A unique coupling mechanism was applied to the design of fundamental-frequency VCOs at frequencies through 4.2 GHz with significant reductions in phase noise.
Voltage-controlled oscillators (VCOs) are the tunable signal sources in a wide range of synthesizers, communications networks, test equipment, and other systems. Ideally, they provide wide tuning ranges with excellent phase-noise characteristics. The resonators in these sources can be formed in a variety of ways, including the use of ceramic and surface-acoustic-wave (SAW) materials. But such three-dimensional (3D) resonators are not conducive to integrated-circuit (IC) realizations and tend to be sensitive to vibration, microphonics, and phase hits.1-12 To overcome the limitations of VCOs based on such 3D resonators, the designers at Synergy Microwave Corp. (www.synergymwave.com) found a way to shrink planar resonators while also applying a unique evanescent mode electromagnetic coupling mechanism to improve the loaded quality factor (Q) of the coupled resonators to lower the phase noise.3-7 The result is a line of patent-pending compact coupled planar resonator VCOs that fit in packages measuring just 0.5 X 0.5 in. but can match the phase-noise performance of SAW-resonator-based oscillators. VCOs have been fabricated with the coupled planar resonator approach at fundamental operating frequencies through 4200 MHz so far.12
These compact coupled planar resonator (CCPR) VCOs were developed to offer a lower-cost alternative to more expensive ceramic and SAWbased VCOs. And unlike the ceramic and SAW oscillators, the planar resonator approach readily lends itself to semiconductor processing methods for fabrication of integrated-circuit (IC) reference oscillators. Several units have been fabricated for use at 144 MHz, 200 MHz, 433 MHz, 622 MHz, 1296 MHz, 2488 MHz, and 4200 MHz using three-layer printed-circuit boards (PCBs) in a package size measuring a mere 0.5 X 0.5 in. Models can be designed for frequencies from 100 MHz to beyond 4.2 GHz.
In these novel CCPR VCO designs (Model DCSR series: Fig. 1), numerous factors impact final phase-noise performance, notably enhancement of the coupling factor between multiple resonators. Typically, self-coupled resonators can be realized by means of a parallel combination of two open stubs having different lengths as some function of the wavelength of the desired resonant frequency. In order to achieve wideband tuning in the design, a dynamically tuned coupling mechanism is needed, which can be implemented, for example, by using a varactor diode as a coupling capacitor across the coupled resonator network. The Q of the printed CCPR resonator network can be enhanced by using an optimum coupling mechanism based on electric, magnetic, or some hybrid combination of electric and magnetic coupling. Work at Synergy Microwave on the various coupling mechanisms led to an effective implementation in the CCPR VCOs and a planar microstrip line/stripline coupled resonator network that can also be fabricated by means of semiconductor processes.11
A 622.08-MHz (SONET/OC-12) VCO was among the first oscillators fabricated with the new design approach. Its phase-noise performance was characterized with resonators in both uncoupled and optimally coupled states using Signal Source Analyzer (R&S), with the results shown in Fig. 2. The typical output power measures +4.721 dBm, while requiring less than 40 mA of current from +12-VDC supply. For the coupled performance, the measured phase noise at 10 kHz from the carrier is –136.0371 dBc/Hz, at 100 kHz from the carrier is –156.8752 dBc/Hz, and at 10 MHz from the carrier is –165.8243 dBc/Hz. In comparison, the uncoupled performance showed noise that was about 16 dB higher in level.1-3 A DC tuning voltage range of 0.5 to 8 V translates to sufficient tuning bandwidth that allows for reliable stable longterm performance. The harmonic rejection is typically 30 dB within the specified tuning voltage bandwidth.
The performance of the CCPR technology is well suited to replace voltage-controlled SAW oscillators (VCSOs) in phase-locked applications. Besides the similar performance in phase noise, these new VCO's also allow for wider tuning bandwidth and tuning sensitivity. The extra tuning bandwidth makes the CCPR technology superior to VCSOs SAW oscillators in terms of thermal stability, aging, and unit-to-unit variations. The CCPR technology is ideal for applications in narrow frequency bandwidth/fixed frequency phase-locked clocks or sources in the IF converters of high-performance test instruments and communication systems. The DCSR series of VCOs from Synergy employ this technology and rival SAW-based oscillators in reaching ultralow phase noise performance, while surpassing SAW oscillator performance in bandwidth, overall development costs for uncommon frequency bands, tuning agility, and long-term reliability. Model DCSR1296-12 (1296 MHz) is another example resulting from this technology. The VCO features measured phase noise of typically –124 dBc/Hz offset 10 kHz from the carrier and requires less than 40 mA current at +12 VDC.
The CCPR VCOs can be fabricated for frequencies to 4 GHz in discrete form. The technology is also adaptable to integrated-circuit (IC) technologies for high-volume applications and extended output frequencies (beyond 4 GHz). The CCPR VCO approach can also be extended to higher frequencies by incorporating mode-injection stubs.2-4 The reported DCSR topology is not limited to this frequency, can be extended for any other fixed frequency (100 to 6000 MHz) without long lead-time of the products.10-12
The current and expanding product offering from Synergy Microwave Corp. offers outstanding performance from packaged oscillators measuring just 0.5 X 0.5 in. It is important to note that these oscillators deliver fundamental output tones, reaching microwave frequencies without benefit of frequency multiplication, where SAW technology has limitations.