A quadrature oscillator, used in various telecommunications circuits, produces pure sine wave signals at two outputs, one of which has a 90 degree phase shift relative to the other. Simple op-amp designs work well but have limited frequency range. Packaged quadrature oscillators offer a low-cost, compact design.
The simplest quadrature oscillator needs only two op-amps. It produces three 90 degree phase shifts, two of which you can use as the main outputs. The second output, called the cosine output, lags behind the first output by 90 degrees.
The bubba oscillator design uses 4 op-amps. While more complex than the quadrature oscillator, it has better frequency stability and lower distortion. Each op-amp stage shifts phase by 45 degrees, so by using every other output, you'll have two sine waves 90 degrees apart.
You can buy quadrature oscillators packaged as a single IC. The single-IC approach is less expensive and more reliable than an oscillator built from discrete parts.