Question

A modem constellation diagram similar to Fig. 2-17 has data points at \((0,1)\) and \((0,2)\). Does the modem use phase modulation or amplitude modulation?

Short answer

The modem uses Amplitude Modulation (AM).

Step-by-step solution

Understand the Modulation Types

Phase Modulation (PM) occurs when the phase of the carrier signal is varied in accordance with the data signal. Amplitude Modulation (AM) changes the amplitude of the carrier signal to transmit data. A constellation diagram helps visualize these changes in phase and amplitude.

Analyze Given Points

The points (0,1) and (0,2) lie on the y-axis. This indicates that the data represented by these points only varies in the 'magnitude' or 'amplitude' aspect along the vertical axis, while the x-coordinate (which represents phase changes) remains constant at 0 degrees.

Determine Modulation Type

Since the points show changes in the vertical axis only and have a constant phase shown by a constant x-component (0 degrees), this indicates that only amplitude is varying. Therefore, the modulation observed on this constellation diagram is Amplitude Modulation (AM).

Key concepts

Constellation Diagram

A constellation diagram is an essential tool in the field of telecommunications. It visually represents different states a signal can have over time.
Each point on this diagram corresponds to a specific signal state, representing changes in modulation. The horizontal axis typically indicates phase changes, while the vertical axis indicates amplitude changes.

• To interpret a constellation diagram, understand that the position of these points reveals how a signal modulates both in terms of phase and amplitude.
• When all points align vertically, it signifies a variation in amplitude without a change in phase.
• Conversely, if points spread along the horizontal axis, it indicates phase changes.

Conveying data through this visual tool allows engineers to quickly grasp how different modulation techniques adjust the wave characteristics to encode information.

Phase Modulation (PM)

Phase Modulation (PM) is one of the key techniques used in analog modulation to encode information. This method modulates the signal by altering its phase, leaving the amplitude uniform throughout.

• In PM, variations in the instantaneous phase of the carrier signal represent the changes in the message signal.
• This technique is robust in terms of resistance to signal degradation, making it useful in environments with variable noise levels.
• The constellation diagram for PM shows data points spread horizontally, indicating that phases are changing.

Phase modulation is particularly helpful in applications where maintaining a consistent signal amplitude is critical, while the ability to encode information through the phase offers added data efficiency.

Amplitude Modulation (AM)

Amplitude Modulation (AM) is a technique where the amplitude of a carrier wave is varied in proportion to the message signal, while its frequency and phase remain constant.
This method is simple to implement and has been historically significant in radio broadcasting.

• In AM, the signal's strength is directly manipulated to represent the information being sent.
• Constellation diagrams display vertical arrangements during AM, as points only vary along the amplitude axis.
• While AM is susceptible to noise, it provides a straightforward method to transmit audio frequencies.

Although technology has developed more sophisticated modulation techniques, AM still finds use in certain applications due to its simplicity and low cost. Its interaction with the amplitude aspect of signals highlights its distinctive approach to encoding data.