oscilloscope platform from Tektronix that offers the traditional benefits of digital storage oscilloscopes such as data storage and advanced triggering. At the same time, it also has the light and dark display and real-time characteristics of the analog real-time oscilloscope, which can produce a fluorescent effect in which the display effect is better than that of the analog oscilloscope. Its structure is shown in Figure 1.

Digital storage oscilloscopes require a certain amount of milliseconds of dead time between the two waveforms due to the need to display the data in the micro-machine; the analog oscilloscope cannot capture waveform information during the retrace time. The DPO's data acquisition and display modules operate in parallel, enabling the DPO to continue to acquire signal data while processing the display data. At the same time, unlike DSO, DPO is displayed once after multiple acquisitions and processing. Since the DPO generally uses a dedicated hardware circuit to perform digital fluorescence processing of the acquired waveform, it is no longer limited by the microprocessor's low-speed processing of the data, so that the update rate of the waveform is qualitatively improved. Therefore, DPO can continuously capture most of the details of the waveform, and can completely reflect the waveform information, and also provide complete data for subsequent analysis and processing. as shown in picture 2.

The application of digital fluorescent display technology enables DPO to display the frequency at which a signal appears at a specific position with different brightness or color. The higher the frequency, the higher the brightness. Digital fluorescent processors are typically constructed of dedicated hardware circuitry (high speed FPGA or ASIC). Like DSO, the input signal is first amplified and A/D transformed to obtain the sampled value of the signal. The sampled value is processed by the digital fluorescence processing unit to form a complete waveform diagram containing the waveform three-dimensional information, without interrupting the acquisition process. Under the control of the microprocessor, the digital fluorescence processing unit transmits about 30 complete waveform images with fluorescent display effects to the waveform display memory, and displays the waveform image on the oscilloscope screen under the control of the microprocessor to achieve the fluorescent display of the analog oscilloscope. effect. At the same time, the microprocessor can perform various functions such as automatic measurement and calculation in parallel.

The number of waveforms captured by the DPO per second can be as high as several hundred thousand frames, thousands of times or even tens of thousands times higher than the average DSO. This fast waveform capture rate combined with superior display capabilities gives the DPO the ability to analyze any detail of the signal. At the same time, due to the use of digital processing, it also has the advantages of a digital storage oscilloscope.

1.3 Project Features

The project's Digital Phosphor Oscilloscope (DPO) is the latest generation of oscilloscopes that combine the advantages of digital storage oscilloscopes and analog oscilloscopes, including waveform storage for digital storage oscilloscopes, transient capture, negative delay triggering and advanced Trigger and other functions, including real-time capture of analog oscilloscopes, high waveform update rate and display effects of gradually changing brightness.

Utilizing FPGA's rich logic resources and powerful data processing capabilities, the main DPX module, namely the digital fluorescence processing module, is implemented in the FPGA module. At the same time, the processed data is transmitted to the PC for processing using the USB interface, and the PC is further used. Analysis processing and display. Due to the FPGA design, the complexity of the system is greatly reduced, and it is also convenient to upgrade and update. At the same time, the FPGA module can be powered through the USB port, which greatly facilitates the debugging of the engineer, and makes the DPO have good portability.

Overall design and demonstration

The virtual digital phosphor oscilloscope can be simply described as a system: the user sets the acquisition trigger parameters through the PC menu, the oscilloscope collects the data according to the user's settings, and performs the digital fluorescence processing on the collected data to generate the waveform image after being processed by DPX. It is transmitted to the PC via USB and finally displayed on the LCD screen. At the same time, the collected data can be further analyzed. Therefore, the oscilloscope can be divided into two parts, one is responsible for monitoring commands and waveforms, the display of menus; the other is responsible for high-speed data acquisition and digital fluorescence imaging.

2.1 overall block diagram

According to the above analysis, the following design scheme is formulated: the oscilloscope adopts the FPGA architecture, and the FPGA as the system control core is responsible for monitoring the key commands sent on the PC and sending corresponding acquisition control commands to the acquisition module according to the current working state, and also controlling the numbers. The waveform image and control menu generated by the fluorescence processing module, on the other hand, are used to implement high-speed data acquisition systems and digital fluorescence processors due to their high speed characteristics. The overall implementation block diagram is shown in Figure 3. The analog-to-digital converter, the clock circuit and the FPGA together constitute the acquisition system of the oscilloscope. The FPGA implements the DPX module internally, and finally uploads it to the PC to process the display through USB.

