From Integrated Circuit Design for High-Speed Frequency Synthesis
8.9 Other Techniques to Generate Quadrature Signals
There are other ways to generate quadrature signals. One common technique is for an oscillator to be followed by a polyphase filter [8]. More than one filtering stage can be used if one desires more phase accuracy and operation over a wider bandwidth. Polyphase filters consume some additional power due to the buffers required. Except for the buffers, they are passive RC circuits, and, as a result, there is typically a 3-dB loss of signal amplitude per filtering stage. Another way to generate quadrature signals is to design an oscillator to run at twice the desired frequency, then to use a divider to generate the quadrature outputs at the desired frequency. The disadvantage is the additional complexity and power dissipation inherent in oscillators operating at twice the frequency. Another technique to generate quadrature signals, which will be discussed in Section 8.12, is the use of differential ring oscillators with two or four stages.
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