Crystal oscillators are also known as clocks, for their importance in keeping system time and frequency. A few decades ago, very high levels of stability required large assemblies with built-in ovens to maintain a narrow range of temperatures for optimum performance. But today's crystal oscillators, including temperature-compensated crystal oscillators (TCXOs) and oven-controlled crystal oscillators (OCXOs), provide improved performance compared to their larger predecessors, many in surfacemount- technology (SMT) packages that can be added to a circuit by means of pick-andplace and automated handling equipment. In terms of small SMT packaging, Ecliptek (www.ecliptek.com) offers the ES52K1 series temperature-compensated, voltage-controlled crystal oscillators (TCVCXOs) in a ceramic housing measuring just 2.5 x 3.2 mm. Available with a variety of output frequencies, including 16.368, 20, 26, and 40 MHz, the surface-mount oscillators operate on +3 VDC with 0.5 PPM maximum frequency tolerance and typical aging rate of 1 PPM/year. The tiny clocks exhibit phase noise for a carrier of 16.368 MHz of -80 dBc/Hz offset 10 Hz from the carrier, -115 dBc/Hz offset 100 Hz from the carrier, -135 dBc/Hz offset 1 kHz from the carrier, and -148 dBc/Hz offset 10 kHz from the carrier.

MMD Components (Monitor & Quartztek, www.mmdcomp.com) offers miniature VCXOs measuring just 5 x 7 x 1.5 mm and delivering outputs to 700 MHz. The low-jitter, low-voltage positive emitter coupled logic (LVPEC) VCXOs are based on inverted-mesa and AT strip crystal, such as the expanded MVIP series for application in harsh environments. Designed for operating temperatures from 55 to +125C, the sources achieve low jitter of about 0.5 ps RMS and operate on supplies of +3.0 and +3.3 VDC.

The FMVC5S series of ceramic SMT VCXOs from Frequency Management International (www.frequencymanagement.com) measure just 7 x 5 mm with output frequencies available from 1.54 to 200 MHz in TTL or HCMOS formats. With standard frequency tolerance of 50 PPM, they can be specified with frequency tolerance of 20 to 100 PPM. The typical jitter is less than 10 ps and typical phase noise is -75 dBc/Hz offset 10 Hz from the carrier, -110 dBc/Hz offset 100 Hz from the carrier, -130 dBc/Hz offset 10 kHz from the carrier, and -140 dBc/Hz offset 100 kHz from the carrier. Designed for operating temperatures from 0 to +70C, the FMVC5S VCXOs operate on a +5-VDC supply and draw maximum current of 30 mA for frequencies to 23.9 MHz, 50 mA for frequencies to 60 MHz, 75 mA for frequencies to 160 MHz, and 90 mA for frequencies to 200 MHz.

The TS5B series of TCVCXOs from Bliley (www.bliley.com) measure a mere 5 x 7 mm in a ceramic SMT package but provide 4.6 PPM frequency stability at frequencies from 5 to 26 MHz, with standard frequencies of 10.0, 12.8, and 25.0 MHz. Available with CMOS and clipped sinewave outputs, the RoHScompliant oscillators can be specified for operating temperature ranges of -20 to +70C or -40 to +85C with 2.5 PPM aging rate at 10 MHz. The phase noise at a 10-MHz carrier is -60 dBc/Hz offset 1 Hz from the carrier, -90 dBc/Hz offset 10 Hz from the carrier, -120 dBc/Hz offset 100 Hz from the carrier, -140 dBc/Hz offset 1 kHz from the carrier, and -155 dBc/Hz offset 10 kHz from the carrier.

The KT2520 series of TCXOs from Kyocera (www.kyocera.com) measure just 2.5 x 2.0 x 0.9 mm with models available from 13 to 52 MHz and standard frequencies of 13.0, 19.2, 26.0, 38.4, and 52.0 MHz. Ideal for CDMA, GPRS, and GSM cellular applications, these minute surface-mount crystal oscillators operate on supplies from +1.7 to +3.5 VDC and consume only 2 mA current. They deliver 2 x 10-6 frequency stability from -30 to +85C.

