Quadrature mixing allows for the complex modulation or demodulation of signals as represented by two orthogonal signals I and Q. Quadrature mixing has the added benefit of an increased complex bandwidth that allows for net bandwidths up to the sample rate of the DACs.

Analog quadrature mixing exposes the signal to impairments because of the imperfections of the circuitry involved. These imperfections can create distortion products or unwanted signals, which may degrade modulation and demodulation quality. The following section illustrates key impairments involved in quadrature mixing and how they affect the respective signal.

The following figure shows a typical quadrature transmission device. The shaded sections represent the presence of several impairments, including DC offsets within the various baseband sections and LO signal power leaking into the RF path because of imperfect isolation.



Offsets and the lack of LO isolation can cause unwanted distortion, defined as residual LO power, which is always centered on the tuned LO frequency. Digital equalization schemes can help suppress residual LO power. This suppression improves the performance of the system for many modulation schemes.

For an I/Q waveform to be represented perfectly, I and Q should have a gain ratio of 1 and be 90 degrees out of phase (orthogonal). Several components in an analog quadrature mixing system can degrade the gain ratio or the phase offset from the ideal values. These impairments are known as gain imbalance and skew.

The shaded sections in the following figure represent several components that affect the phase offset and create skew.



The presence of gain imbalance or skew-related impairments creates a distortion at the negative complex frequency of the tone of interest. This distortion is referred to as the residual sideband image or the image. Gain imbalances also exist in the I and Q paths (not shown), which also contributes to residual sideband image.

Wideband applications drive the necessity to suppress this distortion to avoid unwanted signals that can corrupt transmitted or received data. On the PXIe-5842, calibrated digital equalization suppresses most of the image to improve the performance of the system.