Communication Systems Prof. Chngming Kuo

Chapter 6 Single Tone Modulation Comparisons (cont.)

Single-Tone Modulation Comparisons The forms of DSB, SSB, and conventional AM have been introduced in other modules. In this module, the modulation is restricted to a single-tone sinusoidal function and the behavior of the different methods is studied.

Single-Tone Modulation Comparisons (cont.) Both time and frequency domain forms are reviewed, and power comparisons are made. Results provide certain quantitative criteria for comparing system performance.

Power Relationships

AM Single Tone Power Comparisons

Example 1 A single tone modulated DSB signal has peak voltage of 200 V across 50 . Determine average and peak power levels:

Example 2 An AM transmitter has a carrier power of 1 kW and the antenna input resistance = 50 . Find values of antenna rms voltage and current with no modulation:

Example 2 (cont.) An AM transmitter has a carrier power of 1 kW and the antenna input resistance = 50 . Find values of antenna rms voltage and current with no modulation:

Example 3 For AM transmitter of Example 2, determine for modulation of 50% and 100%, the average power, peak envelope power, and sideband power.

Example 3: Tabulated Values

Example 4 50% modulation: For transmitter of Examples 2 and 3, find antenna rms voltage and current for 50% and 100% modulation.

Example 4 (cont.) 100% modulation:

Summary With single-tone modulation, DSB peak power is twice the average power. With single-tone modulation, SSB peak and average powers are the same. For complex waveforms, peak power may be much greater. With single-tone modulation, AM average power is 1.5 times carrier power. With single-tone modulation, AM peak power is 4 times carrier power.