Analog RF Modulation Techniques
Modulation is the process of impressing an information-bearing signal onto a higher-frequency carrier wave so that it can be transmitted efficiently through a channel such as free space, a cable or an optical fibre. The original low-frequency signal is called the baseband or message, and the carrier is usually a sine wave. By altering one property of the carrier in step with the message, the information is shifted up to a frequency band suited to the antenna and propagation conditions in use.
In amplitude modulation (AM) the instantaneous amplitude of the carrier is varied in proportion to the message, producing a carrier plus two sidebands that mirror the baseband spectrum. Double-sideband (DSB) transmission sends both sidebands, and suppressing the carrier or one sideband gives the more bandwidth-efficient suppressed-carrier and single-sideband variants. In frequency modulation (FM) the carrier amplitude stays constant and its instantaneous frequency is shifted by the message; this trades wider bandwidth for greater immunity to amplitude noise, which is why FM is used for high-fidelity broadcasting.
Animations and applets are valuable here because modulation links a time-domain waveform with a frequency-domain spectrum. Watching the carrier envelope follow the message, or seeing sidebands appear and move as the modulation index changes, makes the connection between the two views concrete. Simulations also allow comparison between AM and FM under noise, and exploration of how the bandwidth of each scheme depends on the message and on the depth of modulation.
Frequently asked questions
- What is the difference between AM and FM?
- AM varies the carrier's amplitude with the message; FM varies its frequency while keeping amplitude constant. FM uses more bandwidth but resists amplitude noise better.
- What are sidebands?
- Sidebands are bands of frequencies above and below the carrier created by modulation. They carry the actual information; the unmodulated carrier itself conveys none.
- Why modulate a signal at all?
- Modulation shifts low-frequency information to a carrier band suited to practical antennas and propagation, and it lets many signals share a channel on different carrier frequencies.
