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Timing requirements for modern commercial electronics are demanding more for the same cost in a smaller footprint. Conversely, military and aerospace applications are looking for timing products that have advanced in thermal, vibration, and aging stability. To meet these demands while satisfying the increasing need for adaptable microwave/RF solutions, frequency-control companies are producing more advanced voltage-controlled oscillators (VCOs) from a myriad of technologies.

The leading technologies for small-footprint VCO technology include traditional quartz-crystal, surface-acoustic-wave (SAW), and microelectromechanical-systems (MEMS) oscillators. These technologies operate using different material properties of disparate resonator devices. As a result, their performance and performance-enhancing technologies vary.

Voltage-controlled crystal oscillators (VCXOs) use a precisely cut quartz-crystal element that resonates at a set frequency--based on the crystal’s geometry--when exposed to an electric field. Crystal oscillators have been manufactured for over 80 years. They are the most common oscillator technology for clock generation in the few hundred megahertz. VCXOs, which offer jitter performance around 0.5 ps-RMS, are used with discrete phase-locked-loop (PLL) circuitry. The crystal resonator within a VCXO is highly sensitive to the noise from the electric field that induces vibration. These oscillators often require filtering and regulation for their power supplies.

Additionally, VCXOs are subject to vibration and temperature sensitivities. Because crystal oscillators have long been the dominant technology in this space, there are many techniques used to compensate for these drawbacks and sensitivities (Fig. 1). Companies like Rakon, Synergy, Kyocera, and Pletronics manufacture VCXOs with optional features that increase the stability of the VCXO in the face of environmental stresses. Yet such stability comes at the expense of cost, footprint, and lead times. These techniques include temperature control, oven packages, low-pass filtering for vibration, and amplification.

Voltage-controlled SAW oscillators (VCSOs) are generally used for applications in the high-frequency domain. They target clock generation or applications requiring low phase noise and jitter performance. Typically, VCSOs operate from a few hundred megahertz to a few gigahertz, depending upon the frequency multiplication used. VCSOs often have phase-noise figures below -90 dBc/Hz at 1 kHz and jitter below 0.3 ps-RMS. Because VCSOs only resonate at a single frequency of operation, some applications may require several resonators packaged together to service multiple frequencies (Fig. 2). Often, VCSOs are used in discrete PLL applications as local oscillators requiring low jitter, low-noise clocks, and clock recovery systems.

Because the resonator material behavior changes with temperature, the resonant behavior of VCSOs is generally susceptible to temperature drift and variations. Additionally, the voltage produced from a SAW resonator is comparably weak and requires high-gain amplification. According to James Wilson, marketing director of timing products for Silicon Labs,  “VCSOs are forced to be designed with high-gain circuitry to provide sufficient or absolute pull range. Also, high control-voltage gain linearity increases variations in PLL bandwidth, which can impact system-level performance including the system bit-error rate.” As a function of the complex manufacturing process used to produce SAW resonators, long and potentially variable lead times beyond 10 weeks are possible. Companies like Api Technologies, Vectron, Silicon Labs, Epson, and Crystek manufacture a variety of VCSOs.

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