Generating Stable RF/Microwave Signals

April 6, 2012
Oscillators are following a trend of smaller packages and lower power consumption while also delivering enhanced spectral purity and leveraging a number of different technologies.

translating the frequency of other signals. Given the number of different oscillator types and technologies currently on the market, making a choice can seem intimidating for someone faced with selecting one for an electronic system. Perhaps the selection can be made a little easier by reviewing the current high-frequency oscillator types and their performance limitations.

In general, high-frequency electronic systems can be thought of as a receiver, a transmitter, or a combination of the two. That is, a system must detect and process signals or generate and send signals. In a radar, the received signals are reflections from targets of signals originally transmitted by the same system. In a satellite communications (satcom) system, a terrestrial earth station sends signals to a space-based satellite and receives return signals from the satellite. Even test equipment, such as a spectrum analyzer, can be thought of as a receiver, with a tunable oscillator and adjustable filters at its core. In all of these systems, the oscillators must meet a certain set of performance parameters for the systems to operate properly.

The most basic of oscillator performance parameters include frequency tuning range (and, for some oscillators that are not tunable, this is a fixed value), output power, output power flatness with frequency and temperature, spurious and harmonic noise, single-sideband (SSB) phase noise, and tuning speed. In some applications, such as in mobile electronic systems, vibration-induced instabilities can be a critical performance parameter, as well as performance over temperature.

In its simplest form, an RF/microwave oscillator consists of a tuned resonant (filter) circuit or element and an active device, such as a transistor, for amplification of the resonant frequencies. Some fixed-frequency oscillators may use passive circuit elements, such as inductors and capacitors, for the tuned circuit, while some will use quartz crystal as the resonant element or resonators based on surface-acoustic-wave (SAW) materials. Tuning circuits with variable elements, such as variable capacitors, also afford a certain amount of frequency tuning. At higher frequencies, yttrium-iron-garnet (YIG) spheres have been used as resonators in YIG-tuned oscillators (YTOs), as have coaxial resonators in coaxial resonator oscillators (CROs) and dielectric materials in dielectric resonator oscillators (DROs). A number of proven oscillator circuits have been developed over the years, often named for their founders (including the Butler, Clapp, Colpitts, Hartley, and Pierce oscillator types).

Surveying the options

High-frequency oscillator suppliers are constantly refining their designs for improvements in key performance parameters, such as phase noise, tuning speed, and even physical size, the better to meet the increased demands of modern electronic systems. Phase noise, for example, is a measure of an oscillator’s frequency stability and must be minimized in systems that rely on digital modulation based on in-phase (I) and quadrature (Q) signal components to transfer information. It is a critical performance parameter for crystal oscillators serving as a clock or reference oscillator in a communication system or test instrument. The highest grade of stability for a crystal oscillator is a family of components known as oven-controlled crystal oscillators (OCXOs); these feature built-in heating circuits that help maintain the oscillator at a fixed temperature during operation, so as to minimize fluctuations in phase and frequency.

A challenge for designers of these type of oscillators is in maintaining the excellent stability while making them smaller in size. For example, the NV45G OCXO from Bliley Technologies provides 100-MHz sinewave output signals at +7 dBm with phase noise of -130 dBc/Hz offset 100 Hz from the carrier. It exhibits -25 dBc harmonics and -75 dBc spurious levels and operates from a +12 or +15 VDC supply. It fits into a surface-mount package measuring 1 x 1 x 0.53 i222n.

One of the lowest-noise OCXOs in the industry, the 10-MHz model OX-045 from Vectron International, is built around a stress-compensated (SC) cut crystal. Although packed into a surface-mount housing measuring only 50 x 50 mm, it achieves impressive phase noise of -113 dBc/Hz offset 1 Hz from the carrier, -140 dBc/Hz offset 10 Hz from the carrier, and a noise floor of -163 dBc/Hz. It boast temperature stability of ±3 ppb from 0 to +70°C and ± 10 ppb from -40 to +70°C. The source offers +10-dBm typical output power with -30 dBc harmonics with an aging rate of only 10 ppb/year.

Even smaller, the model VFOV405 from Valpey Fisher fits in a surface-mount package that is only 14 x 14 mm and only consumes 0.12 W power from a typical +3.3-VDC supply in steady-state operation. It is available with fixed output frequencies from 5 to 50 MHz at HCMOS/TTL levels. The oscillator features an aging rate of only 0.5 parts per billion per day (0.5 ppb/day). The tiny OCXO exhibits exceptional phase noise, making it ideal as a reference oscillator for phase-lock-loop (PLL) frequency synthesizers. For a 10-MHz oscillator, the SSB phase noise is -90 dBc/Hz offset 1 Hz from the carrier, -125 dBc/Hz offset 10 Hz, -155 dBc/Hz offset 1 kHz, and -165 dBc/Hz offset 10 kHz from the carrier. The OCXO is designed for operating temperatures from -40 to +85°C.