Among the smaller crystal oscillators is the DS4-XO family from MAXIM Integrated Products (www.maximic.com) with frequencies from 75 to 622.08 MHz. Leveraging integratedcircuit (IC) fabrication techniques, the miniature oscillators are supplied in a surface-mount package measuring 5.0 x 3.2 mm. The sources integrate a lownoise, high-frequency phase-lock-loop (PLL) circuit fabricated with the company's proprietary silicon-germanium (SiGe) semiconductor process with a fundamental-frequency AT-cut crystal (no overtone performance). The PLLbased design easily accommodates operating frequencies up to 622.08 MHz and higher, if desired. The end result is stable performance from -40 to +85C with just 1 ps RMS jitter. The DS4-XO series also employs advanced thermal techniques with low, 1 PPM aging rate per year and only 7 PPM aging rate over 10 years.

The TG-5005CG series of miniature TCXOs (Fig. 1) from Epson Toyocom Corp. (www.epsontoyocom.co.jp) measure just 2.5 x 2.0 mm and have frequency stability with temperature of 0.5 x 10-6. The oscillators are based on Epson Toyocom's proprietary QMEMS technology, in which oscillators are formed by a combination of quartz-crystal and microelectromechanical-systems (MEMS) technologies. The tiny TCXOs measure just 2.5 x 2.0 mm and can be specified for frequencies from 13 to 40 MHz. They exhibit 2 x 10-6 maximum frequency tolerance and 0.5 x 10-6 frequency stability with temperature for a range of -30 to +85C. The RoHScompliant oscillators are designed to operate on +2.8 VDC.

The Conner-Winfield Company (www.conwin.com) recently announced a new line of compact oscillators for telecommunications applications, including Stratum III and timing references. The firm's T5xx and T6xx lines of TCXOs and VCTCXOs measure just 5 x 7 mm in RoHS-compliant SMT packages (Fig. 2) with choice of LVCMOS outputs from 6.4 t 40 MHz or clipped sinewave outputs from 6.4 to 52 MHz. They offer long-term stability of 4.6 PPM for 20 years with jitter of less than 1 ps RMS. The oscillators can be specified for operating temperatures of 0 to +70C and -40 to +85C with available frequency stability of 0.28 PPM, 0.50 PPM, or 1.00 PPM. They operate at +3.3 VDC and draw 10 mA maximum current. The tiny clocks feature phase noise of -80 dBc/Hz offset 10 Hz from the carrier, -110 dBc/Hz offset 100 Hz from the carrier, -135 dBc/Hz offset 1 kHz from the carrier, and -150 dBc/Hz offset 100 kHz from the carrier.

The new FOX331 series of HCMOS TCXO and VCTCXO sources from Fox Electronics (www.foxonline.com) also feature a small profile, at 7 x 5 x 2 mm, with operation at +3.3 VDC. Designed to provide Stratum III stability, with better than 4.6 PPM long-term frequency stability across 20 years, the oscillators are available with outputs from 10 to 20 MHz. They are ideal for telecommunications networks and in wireless communications base stations. The frequency stability with temperatures from -20 to +70 C is an impressive 0.28 PPM and 0.37 PPM over a temperature range of -40 to +85C. The miniature oscillators offer phase noise of -120 dBc/Hz offset 100 Hz from the carrier, -140 dBc/Hz offset 1 kHz from the carrier, and -148 dBc/Hz offset 10 kHz from the carrier. The RoHS-compliant oscillators are packaged in tape-and-reel format for automated assembly applications.

Fox Electronics also recently enhanced its XpressO line of HCMOS crystal oscillators for use with +2.5-VDC supplies. The oscillators are based on the use of a third-order delta-sigma modulator (DSM) for reduced noise levels. The sources are ideal for synchronous optical network (SONET), wireless communications, and test systems. They are available with frequencies from 0.750 to 180.000 MHz with stabilities as tight as 20 PPM and can be specified for use at temperatures of -20 to +70C or -40 to +85C. The compact oscillators are supplied in RoHS-compliant packages measuring 5.0 x 3.2 mm and 7.0 x 5.0 mm.

The latest line of miniature ceramic surface-mount VCXOs from Bomar Crystal Co. (www.bomarcrystal.com) can be specified for frequencies from 2.000 to 19.999 MHz, 20.000 to 39.999 MHz, and 40 to 60 MHz with frequency stabilities ranging from 25 to 100 PPM stability over a standard operatingtemperature range of 0 to +70 C and an optional operating-temperature range of 40 to +85C. Measuring only 7 x 5 mm, the oscillators provide 100 PPM typical pullability and operate on a standard supply of +5 VDC, with an option to run on +3.3-VDC supplies. Available in package styles with four or six pads, the compact VCXOs draw only 10 mA current for frequencies from 2.000 to 19.999 MHz, 20 mA current for frequencies from 20.000 to 39.999 MHz, and 30 mA current for frequencies from 40 to 60 MHz.

