Sample rate determines the highest frequency component your waveform can accurately reproduce (you need at least 2–5x the highest signal frequency as sample rate). Memory depth determines how long a waveform can be before it repeats. For high-frequency radar or communications waveforms, models like the BNC 686 (20 GS/s, 14-bit) provide high fidelity. For lower frequency biomedical or motor control waveforms, the 645 (50 MHz) or 670C (180 MHz) are more appropriate and cost-effective.