The inverted-mesa crystal paves a route to high oscillator frequency, seemingly in defiance of a crystal's "thinness" limitation. Inverted-mesa oscillators can resonate a region of quartz only 0.00024-in. thick for 250-MHz fundamental operation. This value represents performance anticipated in the near future.
The crystal is created by etching a "well" in a quartz crystal blank (Fig. 1, again). The bottom of the well is the very thin area of quartz needed for high-frequency resonance. The thicker region surrounding it provides mechanical support and electrical connections. Both sides of the crystal are gold-plated to deliver excitation energy to the resonant region. That plating is continued to the crystal's outer edges for bonding to connecting wires.
Inverted-mesa oscillators operate the crystal at its fundamental, albeit high, frequency. Today's designs exhibit jitter below 5-ps rms. Because the technology is new, future improvements are promised in both jitter and frequency. The jitter performance of the overtone and inverted-mesa oscillators definitely stands out when compared with the multiplier type (Fig. 3).
For the most demanding applications, a combination of inverted-mesa crystal and overtone operation will achieve the highest crystal-stabilized frequencies. MF Electronics' through-hole series Model M2944 oscillators apply this technique to achieve operation that reaches 410 MHz.
Looking ahead, the inverted-mesa and overtone method may raise the oscillator's output beyond 1 GHz. Running a 250-MHz inverted-mesa crystal at its fifth overtone would produce a crystal-controlled reference of 1.25 GHz.
Use of through-hole packaging for the M2944 reflects the use of discrete components in the oscillator's drive circuitry. Such an approach is made necessary because existing oscillator ASICs were designed for lower frequencies. They lack the necessary gain and bandwidth for overtone oscillation at 410 MHz. In time, though, newer ASICs will allow oscillators like the M2944 to migrate to surface-mount packages. This pattern will likely be repeated in the future designs of higher-frequency inverted-mesa oscillators.
Global manufacturers must assume their hardware may be used anywhere, whether it's in the Arctic or the Sahara, or any where in between. Instead of building for specific regions, datacom system suppliers are adopting standards that cover the entire range of worldwide environments. That way, hardware built for one environmental extreme won't bring costly field calls when installed in the other.
Component makers are therefore required to meet the −40° to +85°C temperature range specification. For their part, oscillator manufacturers are investing in equipment to cut the time and cost of testing. This will speed the widespread adoption of this rigorous standard.
A reputation for equipment failure can spell doom for a manufacturer's future. The military's historical obsession with reliability is becoming an everyday matter for all manufacturers, but especially for firms shipping across the globe.
A graph of oscillator failure rate over the course of the product life cycle takes the shape of the well-known bathtub curve, reflecting the high incidence of failures at the beginning and end of the product life cycle (Fig 4). Luckily, the manufacturing process can really minimize the impact of early life-cycle failures on customers.
Consider the industry-standard, glass-sealed, 5- by 7-mm surface-mount package. It can accommodate overtone and inverted-mesa oscillator designs, as well as multiplier-based oscillators. During the production phase, oscillators built into it undergo rigorous thermal processing. This confers special thermal-hardening benefits that are otherwise unavailable with different packages.
The basic surface-mount package is formed by bonding multiple circuit traces, pads, and interconnecting vias into a robust one-piece component carrier. Oscillator components are bonded to the carrier's pads in successive production steps. After trimming and testing, the cover is glass-sealed to complete the part. Each bonding and sealing process requires the carrier and its components to pass slowly through the furnace at +420°C.
The surface-mount oscillator's prolonged exposure to a temperature of +420°C during production amounts to a "free" burn-in (Fig. 4, again). This burn-in eliminates infant mortalities, weeding out weak parts.
It carries a price advantage as well. Component manufacturers ordinarily charge a premium for subjecting products to special burn-in treatments. Another benefit gained through high-temperature processing is the stress relief of the crystal, along with its supports.
While undergoing initial manufacturing processes, the crystal and its supports accumulate mechanical stresses. Those stresses ordinarily relax themselves during the first few months of a newly built, green oscillator's operation. Stress relief translates into frequency changes of ±5 ppm/year during the oscillator's first year of use. Due to the multiple processing steps at +420°C, they can provide an annealing effect that eliminates the normal year of stress relaxation. The upshot is removal of the first year's stress relief, as well as its accompanying exaggerated frequency error (Fig. 5).
High temperatures also accelerate the release of vapors and gases entrapped in oscillator components and materials. Ordinarily, outgassing occurs during oscillator lifetime, creating frequency errors as particles land on the crystal's surface. Removing particles before the oscillator's cover is sealed minimizes long-term frequency error. The typical long-term drift specification of ±1 ppm owes a lot to this early temperature-induced outgassing.
From Papers To Production
Advances in packaging and manufacturing processes, which are so critical to the development of higher-performance oscillators, are making it possible to reap the benefits of inverted-mesa crystals. These components, which for decades were merely theoretical abstractions in technical papers, are now beginning to see commercial production.
Moreover, the Internet-driven need for oscillator advances is forcing the pace of inverted-mesa oscillator design. Consequently, equipment builders seeking the very latest parts should not merely consult manufacturers' catalogs. A call to oscillator manufacturers will keep them abreast of the latest developments.