AM DSB-SC

Standard AM wastes huge amounts of continuous power, so engineers completely suppress the carrier wave. This Double Sideband Suppressed Carrier technique saves up to 66 percent of transmission power.

Core Concept

A balanced modulator circuit cleanly multiplies the radio carrier and baseband message signals together. This pure mathematical multiplication directly generates the sum and difference sideband frequencies.

The original central carrier frequency completely cancels out at the final output stage. Power efficiency immediately hits 100 percent because all transmitted power carries active audio information.

A typical 2N3904 transistor balanced modulator needs exact physical tuning. The final receiver must absolutely use sophisticated synchronous detection to precisely rebuild the missing carrier wave.

Key Equations

The time domain equation for a DSB-SC signal represents pure mathematical multiplication. The formula is strictly s(t) = Ac * Am * cos(2*pi*fc*t) * cos(2*pi*fm*t). The total transmitted power is simply the sideband power Pt = Pc * (m^2 / 2).

Given:
Carrier power Pc = 100 W
Modulation index m = 0.5

Why this formula:
DSB-SC transmits only the distinct sidebands, so total power strictly equals the sideband power.

Formula:
Pt = Pc * (m^2 / 2)

Substitution:
Pt = 100 * (0.5^2 / 2)

Calculation:
Pt = 100 * (0.25 / 2)
Pt = 100 * 0.125

Final Answer:
Total DSB-SC Power Pt = 12.5 W

Exam Tip: DSB-SC absolutely requires synchronous detection at the receiver hardware. The local oscillator must exactly match both the transmitter carrier frequency and its precise phase.

Key Properties

• Suppressing the massive carrier wave saves up to 66.6 percent of total transmission power.
• The physical frequency spectrum contains only the active upper and lower sideband impulses.
• A balanced modulator circuit mathematically multiplies the carrier and baseband audio signals.
• The time domain waveform clearly shows phase reversals precisely at the zero crossings.
• Required transmission bandwidth remains exactly 2fm, perfectly identical to standard AM.

Quick Revision

• DSB-SC finally achieves perfect 100 percent power efficiency for raw information transfer.
• Analog ring modulators using matched 1N4148 diodes commonly generate stable DSB-SC signals.
• The physical radio envelope no longer directly resembles the original baseband message signal.
• Synchronous demodulation strictly requires a perfectly phase-locked local oscillator circuit.
• A phase error of exactly 90 degrees causes the demodulated output to drop to zero.
• Exam trap: confusing DSB-SC bandwidth with SSB bandwidth, since DSB-SC still strictly requires the full 2fm spectrum.