Voltage-controlled oscillators (VCOs) continue to add bandwidth with reduced size. As essential frequency elements in communications synthesizers, the latest generation of VCOs covers ever-wider tuning ranges with lower phase noise and superior spectral purity. At the same time, they save critical space in communications systems.

To overcome the size limitations of the ceramic and surface-acoustic-wave (SAW) resonators used in wideband VCOs, Synergy Microwave Corp. has developed compact-coupled-planar-resonator (CCPR) technology. It matches the high-Q performance of ceramic and SAW-based oscillators while enabling operation from high frequencies all the way to the X and low-Ku bands. In addition, it permits a greater-than-octave tuning range. According to Vice President of Marketing Tony Almeida, this technology challenges the performance of yttrium-iron-garnet (YIG)-based oscillators in both tuning bandwidth and power consumption.

Almeida points out, "The technology is now allowing for ultra-small products that are housed in a 0.3-x-0.3-x-0.1-in. package covering octave and optimized bands to reach X and low-Ku bands." He adds that improvements in computer-aided-engineering (CAE) software, circuit-simulation files, and the selection of the right components have allowed this frequency expansion and size reduction. The CCPR resonators employ a unique evanescentmode electromagnetic (EM) coupling mechanism to improve the loaded Q of coupled planar resonators with reduced phase noise. They also boast immunity to high levels of shock and vibration.

While model number DXO900965-5 spans 9.00 to 9.65 GHz, the DCO400800- 5 is crafted to cover 4 to 8 GHz. Although both VCOs are designed for 5-V supplies, they are also available in 3-V versions. According to Almeida, the technology creates a standard and custom product offering that is unchallenged in bandwidth and phase-noise performance. The maker also is in the process of pushing the performance above 20 GHz. By May of next year, it plans to release 5-to- 10-GHz and 6-to-12-GHz DXO versions.

Although the CCPR technology is currently available in discrete form, it is being adapted to a monolithic integratedcircuit (IC) process. The planar resonator technology lends itself to such semiconductor processes as silicon CMOS and BiCMOS, silicon-germanium (SiGe), and gallium-arsenide (GaAs) technologies. In fact, Synergy is considering partnerships in this area as well as licensing its patented technology.

Using clever noise-suppression techniques, Z-Communications has readied its ZRO line of low-phase-noise, wideband ceramic VCOs with innovative bias controls for improved stability. The novel bias techniques minimize voltage fluctuations while helping to reduce performance variations as a function of temperature. The manufacturer recently expanded the frequency range of this VCO series to 4 GHz. Typically, the ZRO series features phase-noise performance of 131 dBc/Hz at 10 kHz offset and 151 dBc/Hz at 100 kHz offset from 738 to 748 MHz. Tuning is very linear from 0 to 6 V with typical sensitivity of 2 MHz/V.

A recent addition to the ZRO family, model ZRO0837H1LF, covers 830 to 845 MHz with tuning voltages of 0.5 to 4.5 VDC. It features typical phase noise of 120 dBc/Hz at 10 kHz offset. The VCO's tuning sensitivity is 7 MHz/V (Fig. 1). It offers typical output power of +1.3 dBm from a +5.5-VDC, 15-mA supply with an operating temperature range of 40 to +85C. The device features second harmonic suppression of 15 dBc.

For narrowband applications that require higher quality factor (Q) at higher frequencies, Z-Comm designers have opted for ceramic resonators. The newest member of the ceramic-resonatorbased CRO series is the S-band model CRO3562ALF. Tailored to operate from 3560 to 3564 MHz with tuning voltages of +1.5 to +4.5 VDC, the CRO3562A-LF features typical phase noise of 112 dBc/Hz offset 10 kHz from the carrier and tuning sensitivity of 8 MHz/V. The VCO provides rated output power of +3.5 dBm from a 5-V supply with a 17-mA current draw. Second harmonic suppression is 35 dBc.

Utilizing a compact enclosure with a coaxial connector, Mini-Circuits has produced a wideband linear-tuning VCO with low phase noise and low pushing. The latest addition to this line of VCOs is the ZX95-2150VW, which spans 970 to 2150 MHz. The oscillator offers phase noise of 120 dBc/Hz at 100 kHz offset. Frequency pushing is a low 1.5 MHz/V. The coaxial VCO tunes with voltages of 0.5 to 25 V and is powered by a +5-VDC supply. It delivers +4 dBm output power across its frequency range.

To overcome the frequency limitations of coaxial resonator oscillators (CROs), Crystek relies on doubling fundamental outputs. As a result, it has developed the CVCO55CXT family of coaxial-based VCOs with an internal proprietary frequency doubler. The CVCO55CXT-5340-5490, for example, exhibits phase noise of 103 dBc/Hz at 10 kHz offset from the carrier. It covers 5340 to 5490 MHz with a tuning voltage range of +0.5 to +4.5 VDC. Typical current consumption is 25 mA from a 5-V supply while minimizing pushing and pulling to 1.5 MHz/V and 0.5 MHz, respectively. Housed in a 0.5-x-0.5-in. package, the CRO offers second harmonic suppression of 30 dBc. The company is in the process of driving CRO-generated frequencies to 6.9 GHz by means of doublers.

