Crystal Oscillators Keep Time In Communications Systems

Reference oscillators are the heartbeat of many electronics systems, as they are counted upon to keep precise time. It is largely due to advances in crystal oscillators that radio communications became possible and practical during World War II. Impressively, such oscillators still hail from many of the original makers in the industry, such as Bliley Electronics and MMD Components (originally Monitor). In addition, military applications remain a driving force behind many oscillator developmentsalthough they now face competition from wireless communications. Today, crystal oscillators take many forms to simultaneously bridge the military and communications markets. At the same time, they are expanding their reach into space and beyond.

From recent product releases, it is clear that WiMAX fever has begun to catch on in the oscillator world. Fox Electronics (Fort Myers, FL), for example, spawned an HCMOS, 14-pin DIP oven-controlled crystal oscillator (OCXO) for use in base stations. Dubbed the FTS310, this OCXO is well suited for the WiMAX, HIPERMAN, and WiBro markets. The OCXO vows to meet the IEEE 802.162004 specification, which dictates that the OCXO have a frequency stability better than 8 ppm for 10 years. The FTS310 flaunts a phase noise of 125 dBc/Hz at an offset of 100 Hz and 145 dBc/Hz at an offset of 10 kHz. It is available with output frequencies from 2.43 to 60.00 MHz. With less than 1.5 W of power, the OCXO promises to warm up to within 4.6 ppm in 2 minutes. The oscillator's supply voltage is 3.30 V 0.16 V. Its operating power is 1.0 W at 0C and 0.6 W at 25C. The FTS310's output load is 15 pF. It operates from 40 to +65C with an option of 40 to +85C. The OCXO measures 20.8 X 13.2 X 11.0 mm.

Bliley Technologies (Erie, PA) was largely responsible for the oscillator advancements that made WWII communications possible (see sidebar, "Tracing Crystals To WWII"). Like other oscillator makers, Bliley now gears much of its product development toward communications. Recently, the company announced a 10-MHz OCXO, dubbed the NVG47S (Fig. 1). Product specifications for this OCXO include an operating frequency of 5 to 13 MHz and typical phase noise of 128 dBc/Hz at 10-Hz offset. The oscillator boasts stabilities as tight as 5 ppb over temperature as well as long-term aging at 20 years. Thanks to its European-style packaging, the NVG47S has a maximum single dimension of 0.57 in. The OCXO supports test equipment, microwave point-to-point and point-to-multipoint communication systems, and all cellular base-station standards including GSM, CDMA, and third-generation (3G) protocols.

Wenzel Associates (Austin, TX) is a supplier of some of the industry's lowest-phase-noise sources. The company's Agate line of crystal oscillators employs a low-noise very-high-frequency (VHF) OCXO that operates from 65 to 130 MHz in fundamental-frequency mode. Through a variety of step-recovery-diode (SRD) multiplication topologies, the company offers versions of the Agate crystal oscillators with frequencies from 650 MHz to 12 GHz.

Available for commercial and military applications, the oscillator provides output levels to +13 dBm (depending upon frequency), temperature stability of 2 X 10⁷ from 0 to +50C, harmonics of 25 dBc, and spurious levels of 80 dBc. The phase-noise levels are 103 dBc/Hz offset 100 Hz from a 1-GHz carrier, 128 dBc/Hz offset 1 kHz from the same carrier, and 150 dBc/Hz offset 20 kHz from the same carrier. Even for a 12-GHz output signal, the phase noise is a low 127 dBc/Hz offset 20 kHz from the carrier.

Although all oscillators deliver claims of durability, the connectorized Red Box series of voltage-controlled oscillators (VCOs) from Crystek Corp. (Fort Myers, FL) also promises ease of integration. These VCOs come in a robust aluminum enclosure that measures 1.25 X 1.25X 0.58 in. with SMA connectors. Most of the company's VCO portfolio will be available in the Red Box series. According to the company, this new line will make the integration of a VCO into any application a simple plug-and-play option. Applications include phase-locked-loop (PLL) frequency translation, test and measurement, point-to-point radios, and multipoint radios.

To meet the myriad requirements of today's oscillators, it has not been uncommon to see different device makers join forces. For example, Frequency Electronics, Inc. (Mitchel Field, NY) acquired an approximate 20-percent interest in Elcom Technologies, Inc. (Rockleigh, NJ) earlier this year. Among other devices, Elcom designs and manufactures high-switching-speed, low-phase-noise ceramic resonant oscillators and dielectric resonant oscillators up to 40 GHz. Essentially, Frequency and Elcom entered into a mutual business and facilities support agreement. Frequency also obtained an exclusive license to use Elcom's technology in space-borne applications.

Vectron International (Hudson, NH) has been leveraging its own oscillators to make inroads into the space market. The surface-mount 5116 voltage-controlled crystal oscillator (VCXO) is now radiation tolerant to 100 krads (Si) total dose. Supposedly, this VCXO expanded the capabilities of MIL-PRF55310/27 clock oscillators into the space market for the very first time. The oscillator offers a nominal frequency range of 1 to 85 MHz. An absolute pull range of 50 ppm is available. The 5116 provides four-point crystal mount and swept quartz, which is the standard architecture for flight models. The VCXO is designed in a 9X 14-mm enclosure. The 5116 VCXO also targets military applications like unmanned aerial vehicles (UAVs).

For oscillators, the number of potential military applications sometimes seems endless. This past spring, French oscillator maker Temex unveiled an ultra-low-phase-noise oscillator that promises to satisfy new-generation radar systems for Western countries, which must be capable of detecting stealthy targets (Fig. 2). The SR W150 RF oscillator features frequency of 500 MHz, although frequencies are possible from 300 to 600 MHz. At 10 kHz from the carrier, phase noise is 165 dBc/Hz. The oscillator's phase-noise floor is 175 dBc/Hz. The SR W150 is frequency stabilized by temperature control and external control voltage. Stability is 1 ppm in the 0 to +50C (+32 to +122F) temperature range with a target of 30 to +70C (22 to +158F). Aging is 1 ppm/year. The SR W150 comes in a 51.5 X 51.5 X 20.5-mm package. Once this oscillator is adjusted in the factory, the company promises that it will never require additional calibration.

Despite all of these impressive oscillator innovations, many wonder whether quartz's limitations will soon stand in the way of future products. An increasing number of productsespecially in the consumer-electronics spacedemand a reliable, scalable, and cost-effective alternative to quartz oscillators. This past February, Vectron and Discera (San Jose, CA) began sampling microelectromechanical-systems (MEMS)-based oscillators to customers. Hailing from Vectron as the VMC2, Discera's MOS1 oscillator family generates frequencies between 1 to 125 MHz at 2-ppm frequency increments.

According to the companies, the product has been successfully tested in 25,000 centrifuge acceleration and 50G vibration operation with no measurable deviation. The oscillator comprises a silicon MEMS resonator and an application-specific integrated circuit (ASIC), which are embedded within a conventional QFN or ceramic package. At the Electronica show in November 2006, Discera demonstrated this technology as a plug-and-play direct replacement of quartz crystal in a camcorder device. Going forward, MEMS-based timing devices may increasingly be found in DVD players, gaming consoles, set-top boxes, camcorders, personal digital assistants (PDAs), and cameras. Although such oscillators are needed in the consumer space, it is hard to imagine crystal being edged out of the applications that it helped create. As with all things, however, only time will tell.