Compact VCOs Expand Bandwidth While Cutting Phase Noise

Nov. 9, 2010
Engineers continuously work to achieve wider tuning ranges at higher frequencies with much lower phase noise and higher stability in voltage-controlled oscillators (VCOs). At the same time, they are pressured to offer such VCOs in miniature ...

Engineers continuously work to achieve wider tuning ranges at higher frequencies with much lower phase noise and higher stability in voltage-controlled oscillators (VCOs). At the same time, they are pressured to offer such VCOs in miniature packages. Meanwhile, a variety of circuit techniquesboth old and neware being exploited to overcome current performance limitations.

Using improvements in proprietary planar-resonator technology (also called compactcoupled- planar-resonator technology), for example, Synergy Microwave Corp. has extended the frequency coverage of its VCOs from low X-band to Ku-band. The new VCOs, which come in 0.3-in. packages, offer octave tuning bandwidths to 15 GHz. "These VCOs are truly unique products made from low-cost, discrete construction with proprietary planar-resonator technology," says Tony Almeida, Synergy's Vice President of Marketing. Aside from a wider bandwidth, the new CCPR-based VCOs are designed for low power consumption and lower phase-noise performance. They are being touted as alternatives to the ceramic and surface-acoustic-wave (SAW) resonators used in wideband VCOs.

Specifically, the product line's two Ku-band additions include the DXO14851515-5, which covers 14850 to 15150 MHz, and the DXO13901400-5 spanning 13900 to 14000 MHz (Fig. 1). For the discrete model DXO14851515-5, phase noise is -74 dBc/Hz at 10 kHz offset from the carrier. It tunes by means of 0 to 15 V with tuning sensitivity of 36 to 72 MHz/V. Typically, it boasts harmonic suppression beyond -25 dBc. The VCO provides frequency pulling and pushing at 80 MHz and 25 MHz/V, respectively.

Its sibling, the DXO13901515-5, offers phase noise of -77 dBc/Hz at 10 kHz offset from the carrier. It tunes via 0 to 8 V with tuning sensitivity of 60 to 90 MHz/V. This VCO boasts typical harmonic suppression beyond -42 dBc. It offers frequency pulling and pushing of 40 MHz and 18 MHz/V, respectively. According to Almeida, this VCO challenges the performance of YIG-based oscillators in tuning bandwidth and power consumption.

Traditionally, VCOs use varactor diodes as part of a tank circuit. Synergy Microwave recently developed a novel topology for monolithic-microwave VCOs, dubbed the tunable active inductor (TAI). According to the firm, the TAI can be integrated as a passive component in a miniature-monolithic VCO circuit with low phase noise and low power consumption (see "Active Inductors Tune Low-Noise VCOs," Microwaves & RF, April 2010, p. 97). Although they are smaller than passive inductors, TAIs often sacrifice linearity and phase noise in VCOs. To overcome these issues, Synergy developed TAI-based VCO circuits that employ dynamic phase injection. As a result, the active inductance value changes monotonically with control voltage without degrading linearity or phase noise.

Employing these techniques, the company has readied an experimental VCO that lends itself to monolithic integration with measured phase noise beyond -110 dBc/Hz offset 1 MHz from a 12-GHz carrier. In this design, the 12-GHz VCO offers a 200-MHz tuning range while requiring only 12 mA and +3-VDC bias. Beyond 12 GHz, discrete construction is not practical because parasitics become more prominent.

In addition, driving it with higher current makes it lossy, explains Synergy's Chief Scientist, Ajay Kumar Poddar. As a result, the developer is investigating a variety of processes to convert it into a monolithic solution including CMOS, BiCMOS, silicon-germanium (SiGe), gallium- arsenide (GaAs), and indium-gallium- phosphide (InGaP) heterojunctionbipolar- transistor (HBT) technology. The firm also is considering marketing the TAI as a standalone, two-terminal component that presents an alternative to traditional varactor diodes.

Broader And Better
Meanwhile, Hittite Microwave has taken the monolithic route to combine low phase noise, low power consumption, and a wide tuning range in a 4-x-4-mm SMT package. The firm has readied two low-single-sideband (SSB) phase-noise, wideband VCOs spanning 6 to 20 GHz. Implemented in GaAs InGaP HBT technology, the HMC732LC4B covers 6 to 12 GHz while the HMC733LC4B spans 10 to 20 GHz.

