A pair of critically coupled tuned circuits, if you current drive one of them you get a voltage on the other which is 90deg phase shifted at centre frequency. The phase slope (group delay) is twice what you'd get with a single circuit. Two pairs, and two differential amplifiers, make an oscillator which gives out four phases. Because there are twice as many tuned circuits as amplifiers, there should be an improvement in phase noise. Four inductors on an i.c. does take up room, but no phase shifting is necessary and it drives quadrature mixers directly. Has anyone done this before? Try the LTSpice circuit.