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Quadrature mixing (direct conversion) 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 bandwidths up to the sample rate of the DAC.
Analog quadrature mixing exposes the signal to impairments because of the complexity of the circuitry involved. These impairments can create distortion products or unwanted signals, which may degrade modulation and demodulation quality. The following sections illustrate several 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 impairments, including DC offsets within the various baseband sections and LO signal power leaking into the RF path because of insufficient 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 corrupt the gain or the phase offset. These impairments are known as gain imbalance and skew. The shaded sections in the following figure represent 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 simply, the image. Wideband applications drive the necessity to suppress this distortion to avoid unwanted signals that can corrupt transmitted or received data. Digital equalization schemes can provide a method for this suppression, which improves the performance of the system for many applications.