Why does my oscilloscope show a noisy signal even when the probe is shorted to ground?

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Short Answer: You're using the long ground lead (alligator clip) — it acts as an antenna picking up ambient EMI.

Detailed: The standard ground lead is 10–15 cm long. At that length, it forms a loop that picks up 50/60 Hz hum from mains wiring, nearby switching power supplies, and radio frequencies.

The fix: Use the short ground spring that came with your probe (or make one from a paperclip).

text
    Probe tip ──────┐
                    │
               ═════╪═════  Short ground spring (1 cm)
                    │
    Ground ─────────┘

Result: Noise drops from 20–50 mV to <2 mV.

Pro tip: In a noisy environment, also enable Bandwidth Limit (20 MHz or 100 MHz) on your scope to filter out high-frequency noise.

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