What is described by the constellation diagram of a QAM or QPSK signal?

Deep Dive: E8C13

The correct answer is B: The possible phase and amplitude states for each symbol. A constellation diagram of a QAM or QPSK signal describes the possible phase and amplitude states for each symbol. The constellation diagram is a graphical representation showing all possible symbol states. In a constellation diagram, each point represents one possible symbol state. The position of each point shows its phase (angle from horizontal) and amplitude (distance from center). For QPSK, there are 4 points (4 possible symbols, 2 bits each). For 16-QAM, there are 16 points (16 possible symbols, 4 bits each). The constellation diagram helps visualize the modulation scheme and is used for signal analysis, troubleshooting, and understanding signal quality. It shows how the signal states are arranged in the I-Q (in-phase/quadrature) plane.

Why Other Answers Are Wrong

Option A: Incorrect. The constellation diagram doesn't show how many carriers are present. QAM/QPSK use a single carrier with I and Q components, not multiple carriers. Option C: Incorrect. The constellation diagram doesn't show frequency response. It shows phase and amplitude states in the I-Q plane, not frequency characteristics. Option D: Incorrect. The constellation diagram doesn't show error correction bits. It shows the symbol states themselves, not the error correction encoding.

Exam Tip

Constellation diagram = Phase and amplitude states. Remember: A constellation diagram shows the possible phase and amplitude states for each symbol in QAM or QPSK signals. Each point represents one symbol state.

Memory Aid

**C**onstellation **D**iagram = **P**hase and **A**mplitude **S**tates (think 'CD = PAS')

Real-World Example

You're analyzing a 16-QAM signal. The constellation diagram shows 16 points arranged in a grid pattern. Each point represents one of the 16 possible symbols (4 bits each). The position of each point shows its phase (angle) and amplitude (distance from center). This diagram helps you visualize the signal states and diagnose problems like phase errors or amplitude distortion.

Source & Coverage

Question Pool: 2024-2028 Question Pool

Subelement: E8C

Reference: FCC Part 97.3

Key Concepts

Constellation diagram QAM QPSK Phase and amplitude states

Official Pool Source

Question text and answer key are synchronized with the current NCVEC Extra Class pool and mapped to the E8C topic.