Quadrature modulators and/or quadrature demodulators are needed for a multiplicity of circuits, such as in modern communications equipment. Such communication systems generally use modulation techniques to transmit signals over a medium such as airways or communication lines. Typical applications include cordless and cellular telephones, pagers, and other radio equipment. Quadrature modulation/demodulation refers to modulation/demodulation of orthogonal carrier signals. Modulators and demodulators are built on a monolithic semiconductor chip. Quadrature demodulation provides an efficient representation of a bandpass signal and allows for the direct extraction of amplitude and phase information. A phase shift keying (PSK) demodulation used in the digital communication system is a method for shifting a phase of carrier to a predetermined value on a signal space diagram according to the information signal. Quadrature demodulators are used to separate quadrature mixed product signals typically generated during quadrature modulation, using a local oscillator. Quadrature demodulators are also used for separating signals having frequencies above or below the local oscillator reference. Quadrature-modulated signals are used for signals which belong together but are independent of each other and have to be transmitted in one transmission channel. The mixing of the signals is usually performed by a pair of modulators modulating a local oscillator signal in quadrature for producing the mixed product signals in quadrature.
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