How to build an oscilloscope

　　Electronics enthusiasts usually use a multimeter to make and repair electronic products, so they will not know where to start when encountering a slightly complicated problem. At this time, it will be very convenient to have an oscilloscope at hand. The price of a common oscilloscope is more than 500 yuan, which is a considerable expense for amateurs. Here is a virtual oscilloscope that can be used as an oscilloscope as long as it is connected to the parallel port of the computer.

　　Computers are now quite popular and have basically become a must-have household appliance for urban families. Electronics enthusiasts can own a good oscilloscope with very little effort. Some virtual oscilloscope software that uses computer sound cards is also common on the Internet, but they cannot measure DC and cannot quantitatively scale, and the sampling frequency rarely exceeds 44kHz. The oscilloscope introduced here has the characteristics of high sampling frequency, supports DC measurement, and can quantitatively measure signals. The figure below shows the display of a 19.2kHz square wave.

　　Its main technical indicators are as follows:

　　Sampling frequency: 224kHz (depending on CPU frequency, the CPU of this machine is P42.93GHz). The signal frequency measured by this machine should be below 40kHz.

　　The maximum input voltage is divided into four levels: ±25V, ±5V, ±1V, ±0.2V. If a 101 oscilloscope probe is connected, the maximum input voltage can reach ±250V.

　　Input impedance: 1MΩ

　　Power supply voltage: AC 9V~15V

　　Trigger range: 0%~100%.

　　Interface: The electrical schematic diagram of the parallel interface supporting bidirectional transmission is shown in the figure below.

　　The whole system consists of a quad op amp TL074, an ADC0820 and some other auxiliary components, including input signal conditioning, signal conversion, analog-to-digital conversion, parallel port interface, trigger level detection and power supply.

　　R10, R11, R12, R13, C19, C20 and C21 form an input AC/DC switching and attenuation network, providing AC/DC input switching and 1:1, 1:5 input signal switching functions. An op amp U1A in TL072 and its surrounding components form a positive-phase amplifier, providing input protection and 1:1, 25:1 amplification switching functions. An op amp U1B in TL072 forms a positive-phase amplifier, providing the function of -5V-+5V to 0~5V conversion. ADC0820 is a CMOS process, sample/hold function, high-speed, 8-bit A converter, with a maximum sampling frequency of 400kHz. This oscillator mainly uses it to realize the collection of analog signals. ADC0820 works in write mode. After the trigger signal is given by pin 1 of the parallel port, the conversion starts. The computer waits for a certain period of time and then reads the converted signal. The working principle of ADC0820 can be found on the Internet. One of the op amps U1D in TL072 works in comparator mode, and provides an external trigger signal by comparing the input signal with the voltage provided by P2. The power supply part provides ±9V and 5V power supplies for op amp ADC0820. Readers can design printed circuit boards by themselves according to the actual situation of the shell they find and refer to the electrical principles. The figure below shows the circuit board installed by the author.

　　The toggle switch on the left is used for DC and AC input conversion, the toggle switch in the middle is used for 1:5 attenuation switching, the toggle switch on the right is used for 25:1 amplification switching, and the potentiometer on the right is used for external trigger adjustment. The input is located on the lower left side of the printed circuit board, the parallel port is located on the right side of the printed circuit board, and the upper left is the power input socket, which requires 9V~15V AC power input.

　　The supporting software is developed with VC6.0. For WindowsNT, Windows2000, Windows2003 and WindowsXP operating systems, the support of IOManager.SYS is required. The software provides functions such as waveform display, printing, time base adjustment, waveform saving, and waveform comparison. Although this is a single-trace oscilloscope, it provides the function of a quasi-dual-trace oscilloscope through waveform saving and reproduction. You can measure the signal at one point, save it, then measure the signal at another point, open the saved signal, and display the two sets of signals on the same screen for comparison.

　　After starting the software, the sampling speed will be automatically tested first, and then a set of demonstration signals will be randomly given (if "show sample screen at startup" is cancelled, it will not be displayed after the next startup). Currently, the software provides three triggering modes: no trigger, internal trigger and external trigger. At the same time, it provides two options for internal trigger: rising edge trigger and falling edge trigger. Select the start button (play) to start signal acquisition, the pause button to stop signal acquisition, and the record button to pause after acquiring a cycle of signals. You can choose to save the signal at any time and save the waveform at that time. All prompts in the software are in Chinese, and you can get started with the application after a simple exploration.

　　Multithreading technology is used in the software compilation process. The foreground thread is responsible for user interaction and waveform display, and the background thread is responsible for signal acquisition. The signal acquisition code is as follows:

　　for (i = 0; i 　　m_MAX_SAMPLES; i++){for(j=1; j<=pDoc-

　　m_WaitingCircle; j++)_outp(890,254); //adc0820 works in write mode, and needs to wait for 600nsp after starting AD conversion Doc-》m_data［i］=_inp(888);_outp(890,255); }