2.2 signal conditioning circuit

The signal conditioning circuit is mainly composed of an attenuation amplifying circuit, a coupling control circuit and a DC bias circuit, and is controlled by the FPGA.

The attenuation amplifier circuit adjusts the amplitude range of the input waveform, and attenuates or amplifies the signals of different amplitudes to adapt to the display range of the screen, which is convenient for observation and measurement.

The coupling control circuit controls the coupling mode of the input signals, which are AC coupling and DC coupling. In the DC coupling mode, all components of the signal (AC and DC) are collected and displayed, and in the AC coupling mode, the DC component of the signal Blocked, only the AC component is captured and displayed.

The DC bias circuit adds a DC component to the signal that controls the signal to move up and down the screen. In addition, the input impedance and analog bandwidth of the oscilloscope are also determined by the signal conditioning circuit. In this project, the input impedance of the signal conditioning circuit is 50 ohms and 1 M ohms. The analog bandwidth is 500MHz.

2.3 data acquisition system

The data acquisition system consists of an Analog Digital Convertor (ADC), a clock chip, and an associated acquisition control module in the FPGA.

2.3.1 Analog to Digital Conversion

This design uses the AT84AD001B analog-to-digital converter from e2v. Its interface is shown in Figure 4. The ADC is a parallel comparison structure that is fast but consumes a lot of power. It integrates two ADCs into one chip. The maximum sampling rate of each ADC is 1GHz, and the quantization precision is eight bits. In addition, the chip also supports the function of interleaving sampling, that is, two ADCs in the same chip simultaneously acquire the same analog signal. And the sampling clock phase is opposite, the sampling data of the two ADCs are spliced ​​together to obtain the highest sampling rate of 2GSPS. The main features of the AT84AD001B are as follows:

Dual ADC with 1GSPS per channel and 2GSPS in interleaved sampling mode;

The output code is optional for Gray code and binary code, and supports 1:1 and 1:2 multiplexed output;

Support analog input switching selection, sampling clock selection;

Support gain control and zero level adjustment;

The bit error rate does not exceed the sampling rate of 1 GSPS

Serial configuration working mode, source synchronous clock data output;

2.3.2 Clock circuit

In this project, NaTIonal Semiconductor's high-precision clock management chip LMK03033C is used. The rms value of its clock jitter is 500 femtoseconds. The chip has a low-noise phase-locked loop and supports eight clock-synchronous outputs for serial configuration. Each output clock features a programmable divide ratio, delay adjustment, and output selection block. The maximum output clock frequency is 1 GHz and the output delay can be adjusted from 0 to 2.25 ns in 150 steps. The chip provides an accurate sampling clock for high-speed ADC acquisition data. The interface is shown in Figure 5.

2.3.3 Acquisition Control and Data Buffering

The high-speed digital signal output by the analog-to-digital converter is written into the data buffer under the control of the acquisition control module, and then subjected to digital fluorescence processing. As shown in Figure 6. The control and buffering of high-speed digital signals is generally implemented using high-speed digital circuits. One solution is to implement high speed control and data buffering using an application specific integrated circuit (ASIC). However, ASICs are extremely costly and cannot be modified and are typically used in well-proven, proven digital circuit designs. Another option is to use a high speed FPGA.

Field Programmable Gate Array (FPGA) is a device with programmable components that is generally slower than an ASIC, but still faster than a general-purpose microprocessor and suitable for high-speed data controllers. And the FPGA's programmable features and low price are suitable for prototyping in the initial development phase of the project.

The acquisition control and data buffer module of the oscilloscope is completed in the FPGA. The module intercepts the part of the user's interest from the infinitely long waveform signal according to the trigger condition set by the user. The access control module of the data storage system is also implemented in the FPGA.

