Reading period from the oscilloscope: A square wave occupies 3 major horizontal divisions per cycle on the screen, and the time scale is set to 50 ms per division (time/cm = 50 ms). What is the waveform period?

Difficulty: Easy

Correct Answer: 150 ms

Explanation:


Introduction:
Oscilloscope time measurements rely on counting divisions across the graticule and multiplying by the timebase setting (time per division). This provides a direct way to determine the period T of a repetitive waveform and, by inversion, its frequency f = 1/T.


Given Data / Assumptions:

  • Horizontal span per division: 50 ms/div (time/cm = 50 ms).
  • Waveform spans 3 major divisions per cycle.
  • Signal is stable and one complete cycle is clearly visible.


Concept / Approach:

The period equals the number of divisions per cycle multiplied by the time per division. Once T is known, frequency can be derived if needed. This is the most fundamental oscilloscope time measurement technique.


Step-by-Step Solution:

Identify divisions per cycle: N = 3 divisions.Time per division: Td = 50 ms/div.Compute period: T = N * Td = 3 * 50 ms = 150 ms.Optional frequency: f = 1 / T ≈ 6.67 Hz.


Verification / Alternative check:

Measuring two or more cycles and dividing by the number of cycles gives the same period and can reduce reading error if the trace is noisy or the trigger is marginal.


Why Other Options Are Wrong:

  • 50 ms / 100 ms / 200 ms / 75 ms: Each assumes a different number of divisions or time/div; none matches 3 * 50 ms.


Common Pitfalls:

  • Mistaking minor for major divisions; always confirm the scale marks.
  • Reading from an oblique or distorted trace; adjust focus and intensity for clarity.


Final Answer:

150 ms

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