To alleviate issues like spurious noise and crystal resonator harmonics, a differential digitally controlled crystal oscillator (DCXO) with sine-wave outputs may present a suitable option for cellular applications. A signal-shaping technique was recently used to produce a 26-MHz differential DCXO. This oscillator, which is the brainchild of a team of engineers from Broadcom Corp., inhabits a total silicon area of 0.15 mm2. It offers a fine-tuning range of 44 ppm while providing about 14 b of resolution and an average step size of 0.005 ppm. The DCXO frequency-tuning function is provided by two banks of identical capacitor arrays.
All of the device's signals that connect externally to input/output (I/O) pins are sine waves. Because the DCXO core generates a pair of true differential sine waves, neighboring I/O pins experience the coupled crystal-signal amplitudes with opposite polarity.
Fabricated in 65-nm CMOS, the 26-MHz DCXO provides phase noise of -149.1 dBc/Hz at 10-kHz offset. The team has measured typical frequency pulling of 0.01 ppm, which arises from turning the sine-wave buffer on and off. The DCXO dissipates 1.2 mA of current. See "A Differentially Digitally Controlled Crystal Oscillator with a 14-bit Tuning Resolution and Sine-Wave Outputs for Cellular Applications," IEEE Journal Of Solid-State Circuits, Feb. 2012.