Digital Oscilloscope Working Principle and Structure
With the development of electronic technology and change, the circuit measurement requirements have become higher, in the electronic production will find that the measurement of many parameters is not a multimeter can be competent, such as a microcontroller I / O port of the output waveform or the production of amplifiers to measure its frequency response and so on. Therefore, oscilloscopes are naturally the same as multimeters, and have become a necessary tool for electronic engineers and enthusiasts.
Working principle and structure introduction
The hardware part of the digital oscilloscope system is a high-speed data acquisition board. It can achieve dual-channel data input, each sampling frequency can reach 60Mbit/s. Functionally, the hardware system can be divided into: signal front-end amplification (FET input amplifier) and conditioning module (variable gain amplifier), high-speed analogue-to-digital converter module (ADC driver, ADC), FPGA logic control module, clock distribution, high-speed comparator, MCU control module (DSP), data communication module, liquid crystal display (LCD). ), data communication module, LCD display, touch screen control, power supply and battery management and keyboard control and several other parts.
The input signal is converted by the preamplifier and gain adjustable circuit into an input voltage that meets the requirements of the A/D converter. The digital signal converted by the A/D converter is cached by the FPGA or the acquisition memory FIFO, and then transmitted to the computer through the communication interface for subsequent data processing, or directly controlled by the microcontroller will be collected and displayed on the LCD screen.
Reference devices are as follows
In these parts, the most important is the programmed amplification (attenuation) circuit and the A/D conversion circuit, because these two circuits are the throat of the digital oscilloscope, the programmed amplification (attenuation) circuit determines the oscilloscope's input bandwidth and vertical resolution, A/D conversion circuit determines the oscilloscope's horizontal resolution, which directly determines the performance of the oscilloscope of the two resolutions. These two parts of the circuit will be measured signals into the back of the processing circuit required for the data signal, this part of the circuit can be used in high-performance integrated circuits plus a small number of peripheral devices constitute a simple circuit design, debugging is also very simple. The most difficult part of the oscilloscope should be the procedure, that is, the software. The software bears all the data processing and control tasks of the digital oscilloscope, including A/D sampling control, horizontal sweep speed control, vertical sensitivity control, display processing, peak-to-peak measurement, frequency measurement and other tasks. It can be realised by using a very common microcontroller on the market nowadays as a microprocessor and programming in C language.
Programmed amplification (attenuation) circuit and power supply circuit
The signal is input from a common X10X1 oscilloscope probe into the amplification (attenuation) circuit. The role of the programmed amplification (attenuation) circuit is to amplify or attenuate the input signal to adjust, so that the output signal voltage in the input voltage requirements of the A/D converter within the range of the best measurement and observation, so the programmed amplifier circuit in the specified bandwidth gain must be flat. As the oscilloscope circuit contains digital and analogue two parts, in order to avoid mutual interference, so the digital part of the power supply and analogue part of the power supply separately, respectively, to provide a set of ± 5V DC power supply, and inductance and capacitance made of filter isolation
Flash memory and clock circuit
Because the A/D converter captures a large amount of signal data, the microcontroller's internal flash memory is not enough to use, so the circuit can choose some external memory to use, but also as a way to write the LCD. The flash memory is also used as a cache for writing the LCD. In order to get the reference clock signal, the microcontroller is also connected to a crystal, which is used to calculate the actual frequency of the external waveform signal.
FPGA Control Unit
FPGAs are semi-custom ASICs that allow circuit designers to program their own application-specific functions. The design uses two different methods: schematic input and VHDL input. The control unit carries out most of the control tasks, providing the appropriate control signals for each functional module to ensure the correct operation of the whole system. Specifically achieve the following functions: frequency divider circuit and generate A/D converter control signals The data acquisition system has a wide measurement range, a frequency divider circuit is designed inside the FPGA to achieve different sampling frequencies for different frequencies of the measured signals to ensure that the collected data is more accurate. The frequency division unit is implemented using the graphical input method and its internal structure is shown in Figure 4. In Figure 4, the use of T-trigger in the input is 1, each clock edge when the output will jump to achieve the frequency division. At the same time, we can see that the input of the T-flip-flop is composed of some logical combinations, which constitutes the gated clock. For gated clocks, the clock function is carefully analysed to avoid the effect of burrs. While the gated clock is guaranteed to be free of dangerous burrs on the clock signal when the following two conditions are met, the gated clock can work as reliably as the global clock.
For the design of the A/D converter, its control signal only two: clock input signal CLK and enable the output signal OE. CLK signal directly through the active crystal input 60M signal, while the OE signal through the FPGA internal and CLK the same frequency and the same phase of the clock signal inverted to get, so that just to meet the conversion of the A/D converter timing relationships.
High-speed A/D conversion; circuit
Digital oscilloscope in the most important circuit is the A/D conversion circuit, its role is to be measured signal sampling and converted into digital signals into memory, said it is a digital oscilloscope throat is not too much, because it directly determines the digital oscilloscope can be measured at the highest frequency, according to Nyquist Theorem, the sampling frequency of at least 2 times the highest frequency of the signal to be measured in order to reproduce the signal being measured. In digital oscilloscopes, the sampling frequency should be at least 5 to 8 times the frequency of the signal under test, otherwise the signal waveform can not be observed.
