Working principle and structure of digital oscilloscope
The hardware part of the digital oscilloscope system is a high-speed data acquisition circuit board. It can realize dual-channel data input, and the sampling frequency of each channel 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 analog-to-digital conversion module (ADC driver, ADC), FPGA logic control module, clock distribution, high-speed comparison processor, microcontroller control module (DSP), data communication module, LCD display, touch screen control, power and battery management and keyboard control.
After the input signal is converted by the preamplifier and gain-adjustable circuit, it becomes an input voltage that meets the requirements of the A/D converter. The digital signal after A/D conversion is buffered by the FIFO in the FPGA or acquisition memory, and then passes through the communication interface. It is transmitted to the computer for subsequent data processing, or the collected signals are directly controlled by the microcontroller to display on the LCD screen.
Reference devices are as follows:
Among these parts, the most important ones are the programmable amplification (attenuation) circuit and the A/D conversion circuit, because these two circuits are the throat of the digital oscilloscope, and the programmable amplification (attenuation) circuit determines the input bandwidth and vertical resolution of the oscilloscope. , the A/D conversion circuit determines the horizontal resolution of the oscilloscope, and these two resolutions directly determine the performance of an oscilloscope. These two parts of the circuit convert the measured signal into the data signal required by the subsequent processing circuit. This part of the circuit can be composed of high-performance integrated circuits and a small number of peripheral devices. The circuit design is simple and debugging is also very simple. The most difficult part of the entire oscilloscope should be the program, that is, the software aspect. The software is responsible for all 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. You can use a very common microcontroller on the market as a microprocessor and use C language programming to implement it.
Programmable amplification (attenuation) circuit and power supply circuit
The signal is input by a common X10X1 oscilloscope probe and enters the amplification (attenuation) circuit. The function of the program-controlled amplification (attenuation) circuit is to amplify or attenuate the input signal so that the output signal voltage is within the input voltage requirement range of the A/D converter to achieve the best measurement and observation effects. Therefore, the program-controlled amplifier circuit operates within the specified bandwidth. The gain within must be flat. Since the oscilloscope circuit contains two parts, digital and analog, in order to avoid mutual interference, the power supply of the digital part and the power supply of the analog part are separated. A set of ±5V DC power supply is provided respectively, and isolated by a filter made of inductors and capacitors.
Flash memory and clock circuit
Because the amount of signal data captured by the A/D converter is large, the flash memory inside the microcontroller is not enough, so the circuit can use some external memory.
At the same time, it is also used as a cache for writing to the LCD. In order to obtain the reference clock signal, the microcontroller is also connected to a crystal oscillator to calculate the actual frequency of the external waveform signal.
FPGA control unit
Programmable logic device FPGA is a semi-custom ASIC that allows circuit designers to program themselves to implement application-specific functions. This design uses two different methods of schematic input and VHDL language input. The control unit carries most of the control tasks and provides corresponding control signals for each functional module to ensure the correctness of the entire system. Specifically, it implements the following functions: Frequency dividing circuit and generating control signals for the A/D converter. This data acquisition system has a relatively wide measurement range. A frequency dividing circuit is designed inside the FPGA to achieve different frequencies. Select different sampling frequencies for the measured signal to ensure more accurate data collection. The internal structure diagram of the frequency dividing unit is implemented using the graphic input method as shown in Figure 4. In Figure 4, when the input of the T flip-flop is 1, the output will jump when each clock edge arrives to achieve 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, carefully analyze the clock function to avoid the effects of glitches. When the gated clock meets the following two conditions, it can ensure that the clock signal does not have dangerous glitches, and the gated clock can work as reliably as the global clock.
For the A/D converter in this design, there are only two control signals: the clock input signal CLK and the enable output signal OE. The CLK signal directly inputs a 60M signal through the active crystal oscillator, while the OE signal is obtained by inverting the clock signal with the same frequency and phase as CLK inside the FPGA, which can just meet the conversion timing relationship of the A/D converter.
High-speed A/D conversion; circuit
The most important circuit in a digital oscilloscope is the A/D conversion circuit. Its function is to sample and convert the measured signal into a digital signal and store it in the memory. It is not an exaggeration to say that it is the throat of the digital oscilloscope, because it directly determines the The highest frequency that a digital oscilloscope can measure. According to the Nyquist theorem, the sampling frequency must be at least twice the highest frequency of the measured signal to reproduce the measured signal. In a digital oscilloscope, the sampling frequency should be at least 5 to 8 times the frequency of the signal being measured, otherwise the waveform of the signal cannot be observed at all.
