Clock Oscillator
Advances in Silicon Technology Enables Replacement of Quartz-Based Oscillators
Introduction
With a market size estimated at more than $650M and more than 1.4B crystal oscillators supplied annually, quartz crystal oscillators have long been the preferred choice for clock generation in consumer, computing, and communication applications. Quartz oscillators are available in a wide range of frequencies, package sizes, and stabilities. In addition to providing excellent jitter performance, quartz oscillators are available from a broad range of suppliers.
Quartz Resonator Based Oscillators
Crystal oscillators require a unique quartz resonator for each frequency. The crystal oscillator assembly process requires the quartz to be cut, x-rayed, lapped, mounted, and sealed into the final package . Fabrication of these resonators becomes increasingly difficult at frequencies over 100 MHz because the resonator must be manufactured to very tight tolerances.
The complexity of the manufacturing process is subject to poor yield at multiple steps within the process forcing material restarts and overall production delays.In addition to lead times, reliability is a chief concern with quartz oscillators. Quartz oscillators are susceptible to contamination which can affect both the center frequency and the ability of the XO to start up reliably. If an oscillator fails in the end application, often the entire system fails because the oscillator provides critical timing for the electronics.
MEMS Resonator Based Oscillators
The industry has long been searching for a technology that enables the replacement of quartz oscillators with a solution that addresses lead time and reliability concerns while providing performance on par with quartz oscillators. Over the last several years, micro-electromechanical system (MEMS)-based oscillators have emerged as a possible replacement technology for quartz oscillators. MEMS-based oscillators provide an alternative solution to quartz by replacing the quartz oscillator with a CMOS-based mechanical resonator.
Si500 Silicon Oscillator
Silicon Lab’s Si500 silicon oscillator leverages a standard IC manufacturing flow (Figure 3). The silicon oscillator is fabricated using standard submicron CMOS technology and standard low-cost plastic packaging that does not require a hermetic seal. The silicon oscillator is factoryprogrammed at test to a specific frequency, signal format, and supply voltage.
Si500 Technology Overview
The heart of the architecture is a low phase noise, frequency flexible LC oscillator. Using innovative mixed-signal analog circuitry, the oscillator is compensated for frequency variation due to operating temperature range, aging, initial frequency accuracy, supply voltage change, and output load change. The silicon oscillator supports a wide frequency range, generating any output clock frequency from 0.9 to 200 MHz. Selection of the frequency, output type, supply voltage, and output enable (OE) is stored in non-volatile memory (NVM). At power-on, the Si500 performs a self-calibration using these stored parameters and configures itself for operation.
Temperature Stability
Temperature stability refers to how much the oscillator frequency varies over the operating temperature range of the device. For the Si500 silicon oscillator, tight temperature stability is achieved through dynamic temperature compensation. The device has an on-board temperature sensor that, upon detection of a temperature change, dynamically adjusts the frequency of oscillation of the LC oscillator to maintain a stable output frequency.
Aging
All oscillators experience drift in the frequency over long periods of time. The effect is called “aging” and is an important specification in the overall frequency stability budget. Aging behavior is dependent on several aging mechanisms including the design of the resonator, the assembly of the oscillator, the contamination level surrounding the resonator, the design of the electronics, and the operating temperature. To determine an upper bound on aging performance, it is necessary to control as many of the mechanisms as possible and to verify conformance through extensive aging studies.
Reliability
Quartz oscillators require hermetic packaging for the crystal. Package leaks or internal contamination can lead to long term frequency aging, or if severe enough, can even prevent oscillation. Consequently, oscillator manufacturers minimize contamination by using costly, hermetically sealed ceramic or metal packaging and special processing. Done properly, reliable operation can be achieved, but package and assembly costs will be considerably higher than with non-quartz CMOS only devices. Being a mechanical device, MEMS resonators are susceptible to the same contamination issues and also require hermeticity.
Shock and Vibration
Shock and vibration can also limit the reliability of quartz-based oscillators. Quartz crystals are mounted above the oscillation electronics using epoxy or metal clips supported on one side only. Attaching the crystal on one side places the crystal’s center of gravity far away from the support point allowing the crystal to swing like a diving board when exposed to vibration.
Jitter Performance
Period jitter is a key specification for oscillators since it impacts the setup/hold time, noise margin, or bit-error rate of systems that require alignment between clock and data. Period jitter describes how much any period may deviate from the ideal clock period and is used to determine the setup/hold time margin within a digital system. The amount of margin required depends greatly on the how many timing violations (i.e., bit-errors) a system can handle. In most designs, no timing violations are allowed over the lifetime of the product, so the amount of margin is quite large. Period jitter is related to phase noise[2] and is often dominated by phase noise at far offset frequencies up to half of the clock frequency.
Programmable Output Buffer
Because the Si500 can generate frequencies across a large range (0.9 to 200 MHz), the output buffer design must provide easy connectivity for the many common receiver formats and voltages used in this range. The Si500 employs a programmable output buffer with support for both differential and single-ended formats while easing the design effort by incorporating common external components.
Conclusion
With advances in mixed-signal CMOS technologies, silicon oscillators like the Si500 are now competitive with oscillators that use traditional quartz or MEMS resonators. By eliminating the need for these mechanical resonators, the Si500 offers significantly improved reliability when shock, vibration, and oscillator start up are considered. In addition, the Si500′s simplified manufacturing flow reduces cost and enables short predictable lead times when compared to traditional quartz based oscillators.
About the Author
Silicon Labs – SLIC and Automotive Microcontroller
Kyocera Clock Oscillators
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