MtronPTI (www.mtronpti.com) offers the XO5120 series of OCXOs with HCMOS or sinewave outputs in a pin package measuring just 1.4 x 1.0 in. The oscillators are available with standard frequencies of 10.00, 12.80, 13.00, 16.384, and 20.00 MHz, with custom frequencies available from 10 to 100 MHz. Ideal for cellular base stations and in test equipment, the OCXOs employ either SC-cut or ATcut crystals with short-term stability of 0.01 and 0.1 parts per billion (PPB), respectively, over operating temperatures from -40 to +85C. The phase noise for AT-cut OCXOs is typically -80 dBc/Hz offset 1 Hz from the carrier, -115 dBc/ Hz offset 10 Hz from the carrier, -140 dBc/Hz offset 100 Hz from the carrier, -145 dBc/Hz offset 1 MHz from the carrier, and -150 dBc/Hz offset 10 MHz from the carrier. For SC-cut OCXOs, the phase noise is typically -90 dBc/Hz offset 1 Hz from the carrier, -120 dBc/ Hz offset 10 Hz from the carrier, -140 dBc/Hz offset 100 Hz from the carrier, -150 dBc/Hz offset 1 MHz from the carrier, and -155 dBc/Hz offset 10 MHz from the carrier. The oscillators can be supplied for voltage supplies from +3.3 to +12 VDC, with typical power consumption of 1.25 W upon warmup.

At least one crystal oscillator supplier sees increasing need for oscillators capable of operating at higher temperatures. Vectron International (www.vectron.com) launched its new VX-400 VCXO at the recent MTT-S in Atlanta, GA. Designed for harsh environment applications, including continuous use at temperatures to +200C, the VX-400 VCXOs are suitable for industrial, military, and aerospace applications that exceed the standard +125C military- standard (MIL-STD) temperatures. Available in a standard four-pin dual-in-line package (DIP) with frequencies from 10 to 30 MHz, the new VCXOs feature phase noise of -150 dBc/Hz offset 10 kHz from the carrier with less than 0.2 ps RMS jitter. The oscillators are designed for +3.3 or +5.0 VDC supplies.

According to Greg Smolka, Vectron's vice-president, Industrial, Military, and Space business unit, "Unlike other electronics serving this market, Vectron's new VX-400 high-temperature VCXO was designed, from the ground-up, to operate in some of the most severe environments on the planet. Rather than simply de-rating our standard products for high-temp operation, we've leveraged our core competencies in quartz crystal circuit design and fabrication, and high-temperature packaging to create a precision frequency control product that operates reliably over industry-leading lifetimes for these markets."

Vectron also announced its new EX-620 Evacuated Miniature Crystal Oscillator (EMXO), an extremely small OCXO, at the MTT-S. Supplied in a low-profile, four-pin hermetic half-DIP package measuring 13.21 x 13.21 mm, it is available with frequencies from 10 to 30 MHz. It features temperature stability of +/-75 x 10-9 at operating temperatures from -20 to +70C and power consumption of less than 0.35 W after warmup, with a short two-minute warmup time.

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According to David Bail, Director of Product Development for Vectron's Mount Holly Springs, PA facility, feedback from customers is invaluable: "Because we are in constant communication with our customers worldwide to help them solve previously unsolved problems, establish competitive differentiation and evolve their platforms, we are acutely aware of their needs and can stay ahead of the curve in developing products that meet and exceed their demands. Our EX-620 series exemplifies that philosophy and adds yet another proof point to our growing product portfolio-the largest in the precision space."

Symmetricom (www.symmetricom.com) recently announced its 9900 series of oscillators featuring basic crystal oscillators (XOs), VCXOs, and TCXOs for demanding space applications. Available with sine wave or PECL outputs from 10 to 1200 MHz, the oscillators are based on third- or fifth-overtone AT-cut crystals in a Colpitts configuration, using optional frequency-multiplication circuitry and output amplification, the firm's precision crystals are contained within hermetic or vacuumsealed packages within the hybrid circuit housing. The sources are manufactured in a MIL-PRF-38534 Class K facility for maximum reliability.

