How do I/Q modulators and demodulators work?
An I/Q modulator multiplies baseband in-phase I(t) and quadrature Q(t) signals by cos ωct and −sin ωct and sums them, producing s(t) = I(t)cos ωct − Q(t)sin ωct, which can represent any modulation. An I/Q demodulator reverses the process with quadrature LOs. Quadrature LOs come from polyphase filters, divide-by-two circuits or quadrature hybrids.[1]
What impairments affect I/Q circuits?
- Gain and phase imbalance: creates an unwanted sideband (image) and limits EVM; see image rejection formula.
- LO leakage / carrier feedthrough: from DC offsets; appears as a tone at the carrier.
- Quadrature skew from LO generation.
- Nonlinearity and baseband filter mismatch across frequency.
How are impairments calibrated?
Injecting DC offsets cancels LO leakage; digital gain and phase correction (a 2×2 matrix on I and Q) cancels sideband. Frequency-dependent mismatch requires equalizing filters. Many transceivers calibrate at startup with a loopback path.[1]
Where are I/Q circuits used?
Every modern digital radio: zero-IF and low-IF transceivers, vector signal generators and analyzers, and SDRs. See direct conversion and SDR.
Frequently asked questions
Why 90 degrees?
Sine and cosine are orthogonal, so I and Q carry independent information on the same carrier.
How is a 90° LO made?
Commonly by dividing a 2× LO by two with flip-flops, or with RC polyphase networks.
What sideband suppression is typical uncalibrated?
Often 30–40 dBc.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “RF Microelectronics, 2nd ed.”. B. Razavi, Prentice Hall. 2012.