Design of Mini-Modular Oscillators using RF and Microwave Design Techniques

Driscoll first presented the concept of using 50Omega modular amplifiers in the design of low noise crystal oscillators (Discroll, MM, 1986). As pointed out by Driscoll, the advantage of using a 50Omega amplifier is the individual configuration and evaluation of the sustaining stage functional elements. In addition to these advantages, the basic approach is frequency independent in the RF and microwave frequency ranges, which can greatly reduce oscillator design time for research or small engineering projects. The approach is also very useful in instructing students in oscillator design principles since functional elements can be analyzed separately and then cascaded. This paper discusses the use of 50Omega block amplifiers, which are small, low cost and commercially available, and functional block elements for design of a mini-modular oscillator which can be used for oven testing of crystals, or for student instruction in oscillator design fundamentals. The approach uses RF and microwave design techniques for the component block design. Depending on the frequency range, a combination of discrete components and microstrip delay lines are used for the analysis, design and synthesis of the oscillator. It is relatively easy to make discrete chip 50Omega attenuators, power splitters, and microstrip delay line phase-shifter. A chip component B-mode trap and phase shift network can be added in the feedback path for stability and tuning. A bias-T using chip capacitors and toroid allows DC input and RF output at the same terminal. Using microwave design techniques for the strip line analysis and impedance transformations, and circuit analysis for the discrete components, the oscillator circuit can be analyzed and designed. Laboratory PC board layout and fabrication is easily achieved using commercially available materials and equipment and can be completed in a few hours. With a little practice, the overall design through first tests can be accomplished in few hours. This paper presents the results of a 160MHz, 3rd overtone SC-cut crystal oscillator design and measurement. The oscillator was designed on a small PC-board suitable for insertion into an oscillator oven test set for pre-aging or aging measurements. Measurement of the individual functional elements and the open loop oscillator characteristics are obtained using S-parameter measurements on an automatic network analyzer and are compared to the theoretical predictions with respect to impedances and power levels. The oscillator performance is evaluated using a spectrum analyzer for output power at various test points. The design-analysis approach, equations and design curves are presented which are applicable to a wide frequency range of applications by simple changes in the functional components to meet a given oscillator requirement