Greenray Industries offers OCXOs with frequencies from 1 to 200 MHz in a range of package styles, including surface-mount and dual-in-line-package (DIP) types. The YH1320 series OCXOs come in a pin package measuring 50.8 x 50.8 x 19.05 mm with HCMOS or sinewave outputs from 10 to 120 MHz. The sources consume 2.5 W during steady-state operation from a +12-VDC supply. They exhibit -20 dBc harmonics and, for a 10-MHz oscillator, phase noise of -125 dBc/Hz offset 10 Hz from the carrier, -160 dBc/Hz offset 1 kHz, and -165 dBc/Hz offset 10 kHz from the carrier.

When a somewhat smaller oscillator package is required, the company recently introduced its T72 series of temperature-compensated crystal oscillators (TCXOs) with clipped sinewave outputs from 10 to 50 MHz. Based on high-performance crystals from Statek, these oscillators are housed in rugged ceramic packages measuring only 5 x 7 mm. They are stable within ±0.2 ppm across operating temperatures from -40 to +85°C. They draw only 1 mA current from a +3.3-VDC supply making them suitable for battery-powered wireless applications. They are also resistant to the effects of vibration and feature a phase-noise floor of -159 dBc/Hz for a 10-MHz source.

Integrated Device Technology recently introduced its fourth-generation FemtoClock® NG crystal oscillators and voltage-controlled crystal oscillators (VCXOs). Housed in packages measuring 5 x 7 mm (Fig. 1), they can be supplied with programmable output frequencies from 15.476 to 1300 MHz.

Additional suppliers of crystal oscillators include Bomar Crystal, Connor-Winfield Corp., Fox Electronics, Freescale, International Crystal Manufacturing, Maxim Integrated Products, MMD Components, M-tron Industries, Silicon Labs, SiTime, and Texas Instruments.

Voltage-controlled oscillators (VCOs) have long been known for their fast tuning speeds and low phase noise over wide bandwidths. Circuitry for these can be made compact enough to fit surface-mount housings and run at lower supply voltages. For example, reviewing the performance of the model MAOC-009264-PKG003 VCO from M/A-COM Technology Solutions may help to understand the essential parameters that come into play when comparing and specifying VCOs for applications.

he MAOC-009264-PKG003 operates from 8.8 to 9.8 GHz by means of tuning voltages from 1 to 13 V. Based on an InGaP heterjunction-bipolar-transistor (HBT) low-noise active device, the oscillator is supplied in a RoHS-compliant, 5 x 5 mm 32-lead PQFN package. It is somewhat unique in providing fundamental-frequency output signals and signals divided by 2 (from 4.4 to 4.9 GHz) at a separate port. The oscillator has a integrated buffer amplifier to provide +9 dBm typical output power from 8.8 to 9.8 GHz and +3 dBm typical output power from 4.4 to 4.9 GHz. The phase noise is typically -88 dBc/Hz offset 10 kHz from any carrier in the fundamental-frequency range, and -115 dBc/Hz offset 100 kHz from the carrier.

The 50-Ω oscillator typically draws 165 mA current from a +5-VDC supply, with -25 dBc harmonics at the main port and -24 dBc harmonics at the divided port. It exhibits frequency pushing, or sensitivity to supply voltage, of 20 MHz/V at the main port and 2 MHz/V at the divided port. The frequency drift with temperature from 8.8 to 9.8 GHz is 0.75 MHz/°C from -40 to +85°C. Peak-to-peak frequency pushing is 10.3 MHz for load VSWRs of 1.95:1 to 2.25:1. The one specification not supplied on the VCO’s data sheet, tuning speed and/or frequency settling time, may be instrumental in selecting an oscillator for an application that requires rapid changes of frequency.

Synergy Microwave Corp., a long-time supplier of microwave VCOs, entered the crystal oscillator market last year with several 10-MHz OCXOs, including model OXO10-1-348. Housed in a compact 25.4 x 22.0 mm surface-mount package, it delivers sinewave outputs with less than -20 dBc harmonics and less than -90 dBc spurious content. It features phase noise of -100 dBc/Hz offset 1 Hz from the carrier, -160 dBc/Hz offset 1 kHz from the carrier, and -165 dBc/Hz offset 10 kHz from the carrier. It consumes less than 200 mA current from a +12-VDC supply during steady-state operation, and has a voltage range of 0 to 5 V for control of a tuning range from ±0.5 to ±1.5 ppm. The OCXO handles operating temperatures from -20 to +70°C.

Mini-Circuits offers more than 3000 wideband and linear-tuning VCOs for frequencies from 3 to 7800 MHz in case styles from 0.25 x 0.25 in. to 0.5 x 0.5 in. It provides surface-mount-packaged VCOs at frequencies from 24 to 6840 MHz. One example, model ROS-2600-119+, tunes from 1650 to 2600 MHz in a metal case measuring 0.5 x 0.5 x 0.18 in. and shielded against unwanted signals and noise. It exhibits phase noise of -102 dBc/Hz offset 10 kHz from the carrier.

