IQdemodulaation

IQ demodulation is a method used to convert a bandpass radio frequency signal into a complex baseband signal by using two orthogonal local oscillator signals. The incoming signal is mixed with an in-phase signal (cosine) and a quadrature signal (sine, 90 degrees apart). After low-pass filtering, two streams are obtained: I(t) and Q(t). Together they form the complex envelope I(t) + j Q(t), which contains the amplitude and phase information of the original RF signal.

The principle relies on quadrature mixing, where the LO provides a pair of carriers separated by 90 degrees. This separation allows the original signal’s real-valued RF content to be represented as a complex baseband signal, preserving both magnitude and phase. In many cases, the baseband representation is used directly in digital signal processing to demodulate various modulation schemes.

Applications include the demodulation of digital and analog communications. IQ demodulation is central to software-defined radios and modern receivers, enabling efficient processing of QAM, PSK, OFDM, and other complex modulations. It also supports instantaneous amplitude and phase tracking, which is useful for carrier recovery and phase-locked-loop operations.

Implementation can be analog or digital. Analog IQ demodulators use mixers, low-pass filters, and a local oscillator to produce I and Q. Digitally, the RF signal is converted to baseband by an analog-to-digital converter, and a digital down-converter performs the mixing and filtering in software or firmware, often with digital signal processing techniques to correct for impairments.

Common practical considerations include IQ imbalance, DC offsets, and phase noise, which can degrade image rejection and accuracy. Calibration and compensation methods are routinely used in SDR and communication systems to mitigate these effects.