CRO/SAW (coupled-resonator oscillator/surface acoustic wave) resonator-based oscillators are widely used as reference sources in phase-locked loop (PLL) applications, since the technology features very low phase noise at fixed frequencies to beyond 3 GHz. CRO/SAW oscillators are also known for high quality factor (Q) and low phase-noise performance. Unfortunately, CRO/SAW oscillators have several disadvantages, including a limited tuning range (which limits the amount of correction that can be made to compensate for the tolerances of other components in the oscillator circuit), and sensitivity to microphonics and phase hits.

Phase hits are infrequent but cause signal degradation in high performance communication systems, and therefore affect data rate increases and can unlock many communication links if they cannot be absorbed. These phase hits are caused by sudden changes of the crystal structure due to temperature and mechanical stress. As a result, phase hits have become a prime target for elimination. While phase hits have plagued communication equipment for years, today’s higher transmission speeds accentuate the problem because of the greater amounts of data affected in a given time period.

Therefore, due diligence must be done upfront by the designer considering using a digitally implemented CRO/SAW oscillator to overcome these problems. In addition, since the design of a new CRO/SAW oscillator is much like that of an integrated circuit (IC), development of an oscillator with a nonstandard frequency requires non-recurring engineering (NRE) costs, in addition to the cost of the oscillators. To counter this problem, the engineering team at Synergy Microwave has developed a low cost oscillator technology based on distributed coupled resonators and novel circuit techniques.

Design Features

Phase hits can be defined as sudden, uncontrolled changes in the phase of the signal source that occurs randomly, and generally lasts for fractions of a second. It can be caused by temperature changes from dissimilar metals expanding and contracting at different rates, as well as from vibration or impact. Microphonics, which are acoustic vibrations that traverse an oscillator package and its circuits, and cause changes in phase and frequency, are dealt with in the DCSOs series VCOs through innovative design topology.1–3

DCSOs, which are depicted much like a high Q planar resonator in an equivalent circuit, can achieve the phase-noise performance of CRO/SAW oscillators but over a wide tuning and operating temperature range at low cost. They can be made quite small compared to CRO/SAWs, especially at frequencies of 1000 MHz and less, and can be readily designed at standard and custom frequencies up to approximately 5 GHz without expensive NRE costs.

Fortunately, the new DCSOs series of VCOs break with tradition and overcome the long-time hurdle of achieving low phase hits, while also reducing phase noise over the tuning range. The low cost, surface-mountable DCSOs are currently available at frequencies of 1000 and 2488 MHz. In addition, the new DCSOs can be delivered at standard and custom frequencies without the non-recurring-engineering standard costs associated with custom CRO/SAW resonator-based oscillator development.

The traditional disadvantages of printed resonator oscillators such as low Q and large PCB area have been overcome by means of a novel topology, which acts as a Q-multiplier, using mode coupling and a regenerative noise filtering approach.1,4 The patent-pending approach includes a methodology for DCSOs to enhance the dynamic loaded Q, and to reduce or eliminate phase hits, while reducing the susceptibility to microphonics to an extremely low level, and retaining low phase noise and broadband tunability.4

Table 1 lists the performance characteristics model of a DCSO 1000 VCO. Figure 1 shows the phase-noise plot of the DCSO 1000 VCO. The measured phase noise is typically –128 dBc/Hz at 10 kHz offset from the carrier. The DCSOs are supplied in surface-mount packages similar to those used for the company’s voltage-controlled CRO/SAW oscillator (VCXO) line. Additional information may be obtained via e-mail at sales@synergymwave.com or from the company’s Web site at www.synergymwave.com.

Fig. 1 The DCSO 1000 VCO's measured phase noise for a carrier frequency of 1000

References

1. U.L. Rohde, “Ceramic Resonator Oscillators Challenges SAW,” Microwaves & RF, September 2003, pp. 100–105.

2. A.K. Poddar, S.K. Koul and B. Bhat, “Millimeter-wave Evanescent Mode Gunn Diode Oscillator in Suspended Stripline Configuration,” IR & MM Wave, 22nd International Conference, July 1997, pp. 265–266.

3. U.L. Rohde, A.K. Poddar, J. Schoepf, R. Rebel and P. Patel, “Low Noise Low Cost Wideband N-Push VCO,” IEEE IMS Symposium 2005, Long Beach, CA.

4. U.L. Rohde, A.K. Poddar and G. Boeck, Modern Microwave Oscillators for Wireless

Applications, John Wiley & Sons Inc., 2005.
Synergy Microwave Corp.
Paterson, NJ
973-881-8800