FE Electrical & Computer Practice Question Solution included after investigation
CASE E-023 · FE

The Vibration Sensor That Invented a 300 Hz Tone

The machine says 900 Hz. The sampled spectrum insists on 300 Hz.

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Inspect, commit, prove, fix, and sign off.

Case E-023 engineering failure visual
HOW THIS WORKS

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Follow the investigation process used in the field — in five guided steps.

1

Inspect

Review the problem, diagram, and evidence.

2

Commit

Choose your hypothesis.

3

Prove

Run calculations and test your idea.

4

Fix

Select and validate a safe correction.

5

Sign Off

See the full debrief and key takeaways.

Case Brief

SAME PRINCIPLES. HIGHER STANDARDS.
PROBLEM STATEMENT

A vibration sensor carries a 900 Hz sinusoidal component into an ADC sampling at 1200 samples/s. The FFT shows a strong 300 Hz tone, and a review note labels it a real machine subharmonic. Diagnose the frequency shift, calculate the alias, and choose a sampling change that preserves the 900 Hz component without aliasing.

Signal Processing Electrical & Computer Design Verification Root Cause Analysis Safe Correction
E-023
INVESTIGATION WORKSPACE WORK THROUGH THE CASE — YOUR CHOICES MATTER.
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CASE CLOSED · ENGINEERING VERDICT

What actually failed — and what should change.

What appeared

A 300 Hz FFT peak appeared from a 900 Hz analog input.

What was missed

The 900 Hz input exceeded the 600 Hz Nyquist frequency.

Corrected model

Check fs/2 before interpreting spectra; undersampled tones fold into baseband.

QUICK ANSWER

With a 1200 samples/s rate, the Nyquist frequency is 600 Hz. A 900 Hz input exceeds Nyquist and aliases to |900 - 1200| = 300 Hz, so the sampled spectrum can show a convincing 300 Hz peak that is not a separate analog vibration.

ENGINEERING TAKEAWAY

Check the Nyquist condition before interpreting a sampled spectrum. Frequencies above fs/2 fold into baseband; increase sample rate or band-limit the input before sampling.

CASE QUESTIONS

Questions this case should settle.

What is the Nyquist frequency for 1200 samples/s?

600 Hz.

What does a 900 Hz tone alias to at 1200 samples/s?

300 Hz.

How can aliasing be prevented?

Sample fast enough for the highest signal of interest and use appropriate anti-alias filtering before the ADC.

NEXT FE PRACTICE QUESTION

The Digital Filter That Lagged a Perfect Step

Signal Processing · 5–10 min

Continue Practice →
WHAT YOU’LL LEARN

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Each case is designed to build the judgment, analysis, and confidence you need for engineering exams — and beyond.

  • Apply core engineering concepts to real-world problems
  • Practice structured troubleshooting and analysis
  • Strengthen exam-ready thinking through realistic scenarios
COMMON QUESTIONS
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