DSO138 Oscilloscope Module
As Figure 1 shows, the DSO138 module consists of a circuit board with a color LCD attached. Soldered to the board are all the user controls (slide switches and pushbuttons) and sockets for signal input and power. On the left-hand side you 55can see that the selection of input coupling and vertical scale is done in the simplest possible way using slide switches. Unfortunately the quality of these switches (at least in the unit I received) is so abysmal that changing the gain setting often results in the complete disappearance of the signal. The buttons on the right control other functions, and the user interface is generally well-designed and intuitive.
The color LCD panel has a resolution of 320×240 (QVGA) and the visible area has a diagonal of 2.4 inches (6.1 cm). Some low-end desktop scopes, much more expensive than this device, have a display with the same resolution, and so the device should be good enough at least for my application. Happily the device comes with a circuit diagram, which is more than you get with many items of test equipment these days. As a result it was easy to see how to add an external TTL trigger input. The module operates from a 9 V power supply, drawing around 100 mA. A simple inverter circuit generates a negative rail, which, like the positive rail, is stabilized using a linear regulator. The microcontroller, an STMicroelectronics STM32F103C8 based on an ARM Cortex-M3 core, is powered at 3.3 V via another regulator.
The design uses the fast ADC built in to the microcontroller. It offers a resolution of 12 bits and an input voltage range from 0 V to 3.3 V; the middle of this range, 1.65 V, corresponds to the zero position on the display. However, the display is already clipped when the unit is fed with a sine wave of much less than the maximum possible amplitude. This, in conjunction with the high resolution of the converter, allows the vertical offset to be adjusted. In other words, the vertical offset is not implemented in the analog preamplifier circuit, but instead is subtracted from the sample values in software.
The fact that only a relatively small part of the ADC’s input range is used obviously makes the device more susceptible to interference and spikes, and these sometimes also affected the horizontal deflection. I tried to fix this by improving the decoupling of the circuit at various points, but without success. ‘You get what you pay for’ as the cliché has it, but in this case you do get quite a lot of hardware for your money, and its minor deficiencies can be tolerated. The DSO138 is not just available as a ready-made module: if you fancy doing some soldering you can also get it slightly more cheaply in kit form, with the SMD components already fitted.