2.4 digital fluorescence processing module

The main work of the digital fluorescence processing unit is as follows:

(1) Perform fluorescence display of the waveform to achieve the fluorescent display of the analog oscilloscope

The digital phosphor oscilloscope displays the acquired waveforms in fluorescence, and displays the frequency of the signals appearing for a long time through the waveform brightness, completely retaining the brightness level information of the waveforms of the plurality of channels, and achieving the fluorescent display effect of the analog oscilloscope. The greater the frequency of occurrence at a certain point, the greater the brightness of the point displayed on the screen; the smaller the frequency of occurrence at a certain point, the smaller the brightness of the point displayed on the screen.

(2) Grid adjustment control

When observing a signal with a digital phosphor oscilloscope, in order to make the observation accurate, the screen is required to display the grid while displaying the waveform signal.

(3) Adjusting control of the brightness of the waveform fluorescence display

When using a digital phosphor oscilloscope to observe signals, the brightness of the displayed waveform will directly affect the observation effect. If the waveform is too bright or too dark, the details of the waveform will be unclear, which is not conducive to user observation. Therefore, it is necessary to add a waveform fluorescent display brightness adjustment function, so that the user can adjust the brightness of the waveform displayed on the fluorescent screen, which is beneficial to waveform observation.

(4) Adjustment control of waveform blanking percentage

When using a digital phosphor oscilloscope to observe a signal, it is sometimes necessary to leave the waveform on the screen for a certain period of time before disappearing, or sometimes it is necessary to display the appearing waveform on the screen forever, such as an audible signal such as a glitch. Therefore, it is necessary to add an adjustment function of the waveform blanking percentage so that the user can adjust the duration of the waveform display on the screen.

(5) Fluorescent display of multi-channel waveforms and control of channel priority

When the oscilloscope observes multiple channels simultaneously, the different channels should be displayed in different colors to distinguish them. At the same time, when multiple channels are displayed at the same time, there should be a prioritization, that is, which channel should be displayed at the top. The user should be able to display the most concerned channel waveforms at the top of all waveforms by adjusting the control to facilitate waveform observation.

(6) Meet the requirements of real-time display of digital phosphor oscilloscope

The oscilloscope is a real-time measuring instrument that needs to meet the requirements of real-time waveform display. Therefore, when performing fluorescence display of a waveform, it is necessary to increase the processing speed as much as possible to improve real-time performance.

This chapter will first introduce the design idea of ​​the digital fluorescence processing unit in detail, and on this basis, the design and implementation of the digital fluorescence processing unit will be introduced in detail.

According to the demand analysis, in order to realize the fluorescence display of the waveform, it is necessary to periodically time a period of time, superimpose the waveforms collected during this period, and record the frequency of occurrence of all waveform points. Therefore, the digital fluorescence processing unit needs to design a memory inside to record the frequency of occurrence of all waveform points in a period of time, which is called analog screen memory. At the same time, a waveform superposition processing module is needed to complete the function of storing the frequency after superimposing a plurality of waveforms on the analog screen memory.

Then, when the timing time expires, all the unit information previously stored in the analog screen memory is read out, and a waveform image is formed by color conversion to be sent to the display for display; and all the unit information in the analog screen memory to be read out at the same time. The blanking process is performed and then written back to the analog screen memory.

The one-frame waveform image sent to the display for display is a pixel image of the same size and resolution as the waveform display area of ​​the display screen. Its original information is stored in the analog screen memory. The analog screen memory is actually a two-dimensional dynamic database. The address unit of the database is in one-to-one correspondence with the pixel points of the waveform display area. Therefore, the size of the database is determined by the total pixel points of the display area of ​​the screen of the digital phosphor oscilloscope. The horizontal axis corresponds to the time axis and the vertical axis corresponds to the amplitude axis. The lower left corner of the specified display area corresponds to the start unit of the database, and the subsequent data units are arranged in order from left to right and bottom to top according to the corresponding screen.

As can be seen from the above, the entire digital fluorescence processing unit should include five sub-units working together: waveform data buffer memory, analog screen memory, waveform activation processing module, waveform blanking processing module, and color conversion processing module, etc., which together complete the multi-channel waveform Digital fluorescence processing, and a frame of waveform image is formed and output to the display at regular intervals, and a blanking process is performed at the same time. The relationship between each subunit is shown in Figure 8.

2.5 PC part

The PC and FPGA exchange data (DPX data and control information) via USB, and the PC supplies power to the FPGA through USB. The DPX data is transmitted to the PC via USB and processed by LABwindows.