Demodulator
Understanding RF Demodulators
The demodulator performs the inverse operation of the modulator: recovering the original baseband information from the received RF signal. IQ demodulation captures both amplitude and phase information, enabling reception of any modulation format.
IQ Demodulator Architecture
- RF signal enters and is split into two paths.
- One path mixes with the LO at 0 degrees (I channel).
- Other path mixes with the LO at 90 degrees (Q channel).
- Low-pass filters remove the sum-frequency components.
- ADCs digitize the I and Q baseband signals.
- Digital signal processing recovers the data.
Demodulator Challenges
- DC offset: LO leakage through the mixer creates DC at baseband. Must be cancelled.
- IQ imbalance: Gain and phase mismatch between I and Q paths distorts the constellation.
- 1/f noise: Low-frequency noise from mixer and baseband amplifiers. Worst in zero-IF.
Frequently Asked Questions
What is a demodulator?
A demodulator recovers original information from a modulated RF signal. Modern IQ demodulators use two mixers with 90-degree LO offset to recover I and Q baseband components, enabling reception of any digital modulation format.
What is the difference between coherent and non-coherent demodulation?
Coherent demodulation uses a local carrier phase-locked to the received signal. Provides best performance but requires carrier recovery. Non-coherent does not need phase reference; simpler but 1-3 dB worse sensitivity.
What causes constellation distortion?
IQ imbalance (gain/phase mismatch), LO phase noise, DC offset (zero-IF), nonlinear distortion (compression), and multipath channel. Digital compensation (equalization, IQ calibration) corrects most impairments.