This document describes the impact of amplitude and phase mismatch in an I/Q (in-phase/quadrature-phase) signal pair. The result of a mismatch is an unwanted spectral component at the negative signal frequency.
An ideal sinusoidal I/Q signal pair with no amplitude or phase mismatch can be represented by the following complex function:
y(t) = A exp(i(ωt + φ)) = A cos(ωt + φ) + i A sin(ωt + φ)
The real part (the first term in the second expression) of y(t) is the I signal, and the imaginary part (the second term except the "i") is the Q signal. ω, A, and φ are the angular frequency, amplitude and phase, respectively.
If an amplitude and phase mismatch is introduced, the function becomes:
y(t) = A cos(ωt + φ) + i α A sin(ωt + φ + ε)
Here, A and φ are the amplitude and phase of the I signal, α is the amplitude mismatch (the ratio of Q amplitude to I amplitude), and ε is the phase mismatch (the Q phase plus 90 degrees minus the I phase). For α=0 dB (α=1) and ε=0 degrees there is no mismatch.
By rewriting Equation 2 we can split the mismatched I/Q signal into its spectral components:
y(t) = 0.5 (1 + α exp(i ε)) A exp(i(ωt + φ)) + 0.5 (1 - α exp(-i ε)) A exp(-i(ωt + φ))
The first term is the wanted signal (including both I and Q parts) - a pure complex sinusoid at the frequency ω. The second term is an unwanted signal at the negative frequency -ω. The wanted signal is simply the ideal I/Q signal from Equation 1 multiplied by a complex constant, which modifies the amplitude and phase (here, the I and Q parts are modified equally).
From Equation 3 the ratio of the unwanted signal magnitude to the wanted signal magnitude can be found:
It has been shown that the introduction of an amplitude and/or phase mismatch between the I and Q signals causes an unwanted spectral component to appear at the mirror frequency. Also, the amplitude and phase of the wanted spectral component will change. Equation 3 quantifies these effects.