According to Doug Lowrie, product marketing manager for Symmetricom, "Symmetricom's thirty-five years of legacy in high reliability and performance quartz oscillators are now available in hybrid construction for applications that require minimal size, weight and power. Our combination of space-qualified OCXO products and these new hybrid oscillators position Symmetricom as the most diverse supplier of high-performance oscillators for space applications. These products perform in environments of exposure to high levels of shock, vibration and radiation, vigorous requirements for space applications."

Long-time oscillator supplier Wenzel Associates, Inc. (www.wenzel.com) has integrated several components into a compact housing to essentially create a crystal oscillator capable of output frequencies from 650 MHz to 12 GHz. The company's Agate oscillator houses an ovenized VHF crystal oscillator with output frequency from 65 to 130 MHz with various step-recovery-diode (SRD) multiplier topologies to generate outputs to 12 GHz with integral filters to remove noise sidebands. Capable of output levels as high as +13 dBm, the Agate oscillators are suitable for applications in radar, communications, and test systems. For a 650-MHz carrier, the phase noise is -110 dBc/Hz offset 100 Hz from the carrier, -135 dBc/Hz offset 1 kHz from the carrier, -150 dBc/ Hz offset 10 kHz from the carrier, and -152 a dBc/Hz offset 20 kHz from the carrier. For a 10-GHz carrier, the phase noise is somewhat degraded but still respectable, at -82 dBc/Hz offset 100 Hz from the carrier, -107 dBc/Hz offset 1 kHz from the carrier, -127 dBc/Hz offset 10 kHz from the carrier, and -129 dBc/Hz offset 20 kHz from the carrier. The Agate oscillators measure 2.5 x 5.0 x 0.8 in. (25.4 x 127 x 20.32 mm).

Oscilloquartz SA (www.oscilloquartz.com) has also responded to customer demand for higher levels of integration in its crystal oscillator products, developing a line of GPS-based clock oscillators for WiMAX base stations. These time keepers were developed for systems employing time-division-duplexing (TDD) WiMAX where base stations require precise phase timing. These GPS receiver clocks make it possible to phase synchronize all base stations in a network. The OSA WiMAX series clocks, based on a stable double OCXO design, exceed TDD WiMAX requirements in terms of phase alignment and phase noise. The sources provide 1 pulse per second (PPS), 10-MHz, and time-of-day (TOD) outputs references to universal time code (UTC), with typical frequency stability of better than 1 x 10-10/day. The OSA 4530 WiMAX series sources measure 50.8 x 101.5 x 127.0 mm, with multiple 10-MHz and 1-PPS outputs available upon request.

Networks International Corp. (www.nicke.com) offers custom TCXOs and VCTCXOs for rugged environments. The VCTCXOs are available for frequencies from 10 to 40 MHz while the TCXOs can be specified for frequencies from 10 to 100 MHz. The oscillators employ a thermistor network to generate a correction voltage that reduces the frequency variations over temperature. The correction voltage is usually applied to a varactor diode in the crystal circuit such that the crystal frequency may be varied by a small amount. These oscillators feature a short warmup time, requiring only the time for the components to reach thermal equilibium.

Raltron Electronics Corp. (www.raltron.com) features several lines of TCXOs and OCXOs, including the 5100 series of OCXOs in a compact CO-15-type package measuring 22 x 22 x 11.5 mm. The OCXOs are available for frequencies from 1 to 155.520 MHz based on AT-cut or SC-cut crystals. The frequency stability can be as good as 0.25 PPM with the former and 0.03 PPM with the latter for operating temperatures from -40 to +85C. The phase noise with the AT-cut OCXOs is -90 dBc/ Hz offset 10 Hz from the carrier, -135 dBc/Hz offset 1 kHz from the carrier, and -150 dBc/Hz offset 10 kHz from the carrier. The phase noise for the SC-cut OCXOs is -110 dBc/Hz offset 10 Hz from the carrier, -140 dBc/Hz offset 1 kHz from the carrier, and -150 dBc/Hz offset 10 kHz from the carrier.

The OSC 10-001 and 10-101 AT-cut surface-mount crystal oscillators from TRAK Microwave Corp. (www.trak. com) provide fundamental-frequency and multiplied-frequency outputs from 60 to 1440 MHz with frequency stability of 30 PPM for operating temperatures from 0 to +70C. Typical spurious is -80 dBc with phase noise of -110 dBc/Hz offset 100 Hz from a 100-MHz carrier, better than -130 dBc/Hz offset 1 kHz, and better than -155 dBc/Hz offset 10 kHz from the same carrier. The oscillators run on +12 to +18 VDC.