An oscilloscope visualizes electrical signals over time — it's the single most important tool for electronics debugging and design. The used market is rich with ex-corporate and university equipment. A 10-year-old Tektronix or Keysight scope still outperforms most new budget scopes. Know your bandwidth and sample rate needs before buying.
Bandwidth is the frequency at which a sine wave appears 3dB quieter — at this point your measurements are already -30% off. Rule of thumb: get 5x the highest frequency you need to measure. For digital signals, it's the rise time that matters, not the clock frequency. 50-100 MHz: Audio, Arduino, basic digital. Hobbyist sweet spot. 200-500 MHz: Medium-speed digital, switch-mode power supplies, I2C/SPI/UART debugging. 1 GHz+: High-speed digital, DDR memory, RF. Professional.
Sample rate determines how many data points the scope captures per second. Need at least 2.5x bandwidth (Nyquist + margin). A 100 MHz scope should have 500 MSa/s minimum. With multiple channels active, sample rate divides — 4 channels at 1 GSa/s total = 250 MSa/s per channel. Check the spec sheet for per-channel rates, not just the headline number.
2 channels: Most work. Serial decode (clock+data), differential measurements, input vs output. 4 channels: SPI bus (3 signals + trigger), power sequencing, multi-rail power supplies. Worth the premium for professional work. MSO (Mixed Signal): Adds 8-16 digital logic channels. Protocol analysis on parallel buses.
DSO (Digital Storage): What you want. Captures and stores waveforms. Measurements, math, serial decode. Analog: CRT-based, no storage. Beautiful waveform display, no measurement features. Nostalgia/collector item, not a daily driver. USB/PC-based: Picoscope, Analog Discovery. Small, PC-dependent. Excellent for field work, limited by USB bandwidth.
Run self-calibration (all digital scopes have this). Check all channels show a clean trace with probes connected to the compensation output. Look for dead pixels on LCD screens. Check encoders (knobs) — dirty encoders cause jumping values. Ask if probes are included (good probes cost $50-200 each). Check the fan isn't dying (loud bearing noise).
Entry ($100-400 used): Rigol DS1054Z (hackable to 100 MHz), Siglent SDS1104X-E. Amazing value, 4 channels, serial decode.
Mid-Range ($500-2000 used): Keysight DSOX, Tektronix MDO/DPO, older Agilent. Professional build, better UI, more responsive.
Premium ($2000+): Keysight InfiniiVision, Tektronix 5-series, R&S RTO. High bandwidth, deep analysis.
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