1. Preamble

    The oscilloscope is one of the the most important tools to be used by any electronics hobbyist but not everybody can afford to have one. As commercial scopes are often too expensive, almost every electronics hobbyist thought at a certain time to build one from scratch. The classical oscilloscope (cathode ray tube) is difficult to build at home because of its size, mechanical fragility, high voltages presence, etc. An alternative solution is the modern "PC oscilloscope", having the advantage of post-processing and recording capabilities, and kind of reduced complexity. However, this solution is often non-portable, expensive (requires an PC) and dangerous for the PC if not isolated from it's chassis. The third solution, commonly used these days by all commercial oscilloscope manufacturers, is the digital oscilloscope with LCD screen. Therefore, the authors decided to use this solution, and tried to develop it using common parts from today's component retailers.

    2. Features

• Maximum sample frequency: 40MSPS
• Maximum input frequency: 5MHz
• Maximum displayed frequency without aliasing: 10MHz
• Input circuit bandwidth: 20MHz
• Display resolution: 240x128 total, trace resolution 200x125
• Sensitivity: 40mV/div
• Coupling: DC
• Input impedance: 10K
• Power supply: single DC source 8V..10V, 1A
• No incremental mode
• Time base: 1s/div, 500ms/div, 200ms/div, 100ms/div, 50ms/div/, 20ms/div, 10ms/div, 5ms/div, 2ms/div, 1ms/div, 500us/div, 200us/div, 100us/div, 50us/div, 20us/div, 10us/div, 5us/div, 2us/div, 1us/div, 500ns/div
• Trigger: digitally adjustable
• Trace offset: digitally adjustable

    Short description:
    The Input circuit is built with one OPA2652 operational amplifier from Texas Instruments, and together with a low-pass RC filter sets the bandwidth to 20MHz. Additionally, the input circuit handles the vertical trace shift (offset) using the input from a PWM signal generated by the microcontroller (pin 15). The ADC converter is an 8bit ADS830 from Texas Instruments, capable of working up to 60MSPS. In this design, the ADC works at a maximum of 40MHz, and this clock is generated by the QOS40 (plastic) or QOM40 (metal) oscillator, wich is programmed (divided) through the CPLD circuit (XC9572 from Xilinx). To handle the high throughput of the ADC at high-rate acquisition, the digital output of the ADC is connected to a high speed FIFO memory IDT7201 (512 bytes long) from IDT. After a full memory buffer is reached, the whole memory content is discarded to microcontroller's memory, where the samples are further processed and then displayed on the LCD. The microcontroller is an ATMega162 from Atmel, a member of the popular AVR family. The display is a LMG6402PFLR from Hitachi, but any other HD61830B compatible LCD displays can be used (the pinout should be checked for compatibility, of course). The power supply circuit is built with two 7805 series regulators, and the negative voltages (needed for the LCD and the input circuit) are obtained using three ICL7660A integrated circuits from Intersil.

    4. PCB

    The PCB is organised as two separate boards: the mainboard and the keyboard. Both of them are designed as single side boards, easy to manufacture using amateur methods. The authors had realised them using the Press'n'Peel method, but any other method can be used if enough accuracy is obtained (SMD parts were used on the board).

    Because of the high complexity of the design and routing constraints derived from single side routing, a few of the connexions must be soldered as "air wires". You can find instructions for "air wires" connections in the picture above.

    5. Software

    The software for the microcontroller was developed in C and compiled with GCC for AVR. Additionally, in the development phase, Avrstudio 4.xx debugger was also used (for the connection with an JTAG ICE emulator). The microcontroller can be programmed using any AVR programmer, like STK200, PonyProg, AVR910, etc.

    6. Firmware

    The CPLD circuit was used in the design to generate the clock for the ADC (time base is devided from the XTAL circuit) and also for keyboard interfacing (serialisation). The CPLD circuit must be programmed using the IMPACT module from Xilinx ISE WebPack software (free of charge) and a simple programmer circuit, XilinxCable (very easy to build).