How it works
The CRT-based CRO uses an electron gun to produce a focused beam; the X-plates get a sawtooth time-base signal (generated internally) while the Y-plates carry the input signal after passing through a calibrated attenuator and wideband amplifier (bandwidth typically 20 MHz to 100 MHz). Triggering synchronises the sweep with the input signal — edge triggering fires when the signal crosses a set level with a defined slope. The Digital Storage Oscilloscope (DSO) samples at ≥2× the highest frequency (Nyquist), stores samples in memory (512 to 10k points), and can capture single-shot events that a CRO would miss.
Key points to remember
Lissajous figures on a CRO: apply signal A to X and signal B to Y — if frequency ratio is 1:1 and phase difference is 90°, you get a circle. Phase difference φ = sin⁻¹(Y0/Ymax) where Y0 is Y-intercept and Ymax is maximum Y deflection. Bandwidth of the Y-amplifier determines the highest frequency measurable; a 10:1 probe increases input impedance from 1 MΩ to 10 MΩ and reduces loading on high-frequency circuits but attenuates the signal by 10×. Rise time of a CRO is related to bandwidth: tr = 0.35/BW; for a 100 MHz scope, tr = 3.5 ns.
Exam tip
The examiner always asks you to determine frequency ratio and phase difference from a Lissajous figure — count the tangent points on the horizontal and vertical envelopes and remember fy/fx = nx/ny.