Oscilloscopes

What an oscilloscope shows

An oscilloscope plots voltage versus time or one channel against another. Important settings include vertical scale, time base, trigger level, coupling, probe attenuation and acquisition mode. A stable trigger aligns repeated waveform events; it does not fix aliasing or inadequate bandwidth. Bandwidth describes analog response, while sample rate describes time sampling. Record length, sample rate and time span are related. A high sample rate does not compensate for an unsuitable probe or analog bandwidth.

Probe and connection effects

A probe loads the circuit through resistance and capacitance. A compensated 10× passive probe commonly reduces loading relative to a 1× connection, but exact behavior depends on the probe and scope. Set the attenuation consistently. Many bench-scope probe grounds are connected to protective earth. Do not attach such a ground clip to an arbitrary live node or defeat protective earth. Use appropriately rated differential or isolated measurement equipment and the approved procedure when required.

Worked example

A periodic waveform spans four horizontal divisions at 0.5 ms/div. Period T = 4 × 0.5 = 2 ms and frequency f = 1/T = 500 Hz. It spans three vertical divisions at 2 V/div. If the displayed scale already includes the 10× probe factor, Vpp = 6 V. Do not multiply by 10 again. For a zero-offset sine wave, Vrms = Vpp/(2√2) = 2.121 V. This sine-wave relation does not hold for arbitrary waveforms.

Rise time

For a roughly single-pole or suitably Gaussian measurement response, tr ≈ 0.35/BW is a useful approximation. Measured rise time combines signal, probe and scope behavior; it cannot always be assigned wholly to the signal.

Quick check

1. What stabilizes a repeated display? Triggering.
2. Can undersampling produce a misleading apparent frequency? Yes.
3. Is probe attenuation always an extra multiplier? Only if the scope scale has not already accounted for it.