To simplify their use in PLL circuits, earlier this year Hittite Microwave Corp. launched a pair of surface-mount PLLs with integrated VCOs, models HMC833LP6GE and HMC834LP6GE (Fig. 2). Model HMC833LP6GE is a fractional-N PLL and VCO that spans 1500 to 3000 MHz with an integral VCO divide by 1 through 64 output divider and frequency doubler, allowing the device to generate frequencies from 25 MHz to 6 GHz. The HMC834LP6GE also combines a PLL, VCO with range of 2.8 to 4.2 GHz, output divider, and doubler; it generates frequencies of 45 MHz to 1050 MHz, 1400 MHz to 2100 MHz, 2800 MHz to 4200 MHz, and 5600 MHz to 8400 MHz. The devices exhibit noise floor of -170 dBc/Hz and can operate from supply voltages of +1.8 to +5.2 VDC.

Additional suppliers of VCOs include Linear Technology, Micronetics, ON Semiconductor, Phase Matrix, Raltron Electronics, RF Micro Devices, Sivers IMA, Skyworks, Spectrum Microwave, TriQuint Semiconductor, and Z-Communications.

When tuning speed is not critical, YIG-based oscillators provide broad frequency coverage with low phase noise. One of the longest-running YIG-based component suppliers, Omni YIG, Inc., in response to the growing demands for smaller oscillators, recently introduced a line of miniature YIG oscillators that includes the model YOM3824DD for applications from 2 to 6 GHz. It measures just 1.4 x 1.4 x 3.1 in. including an integral 12-b digital driver, and is capable of delivering +15-dBm typical output power across the frequency range. The YIG oscillator is usable at temperatures from -54 to +85°C with low spurious levels of typically -70 dBc and phase noise of -120 dBc/Hz offset 100 kHz from the carrier.

When even smaller YIG oscillators are needed, Micro Lambda Wireless offers its MLTO series of TO-8-packaged oscillators in 2-GHz bandwidth models from 2 to 8 GHz. With a package height of only 0.27 in., these YIG sources are built for operating temperatures from 0 to +65°C. For example, YIG oscillator model MLTO-20204 tunes from 2 to 4 GHz (with a free-running frequency of 3 GHz) with +10-dBm typical output power. It operates from +8 V and -5 V supplies and exhibits 2-MHz pulling into a 12-dB return loss load and ±2 MHz/V pushing with power-supply variations.

It delivers -15 dBc minimum harmonics, -70 dBc minimum spurious levels, phase noise of -100 dBc/Hz offset 10 kHz from the carrier and -125 dBc/Hz offset 100 kHz from the carrier. The main coil sensitivity is 6 MHz/mA while the FM coil sensitivity is 300 kHz/mA. The TO-8 YIG oscillator draws 60 mA current from a +8-VDC supply and 15 mA at -5 VDC.

To overcome the characteristic slow tuning speeds of YIG oscillators, Giga-tronics developed their model FTO-0408-540-01 source for use from 4 to 8 GHz. It offer phase noise of -104 dBc/Hz offset 10 kHz from the carrier with about five times the tuning speed of traditional YIG oscillators.

Teledyne Wireless, which acquired the YIG component technology of Ferretec in 2004, maintains low phase noise in its higher-frequency oscillators through the use of bipolar transistor active devices. Its model FS2637 oscillators cover the frequency range from 8 to 18 GHz in a housing measuring 1.25 x 1.25 x 0.84 in. The typical phase noise is -128 dBc/Hz offset 100 kHz from the carrier. The oscillator is specified for +13 dBm output power at temperatures from 0 to +60°C and +11 dBm output power at temperatures from -55 to +85°C. The output-power flatness is within ±3.5 dB at temperatures from -55 to +85°C. Harmonics are typically -12 dBc while spurious levels are typically -60 dBc or better. The oscillator suffers maximum drift of 20 MHz with temperature, with 0.1% tuning linearity, and 0.5 MHz/V pushing. It draws 180 mA at +15 VDC and 30 mA at -5 VDC. Additional suppliers of YIG oscillators include Microwave Dynamics and Vida Products.

About the Author

Jack Browne | Technical Contributor

Jack Browne, Technical Contributor, has worked in technical publishing for over 30 years. He managed the content and production of three technical journals while at the American Institute of Physics, including Medical Physics and the Journal of Vacuum Science & Technology. He has been a Publisher and Editor for Penton Media, started the firm’s Wireless Symposium & Exhibition trade show in 1993, and currently serves as Technical Contributor for that company's Microwaves & RF magazine. Browne, who holds a BS in Mathematics from City College of New York and BA degrees in English and Philosophy from Fordham University, is a member of the IEEE.

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