Electronic Design

Inverted-Mesa Crystals Carry Oscillators Into The Internet Age

Crystal technology moves from the lab to the production line, boosting frequencies while reducing jitter.

Communications applications, fueled by the Internet and e-commerce expansion, are emerging as the driving force in oscillator development. The need for ever-wider data "pipelines" brings along the demand for ever-higher reference frequencies. Unfortunately, at roughly 40 MHz, conventional crystal oscillators hit a frequency ceiling.

A crystal's frequency limitation stems from the relationship between resonant frequency and crystal thickness. The ratio is approximately T = 60/F, where T is crystal thickness in thousandths of an inch and F is fundamental frequency in megahertz. The thickness of a crystal designed for 40 MHz is therefore 60/40 = 0.0015 in. = 1.5 mil. Crystals thinner than this 1.5-mil threshold are too fragile to withstand the mechanical stresses of oscillator production.

Several available techniques outflank the 40-MHz limit. Using these methods, commercial crystal oscillators can generate outputs of up to several hundred megahertz. Yet there's still a significant gap between the crystal-stabilized reference frequencies available and the needs of today's Gigabit Ethernet, ATM, and SONET systems. In these applications, the datacom equipment builder must multiply, or upconvert, the crystal-oscillator reference frequencies to reach the gigahertz range.

Frequency multiplication tends to increase jitter as well as frequency, however. To minimize jitter at the gigahertz level, equipment builders have to start with super-low-jitter reference oscillators. But the higher the reference frequency climbs, the lower the final gigahertz signal's jitter. Naturally, crystal-oscillator manufacturers are developing generations of clock oscillators that raise frequency while simultaneously reducing jitter. Doing so requires advances in quartz-crystal technology, oscillator circuit design, packaging, and manufacturing (see "Oscillators Are More Analog Than Digital," p. 118).

Those manufacturers have long depended on two basic techniques, overtone and multiplier, to overcome an ordinary crystal's 40-MHz frequency ceiling. In the past year, a third and until now theoretical concept—inverted mesa—has proven eminently practical for achieving high oscillator frequencies (Fig. 1).

The clock oscillators rooted in frequency-multiplier techniques have long been the workhorses of modern time- and frequency-reference applications. These very economical devices use custom ASICs for crystal excitation, frequency multiplication, and output waveshaping. They're available for wide temperature ranges and achieve room-temperature stability to 10 ppm without compensation.

Multiplier-based oscillators also can operate over the −40° to +85°C extended industrial temperature range. They're available in through-hole and robust surface-mount packages. In addition, they can be provided for commercial off-the-shelf (COTS) use.

Phase-locked loops (PLLs) provide the basis for frequency multiplication in commercial oscillators. The PLL configuration is based on a simple voltage-controlled oscillator (VCO).

A 1/N digital divider is interposed between the VCO's output (FOUT) and the phase detector's input (FOUT /N). An error signal locks the VCO's output frequency to N times the reference frequency input so that FOUT = N ×FIN. Because the VCO operates at frequencies beyond crystal stabilization, it lacks crystal stability. The multiplier's VCO contributes phase noise, or jitter, to the output signal, resulting in jitter measurements of about 10- to 15-ps rms for multiplier-based oscillators. As a result, these oscillators can't satisfy the low-jitter requirements of commercial SONET, ATM, and Gigabit Ethernet transceivers.

Fortunately, a crystal has the ability to resonate at odd harmonics of its fundamental frequency. Overtone oscillators exploit this capability by using filters in the excitation circuit, eliciting resonance at odd harmonics. When properly excited at a specific overtone, crystal vibration contains negligible components at fundamental or other sub-overtone components.

Compared to multiplier types, the major benefit of overtone oscillation is its excellent jitter performance. A jitter specification below 5-ps rms can be readily achieved. Nevertheless, crystal fragility limits the maximum frequency of overtone oscillators to roughly five times the 40-MHz fundamental-mode limit, or 200 MHz. This range is sufficient for many existing applications. The combination of performance, reliability, and cost provided by overtone oscillators often makes designers choose them.

Of course, when using them, developers must be aware of some pitfalls. A crystal "prefers" to resonate at its fundamental frequency. While it can be resonated at a higher overtone, circumstances do exist that cause it to revert to the fundamental. One such instance can arise on startup, if the oscillator's dc supply voltage ramps up gradually rather than reaching the rated voltage "instantly" (Fig. 2). During such undervoltage conditions, the oscillator transistors experience reduced gain and bandwidth. Lacking bandwidth at the overtone frequency, the oscillator may excite oscillation at the fundamental frequency.

High operating temperature also may lower transistor gain and bandwidth, triggering a reversion to fundamental frequency. Quite mysteriously, though, the crystal may lack "activity" at a specific combination of rated temperature and supply voltage. It could then refuse to oscillate under those conditions.

The decrease or absence of crystal activity, along with reduced transistor bandwidth at low voltage and high temperature, all pose future challenges for oscillator designers. Innovative designs, superior transistors, and ASICs will be required for systems operating at 2.5 V.

Eventual operation at 1.8 V will present even stiffer challenges. The design of 2.5- and 1.8-V oscillators might even require that outboard components augment the capabilities of oscillator ASICs. This is especially likely for overtone oscillators built to meet the −40° to +85°C extended, industrial temperature-range specifications.

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.

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