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Two most common methods is to have an LO source of four times or twice the primary mixing frequency.
The four times signal goes through two divide by two flip flops to generate quadrature. The disadvantage is it requires 4x LO. The advantage is the 4x signal does not have to have 50% duty cycle. Achieving a strict 50% duty cycle from an oscillator is not easy.
The scheme with twice the freq requires the 2x signal to have 50% duty cycle quality. It uses two parallel divide by 2 flip flops, one driven on positive edge, one driven on negative edge of the 2x input. In this approach the VCO typically has extra circuitry to control its duty cycle to 50%. It is similar to an AGC circuit.
If all you need is a relatively narrow frequency range you can accomplish quadrature with passive L-C filters. It usually takes more IC silicon area then the flip-flop methods and typically has more sideband noise degradation.
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