Sideband generation in communication systems Which stage in a standard radio transmitter–receiver chain is responsible for creating the upper and lower side frequencies around a carrier?

Introduction / Context:
Amplitude and angle modulations produce spectral components known as sidebands. Knowing which block creates sidebands is essential for understanding bandwidth, channel spacing, and spectral masks.

Given Data / Assumptions:

• Classical transmitter chain: audio source → modulator → RF power amplifier → antenna.
• Receiver chain: antenna → RF/mixer/IF → detector → audio.
• We consider conventional AM (double-sideband) or other linear modulations.

Concept / Approach:
In AM, a carrier at frequency fc is combined with a baseband signal at fm to produce components at fc + fm (upper side) and fc − fm (lower side). This spectral translation occurs in the modulator, which multiplies (or otherwise combines) the carrier and the modulating signal.

Step-by-Step Solution:
Identify the operation needed: frequency translation by multiplication of signals.Locate the stage performing multiplication: the modulator in the transmitter.Confirm other blocks: microphones only produce audio; demodulators reverse the process; oscillators generate carriers but do not create sidebands alone.

Verification / Alternative check:
Spectral plots of a modulated carrier show the sidebands appear at the modulator output; removing modulation collapses the spectrum to the unmodulated carrier.

Why Other Options Are Wrong:

• Microphone: no RF sidebands; only audio generation.
• Demodulator: removes sidebands to recover baseband.
• Local oscillator: provides a pure carrier; sidebands arise only when it is modulated or mixed with information.
• Audio power amplifier: operates at audio frequencies, not at RF.

Common Pitfalls:

• Confusing mixing (frequency conversion) in receivers with modulation in transmitters; both are nonlinear but serve different purposes.
• Assuming any nonlinear stage makes sidebands; controlled modulation creates the intended sidebands.

Final Answer:
Modulator in the radio transmitter