By exploiting smaller discrete RF components, Crystek Microwave has designed a compact package for a broadband CRO without penalties in phase-noise performance. Housed in a 0.380-x-0.380-x-0.220- in. SMD package, the CVCO38 covers 3660 to 3700 MHz. It offers typical phase noise of 107 dBc/Hz at 10 kHz offset.

Instead of taking the discrete route to switch multiple VCOs, Skyworks Solutions is developing a monolithic solution. By combining multiple VCOs with a switch on the same die, the firm is developing a complete broadband synthesizer using a SiGe- BiCMOS process. Currently, it is readying a broadband synthesizer for base-station infrastructure applications. The design incorporates four distinct VCOs, a multiplexer, and phase-locked-loop (PLL) circuitry on the same die (Fig. 2).

The four VCOs combine to cover 400 MHz to 5.6 GHz with each oscillator handling about a 30-percent bandwidth. In this scheme, the first VCO spans 2.7 to 3.5 GHz while the second version delivers 3.3 to 4.3 GHz. The third and fourth VCOs cover 4.1 to 5.3 GHz and 5 to 6 GHz, respectively. The device offers a tuning range of 0.5 to 2.5 VDC. It features 146 dBc/Hz phase noise at 1 MHz offset from a 900-MHz carrier. Frequency pushing is less than 3 MHz/V. The four-VCO design consumes 115 mA current from a +3-VDC supply. Wesley J. Boyd, the firm's Director of Marketing and Applications for Linear Products, adds, "Great care is taken to ensure that the low-loss switches do not degrade the Q of the VCOs."

To address the data demands of forthcoming fourth-generation (4G) radio systems, point-to-point radio developers are being challenged to offer higher-data-rate radios using more complex modulation schemes. RFMD has readied VCO and PLL modules with a variety of resonator structures on Rogers substrates, which provide very low phase-noise performance. "Because pointto- point radio makers multiply the fundamental frequencies of our modules up to the output frequencies of their radios, excellent phase noise performance is absolutely critical," states Clark Ragan, RFMD's Product Line Director for Signal Sources. "We design and manufacture our modules to be phase-transient free so that users do not experience increased bit errors as their transceivers ramp over temperature." Meanwhile, RFMD has also released dual-band VCOs and PLL modules for applications like professional mobile (PMR) and specialized mobile (SMR) radios as well as precision GPS applications.

Another proponent of MMIC VCOs for point-to-point radios is Hittite Microwave. Leveraging its gallium-arsenide (GaAs) heterojunction-bipolar-transistor (HBT) process, the supplier recently released four VCOs for applications to 15.5 GHz. The HMC734LP5E and HMC735LP5E deliver output frequencies from 8.6 to 10.2 GHz and 10.5 to 12.2 GHz, respectively, while the HMC736LP4E and HMC737LP4E cover 14.5 to 15.0 GHz and 14.9 to 15.5 GHz, respectively. The HMC734LP5E and HMC735LP5E provide +18 dBm output power from a +5-VDC supply. Both devices offer single- sideband (SSB) phase noise of 100 dBc/Hz at 100 kHz offset from any carrier in the frequency range. To conserve power, these MMIC VCOs permit the designer to disable an integrated divideby- 4 prescaler. The HMC736LP4E and HMC737LP4E provide output power of +9 dBm at +4.2 VDC with SSB phase noise of 105 dBc/Hz offset 100 kHz from any carrier in the frequency range. All four devices tune from 1 to 13 V.

For its wideband VCOs, Spectrum Microwave has taken the hybrid route. According to Marketing Manager Walter Witt, "The main advantage of hybrid VCOs is environmental integrity and protection from moisture and corrosive oxidation. Other benefits over non-hermetic design include greater immunity to microphonics, a wider operating-temperature range, and finer substrate geometry." Witt confirms that finer substrate geometry allows more flexibility in providing multiple outputs or inputs with higher output power and greater customization. He adds that higher power ability is attributed to better thermal design control at the die level. Plus, available screening in hybrid VCOs ensures performance in rugged military environments.

Using varactor diode and bipolar transistors in hybrid construction, Spectrum's wideband HVA107T-1 offers a full octave bandwidth of 4 to 8 GHz with output power of +7 dBm and SSB phase noise of 90 dBc/Hz at 50-kHz offset from any carrier. Typically, the oscillator provides 1.0 dB output-power variation with temperature. Total frequency drift over the temperature range is 50 MHz. With 12 dB return loss, the unit exhibits frequency pulling of 25 MHz and 10 MHz/V pushing. It operates with a tuning voltage range of 0 to +21 VDC. The oscillator requires 86 mA current at +15 VDC 1 percent.

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