Oscilloscope is quite universal instrument which I would recommend to invest first. Normal bench scope does job pretty good. I am not big fan of DIY scopes that are built of microcontroller and LCD or interfaced to PC via serial interface. You will never get decent sampling and functionality with low end parts. But in other hand, building such scope can be fun and be a good choice for student project. prem_ranjan shares his Arduino based scope project where hi outputs waveform to MATLAB plots.
The investment in to this project is really minimal. O course you could capture signal directly to Arduino analog pin, but op-amp based signal conditioning could make life easier. In the end here are few features of this scope:
- no additional hardware is needed for basic functions;
- max plotted frequency 7kHz;
- up to four channels max 7/4 kHz each;
- 8-bit resolution;
- variable trigger on channel 0;
- sampling memory is only limited by computer.