FE Electrical & Computer Practice Question Solution included after investigation
CASE E-037 · FE

The Four-Channel FDM Link That Needed 95 kHz

Four 20 kHz channels looked like an 80 kHz job—until the guard bands were counted.

▶ INTERACTIVE CASE FILE • 5–10 MIN • CLICK TO INVESTIGATE

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

Case E-037 engineering failure visual
HOW THIS WORKS

Think like
a real engineer.

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 frequency-division multiplexing plan combines four channels, each occupying 20 kHz, with a required 5 kHz guard band between adjacent channels. The planner reserves 80 kHz by summing only the channel bandwidths, but the fourth channel cannot fit without violating spacing. Calculate the required spectrum and identify the correct number of guard bands.

Communications Electrical & Computer Design Verification Root Cause Analysis Safe Correction
E-037
INVESTIGATION WORKSPACE WORK THROUGH THE CASE — YOUR CHOICES MATTER.
🔒
DEBRIEF (LOCKED)

Complete the investigation
to unlock the full debrief.

Find the root cause, confirm the fix, and see how this connects to the exam.

🔒Root causeComplete the case
to reveal
🔒Correct fixComplete the case
to reveal
🔒Exam takeawayComplete the case
to reveal
CASE CLOSED · ENGINEERING VERDICT

What actually failed — and what should change.

What failed

An 80 kHz allocation was too small for the four-channel plan.

What was assumed

Guard bands were treated as if they consumed no spectrum.

Corrected model

Total span = NB + (N-1)G when guard bands occur only between adjacent channels.

QUICK ANSWER

For four 20 kHz FDM channels with 5 kHz guard bands only between adjacent channels, there are three guard bands. Total spectrum is 4×20 + 3×5 = 95 kHz.

ENGINEERING TAKEAWAY

For N equal FDM channels with guard band G only between adjacent channels, total occupied span is N B + (N-1) G.

CASE QUESTIONS

Questions this case should settle.

How many internal guard bands are there between four FDM channels?

Three, assuming guard bands are required only between adjacent channels.

What total span is needed for four 20 kHz channels with 5 kHz guard bands?

95 kHz.

NEXT FE PRACTICE QUESTION

The PCM Link That Needed 64 kbps, Not 32

Communications · 5–10 min

Continue Practice →
WHAT YOU’LL LEARN

Real failures. Lasting skills.

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
Do I need prior knowledge to use these cases?

Basic subject familiarity helps, but every case is designed to teach through the investigation itself.

How long does a case take?

Most cases are designed for a focused 5–10 minute investigation.

Are the cases aligned with engineering exams?

Each case is mapped to a verified exam, subject, topic, and misconception before publication.

What happens after I complete a case?

The sealed debrief unlocks with the root cause, corrected reasoning, fix, and takeaway.