Demodulation
Understanding Demodulation
Demodulation is the final step in RF signal reception: recovering the original information (voice, data, video) that was encoded onto the carrier at the transmitter. The choice of demodulator is determined by the modulation scheme and directly impacts receiver sensitivity, data rate, and error performance.
Demodulation Techniques
- Envelope detection (AM): A diode rectifies the modulated signal, capturing the amplitude variations. Simplest demodulator.
- Frequency discrimination (FM): Converts frequency variations to amplitude variations using a slope detector or PLL.
- I/Q demodulation (digital): Multiplies the signal by a cosine (I) and sine (Q) reference to extract both amplitude and phase. Required for QAM, OFDM, and all modern digital modulation.
- Coherent detection: Requires a reference carrier synchronized to the transmitted carrier. Provides best sensitivity (3 dB better than non-coherent for BPSK).
I/Q Demodulation
Modern digital demodulation uses quadrature (I/Q) mixing to extract the in-phase and quadrature signal components. These are digitized and processed by DSP to recover the transmitted symbols.
Frequently Asked Questions
What is demodulation?
Demodulation extracts the original information from a modulated carrier. It is the reverse of modulation and the final processing step in a receiver. The demodulator type must match the modulation scheme: envelope detection for AM, frequency discrimination for FM, I/Q detection for digital modulation.
What is I/Q demodulation?
I/Q demodulation splits the received signal into in-phase (I) and quadrature (Q) components by mixing with cosine and sine versions of the carrier. This recovers both amplitude and phase information, which is essential for all modern digital modulation formats.
What is coherent vs non-coherent demodulation?
Coherent demodulation uses a reference carrier perfectly synchronized to the transmitter. It provides 3 dB better sensitivity than non-coherent (which requires no carrier reference) but is more complex. Most modern digital systems use coherent demodulation.