Evolving Oscillator Technology Answers Microwave Application Needs
By Roger Burns, Field Applications Engineer, Fox Electronics

With higher frequencies and extreme stability being prominently featured on the list of oscillator criteria for designers of critical microwave communications applications, having timely options that can deliver the highest desired performance with minimal need for compensating design steps is a key to cost-effective solutions. Fortunately, in the ongoing battle to push the limits of technology, component package size and component costs, oscillator manufacturers continue to close the gap between high-level performance and cost-effective purchasing.

Several recent advancements in oscillator technology offer new options for equipment designers and manufacturers, such as wireless telecom and wireless data networks, and the precision test equipment used to monitor those systems. These developments include configurable oscillator technology, which makes delivery of high-frequency oscillators more timely and affordable than ever before. Smaller packaging is also provided for some of the most stable oscillators available – oven-controlled crystal oscillators (OCXOs) – with stability ratings at a fraction of a part per million.

Configurable technology cuts conventional oscillator costs and lead times
The introduction of the latest configurable oscillator technology offers technical and business advantages in terms of performance and delivery times across a wide spectrum of oscillator uses. But it is the ability to deliver those advantages at frequencies up to 1.1 GHz, with low-jitter and phase noise characteristics comparable to conventional custom oscillators, that makes this technology particularly attractive for subscriber applications in high-frequency microwave systems such as wireless WiMAX or WiBRO data networks. It can also be used in telecom environments for central office or remote optical network switching applications, in the interface between fiber-optic and microwave systems, or in the conversion of digital signals back to voice communications.

Until the evolution of configurable oscillator technology, shortening the long lead times typically associated with custom-made conventional oscillators of any frequency usually involved compromising some aspect of performance. While it is possible to use multipliers to tune a stock fixed-frequency oscillator to a higher frequency for a specific application, that approach tends to magnify noise problems proportionally. Programmable oscillators (See Figure 1), developed in the early 1990s as a solution for quicker oscillator turnaround, also offer an alternative for shortened lead times. Unfortunately, their noise characteristics often limit them to low-volume production runs where noise is not a problem, or to prototype applications where noise problems can be rectified by ordering conventional custom oscillators for the actual production run.

New approach yields new preformance results
Instead of using a “one-solution-fits-all” approach like programmable oscillators do, the new configurable oscillator technology uses a modular building-block architecture to provide the desired frequency without compounding the noise in the performance of the finished product.

Like a programmable oscillator, a configurable oscillator also starts out with a conventional oscillator crystal blank that runs the output through a series of functions. But, instead of using a conventional integer phase-locked loop (PLL) that covers a wide frequency range, it uses one of several fractional-N PLLs chosen based on the desired frequency for the final application. (See Figure 2) This fractional-N PLL doesn’t divide the reference frequency and therefore, eliminates one of the noise problems associated with programmable oscillators.

A problem introduced by the fractional-N PLL, however, is the fact that irregular divisors create spurs as noise elements. To compensate for this, the configurable oscillator design adds a 3rd order Delta Sigma Modulator (DSM) block to lower the overall amplitude of the spurs by spreading them out to different spots over time. This creates a signal that is the “mirror image” of the noise in the oscillator, effectively canceling it out. (A familiar everyday example of that principle at work would be the performance of “noise canceling” headphones used by music aficionados).

The final step in the configurable oscillator process adds one of three types of output buffers, depending on the needs of the application. HCMOS is the most popular and is used for the majority of lower frequency applications. However, LVPECL and LVDS outputs are available to satisfy higher frequency applications.

By using common components in such a modular approach, it is possible to choose which crystal blank, fractional-N PLL, DSM and output buffer are needed to configure an affordable oscillator package that satisfies the frequency, noise requirements and output type needed by the application. The available combinations create HCMOS oscillators from 1 MHZ to 250 MHz, LVPECL oscillators from 1 MHz to 1.1 GHz, and LVDS oscillators from 1 MHz to 1.1 GHz.

In each case, the result is a functional oscillator featuring the quick delivery time of a programmable oscillator, yet operating at a custom-specified frequency with low-jitter and noise characteristics similar to those of conventional oscillators. Application-specific integrated circuits (ASICs) used in the various configurable oscillator modules provide precise performance and cost-effective production. In addition, with the small die sizes for oscillator circuitry, combined with the use of common-frequency crystal blanks, the desired results can be achieved at price points that are even lower than those of conventional oscillators.

Yet another advantage of this new technology is the fact that these good things do indeed come in small packages. High frequency configurable oscillators (up to 1.1 GHz) are now available in package sizes down to 5 mm x 3.2 mm. (See Figure 3)

Technical improvements deliver business benefits, too
In addition to its desirable performance characteristics, the new configurable oscillator technology’s manufacturing costs and lead times make it an attractive alternative for applications with volumes too low to benefit from any economy of scale in custom-manufactured conventional oscillators. This is particularly true for specialized applications where order volumes might be as low as one or two thousand units, as opposed to consumer electronics applications, where order volumes are often in the hundreds of thousands or greater.

But even more important, the efficiencies and economies of scale in using common modular components make configurable oscillators a quicker and more cost-effective solution than conventional oscillators across the board. They deliver low-jitter and phase noise characteristics comparable to those of fixed-frequency oscillators, at a lower cost. And, since a configurable oscillator can be produced to custom specifications in just a fraction of the time of a conventional oscillator, it cuts down typical delivery times from 8 or 10 weeks to less than 2 weeks.

Ultra-stable OCXO oscillators grow smaller
Another advantageous oscillator development for critical microwave applications is the availability of ultra-stable oven-controlled crystal oscillators (OCXOs) in half-size 8-pin DIP formats to meet the ever-present desire for more compact component packaging.

Because the resonance frequency of an oscillator is affected by temperature, OCXOs create a constant-temperature environment that enables the crystal to perform at a very consistent level, in spite of any temperature fluctuations in the surrounding working environment. The typical OCXO consists of a piezoelectrical crystal mounted within an insulated housing, which incorporates a resistive heating element and a temperature sensor that work together to maintain a consistent temperature within the housing – typically an elevated temperature between 70ºC and 80ºC.

The ability to transmit at high speed and high frequencies makes OCXO oscillators attractive for applications like GSM base stations for voice communications. OCXO oscillators with frequency stability of less than 0.3 ppm (vs. temperature), when compared to the more common 25 ppm stability (vs. temperature) for other types of oscillators, provide the precise timing that enables the cellular base station to synchronize with large numbers of individual cell phones. OEMs can use these oscillators to maintain high stability even when incorporating multiplier or phase-locked loop techniques to generate frequencies higher than the component’s factory rating.

While earlier OCXO designs offered 10 MHz and 14-pin DIP packaging as the de facto standard, newer models that operate at up to 80,000 MHz are now available in 8-pin DIP packaging as small as 13.2 mm squared. This small size is made possible by the use of smaller crystals that, in turn, permit the use of other smaller components within the package. (See Figure 4.)

Keeping the leading edge moving forward
Whether oscillators are used at their specific frequency, or tuned to higher frequencies through the use of multipliers or phase-locked loops, the need for accurate, affordable, low-noise, stable performance never changes. That is why staying aware of the latest options enables system designers to develop more precise and more competitive product solutions. And, for those applications that don’t demand the ultra-high stability of OCXO oscillators, the new configurable oscillators offer significant promise in terms of performance, price and product delivery to satisfy both the technical and business needs of microwave product designers and manufacturers.

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