Amplitude Modulation - AM

Amplitude modulation (AM) is a technique used in electronic communication to transmit information via a carrier wave. The carrier wave is usually a radio frequency sinusoidal signal generated by an oscillator circuit. The amplitude or strength of this carrier wave is varied in proportion to the message signal being transmitted.

The message signal, which contains the information being sent, can be an audio signal from a microphone, a video signal from a TV camera, or a digital data signal from a computer. This modulating message signal is superimposed onto the carrier wave by an electronic circuit called a modulator.

In the modulator, the amplitude of the carrier is varied in direct proportion to the instantaneous amplitude of the modulating message signal. So if the message signal amplitude increases, the carrier amplitude also increases. When the message signal amplitude decreases, the carrier amplitude decreases accordingly. This amplitude variation encodes the message into the carrier.

The carrier wave with its amplitude modulated by the message signal is then amplified to boost its power using a radio frequency power amplifier. This amplified AM signal is then transmitted as radio waves by feeding it into an antenna.

The AM radio waves propagate through space and can be intercepted by a receiver antenna tuned to the same frequency as the transmitter. The receiver demodulates the signal to extract the original modulating message signal from the carrier wave. This recovered message can then drive a speaker to reproduce the transmitted audio, video, or data.

By varying the carrier amplitude to mirror the message waveform, the message information gets encoded onto the carrier in AM. This allows the transmission of information over the radio by modulating the properties of a sinusoidal carrier wave.

The receiver demodulates the signal by detecting the changes in amplitude and reconstructing the original modulating signal. AM is a relatively simple and inexpensive modulation technique, but it is also relatively susceptible to noise and interference.

AM Modulation Waveforms
AM Modulation Waveforms

Principles of AM

AM modulation can be described mathematically as follows:

AM(t) = Acos(2πfct) + Bcos(2πfmt)

where:

  • AM(t) is the modulated signal
  • A is the amplitude of the carrier wave
  • fc is the frequency of the carrier wave
  • B is the amplitude of the modulating signal
  • fm is the frequency of the modulating signal

The modulated signal consists of three components:

  • The carrier wave
  • The upper sideband
  • The lower sideband

The carrier wave is the original unmodulated signal. The upper and lower sidebands are created by the modulation process. The upper sideband contains frequencies that are equal to the carrier frequency plus the modulating frequency, while the lower sideband contains frequencies that are equal to the carrier frequency minus the modulating frequency.

Am Modulation

Applications of AM

AM is used in a wide variety of applications, including:

  • AM radio broadcasting - AM is used for medium and long wave broadcast radio transmissions in the 530-1710 kHz range. It allows the transmission of audio programs over long distances.
  • Two-way radio communications - Handheld walkie-talkies, aviation radios, maritime radios, and emergency service radios use AM to enable two-way voice communications.
  • Radio control systems - AM is used in radio-controlled vehicles like cars, boats, and airplanes to transmit control signals from the handset to the model.
  • Broadcasting telemetry data - AM enables wireless transmission of telemetry measurements like temperature, pressure, humidity, etc. from remote sensors.
  • Magnetic tape recording - Analog audio signals are recorded onto magnetic tapes using AM. The tape head magnetization is modulated by the audio signals.
  • Signal transmission through optical fibers - AM enables transmission of analog signals through fiber optic links using light intensity modulation.
  • Sensitive equipment testing - AM test signals at precise frequencies are used to evaluate amplifiers, receivers, and other equipment for frequency response.
  • RFID tags - Some RFID tags use AM backscatter modulation to transmit stored data to readers.
  • Electrical measurements - LCR meters use AM internally to measure inductance, capacitance, and resistance parameters.

Advantages and Disadvantages of AM

Advantages of AM

  • Simple and inexpensive - AM transmitters and receivers are relatively simple to implement and inexpensive to build. Only basic electronics are required.
  • Long-range transmission - AM radio waves can propagate over very long distances, especially in the low-frequency and medium-frequency bands. This makes AM suitable for long-range broadcasting.
  • Bandwidth efficient - An AM signal has a narrower bandwidth compared to other analog modulations like FM. This allows more AM stations to be accommodated in a given spectrum band.
  • Works in noisy conditions - AM receivers can recover signals even in noisy conditions like bad weather which disrupt amplitude. FM would get distorted and become unusable.
  • Compatible with large antennas - The large transmitting antennas required for long-range AM are simpler and cheaper to construct compared to antennas for higher frequencies.
  • Detection is simple - AM signals are easy to detect using standard diode envelope detectors. High-frequency demodulation is not required.
  • High signal-to-noise ratio - AM can achieve a very good signal-to-noise ratio with powerful transmitters and receivers. This enables quality audio entertainment.
  • Useful for radiation transmissions - Ground waves and sky waves, used in AM propagation, can provide service to rural areas without local transmitters.

So in summary, the key advantages of AM like simplicity, cost-effectiveness, compatibility with large antennas and capability for radiation transmission make it very suitable for widespread domestic broadcast services.

Disadvantages of AM

  • Susceptible to interference - AM signals are easily affected by external electromagnetic interference from natural sources like lightning or electrical equipment. This can cause distortion and noise in the received audio.
  • Low fidelity - The transmitted audio bandwidth in AM is limited to 5-10 kHz, restricting high-fidelity music reproduction. FM and digital radio offer better sound quality.
  • Nighttime interference - AM radio waves bounce back to Earth at night and get reflected. This causes interference and signal distortion at night.
  • Affected by weather - Rain, snow, clouds, and other weather disrupt AM signal propagation, causing fading and weak reception during bad weather.
  • No privacy - AM receivers do not need special decoders. Anyone can receive signals, so public content needs to be transmitted. Private communication is not possible.
  • Wasted transmitter power - The AM transmitter outputs significant unused power in the non-information-carrying carrier wave which is filtered out. This reduces efficiency.
  • Modulation instability - AM modulation becomes nonlinear and distorted when transmitter amplifiers are overdriven near saturation trying to obtain higher powers.
  • Limited capability - Unlike FM, AM cannot be used for two-way radios or transmission of stereo audio or digital data. Only one-way analog audio is feasible.

So in summary, noise interference susceptibility, low fidelity, wastage of transmitter power, and lack of advanced capabilities like digital transmission are some of the drawbacks of basic amplitude modulation.

Future of AM

Here is an overview of the future outlook for amplitude modulation (AM) technology:

- AM radio broadcasting is likely to decline but not disappear. FM provides better sound quality, while digital modes like DAB+ offer more channels. But AM retains some advantages for broader national and international coverage.

- AM will continue to be used for niche radio services like aviation, marine, emergency response, and specialized community or rural stations where frequency efficiency and long-range coverage are required.

- Usage of two-way radios like walkie-talkies may reduce as digital modes get adopted. But simplicity and low cost will ensure AM remains in some usage.

- Magnetic tape recording has declined but AM is still a viable option for specialized instrumentation recorders in areas like seismology.

- Fiber-optic communications have mostly transitioned to digital modulation. But AM retains a few advantages and may persist in some legacy links.

- AM is likely to remain popular for simplicity in applications like ham radio, model control, and hobby electronics. The tinkerer community continues to innovate AM-based projects.

- AM test signal generators will continue to find usage in RF device characterization and receiver testing applications.

- AM RFID has been largely superseded by superior techniques but may find occasional usage in niche asset-tracking applications where simplicity and cost are priorities.

- AM has declined for general broadcasting and communications but retains some utility for specialized applications benefiting from its advantages. It is expected to remain embedded in certain niche usage domains. While no longer the mainstream tech, AM modulation endures in the radio transmission ecosystem.