The VCOs integrate a resonator, negative-resistance device, and varactor diode in a single SMT package. They tune from 0 to +23 V with a very fast tuning bandwidth (Fig. 2). The HMC732LC4B provides -65 dBc/Hz SSB phase noise at 10 kHz and -95 dBc/Hz at 100 kHz offset from the carrier. In contrast, the HMC733LC4B offers -60 dBc/Hz and -90 dBc/Hz SSB phase noise at offset frequencies of 10 and 100 kHz, respectively. Both provide a single-ended RF output, which is matched to 50 O. The HMC732LC4B and HMC733LC4B operate from a single +5-VDC power supply while drawing 54 and 70 mA of current, respectively. According to the firm, their SMT footprint requires up to 90 percent less printed-circuit-board (PCB) area when compared to traditional MCM and discrete hybrid VCOs.

Concurrently, Endwave Corp. has launched a series of VCOs that sports dual outputs with total frequency coverage of 7.3 to 8.8 GHz and 3.65 to 4.40 GHz. Based on 2- m indium-gallium-phosphide (InGaP)/gallium- arsenide (GaAs) heterojunctionbipolar- transistor (HBT) technology, the monolithic-microwave-integratedcircuit (MMIC) oscillators feature onboard, divide-by-two prescalers. They can therefore create a second tuned output range at one-half the frequency of the fundamental outputs. For example, the model EWV0801YF tunes from 7.3 to 8.3 GHz with +13 dBm typical output power. It covers a divided frequency range of 3.65 to 4.15 GHz with +8 dBm typical output power.

Typically, the VCO tunes with a control-voltage range of 2 to 13 V and supply current of 260 mA. It can then attain phase noise of -116 dBc/Hz offset 100 kHz from the carrier in either band with subharmonic levels of -42 dBc. For a tuning voltage of 5 V, pushing is typically 10 MHz/V. Pulling into a 2.0:1 VSWR load is typically 5 MHz peak to peak. From -40 to +85C, frequency drift with temperature is typically only 0.75 MHz/C. The device exhibits output return loss of 5 dB. The higher-frequency model EWV0802YF tunes across a fundamental-frequency range of 7.8 to 8.8 GHz with +13 dBm typical output power. It covers a divide-by-two outputfrequency range of 3.90 to 4.40 GHz with +9 dBm typical output power. The VCO matches the phase-noise performance of the lower-frequency model. Both oscillators are supplied in low-loss, 32-lead QFN packages measuring 5 x 5 mm.

Wideband Tuning
The drive to minimize phase noise in VCOs at higher frequency bands with octave tuning rangesespecially at S- and C-bandsalso has been felt at Z-Communications. The firm's designers are leveraging high- Q components and embedded microstrip resonator design to obtain very low phase noise and linear tuning from 400 MHz to 4 GHz. Plus, the circuits are balanced across the band to eliminate subharmonics in the desired band, explains Applications Engineer Basilides Cruz. An example is the V600ME14-LF.

AABroadband joins the fray with a new line of linear VCOs covering 20 MHz to 10 GHz in up to octave bands. The VCO2-1750 is a fundamental-mode reflection oscillator utilizing bipolar transistors and hyperabrupt silicon varactor diodes to achieve a highly linear VCO with wideband tuning characteristics. Likewise, Spectrum Microwave continues to refurbish its line of varactor-tuned, hybrid low-phase-noise wideband VCOs while Phase Matrix expands its broadband family. Utilizing a compact surface-mount enclosure, Mini-Circuits has released a wideband low-phase-noise VCO covering 1970 to 2770 MHz.

Crystek continues to exploit smaller, discrete surface-mountable components with the right combination of varactor diode and bipolar transistors. Its goal is to push the performance envelope of coaxial-resonator VCOs while improving the performance of SAW-based VCSOs, thus delivering sinewaves to 1 GHz in smaller surfacemount packages.

At the lower end of the spectrum, Si Time has leveraged microelectromechanical systems (MEMS) technology to overcome the limitations of quartz-based VCXOs and deliver a programmable silicon solution from a miniature package. With an order-of-magnitude improvement in robustness and reliability over quartzbased VCOs, SiTime's voltage-controlled MEMS oscillators (VCMOs) are designed to offer pull-range linearity from 30 to 240 ppm.

Two new members in this line include the Si3701/02. Designed for applications that require clock synchronization and recovery, the Si3701 covers 1 to 110 MHz with a programmable frequency range of 30 ppm, 60 ppm, 120 ppm, or 240 ppm. It tunes from 0 to 1.75 V. The VCMO comes in a 4-pin, 2.5-x-2.0- mm package. Tailored for extremely low jitter (

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