Novel Approaches To Wide Electronic Tuning Bandwidth In Solid State Millimeter Sources

Abstract Wideband mechanically tunable oscillators, having greater than 20 percent bandwidth, have been routinely constructed. The second or third harmonic output from an inductive ra­ dial mode resonant disc, or from a high Q cavity, tunable at both fundamental and harmonic frequencies, results in substantial power levels suitable for most local oscillator appli­ cations in the millimeter range extending up to approximately 100 GHz. When low noise per­ formance is required the active device is a Gunn diode. In most applications packaged GaAs Gunn elements are employed. The use of InP Gunn devices promises to extend the fundamental operating range well beyond the upper limit of the W-Band (75-110 GHz) region. Correspond­ ingly useful harmonic power output into the 300 GHz region is predicted from circuits util­ izing this more recently developed device. On the other hand, electronic tuning of these sources has been limited to, at best, a few percent. Several configurations of series ar­ ranged varactor and Gunn diodes, along with novel applications of radial mode disc capaci­ tors in full and reduced height rectangular waveguides, have led to greater than 10 percent electronic tuning range through the upper end of the V-Band (50-75 GHz) region. Another arrangement of series connected varactor and Gunn devices having quarter wave cylindrical resonators attached to the diodes, and constructed in coaxial cavities, have similarly pro­ duced wideband electronic tuning characteristics. Theoretical calculations have been car­ ried out, for circuits using cascoded varactor and Gunn elements in conjunction with a non­ linear radial mode disc capacitive transmission line, and compare favorably with measured results up through V-Band frequencies. Similar performance is predicted, for sources ar­ ranged in series geometries, well into the millimeter region.IntroductionA new class of millimeter local oscillators has been developed to exploit the inherently wide tuning bandwidth capabilities, and low noise performance, of high Q series connected Gunn and varactor circuit utilizing a capacitive non-linear radial transmission line be­ tween these devices. The advantages of series arranged circuit configurations versus par­ allel tuned geometries have been theoretically analyzed by Cawsey1 . Waveguide band varac- tor-tuned Gunn sources in coaxial structures were constructed at X-Band (8.2-12.4 GHz) fre­ quencies by Large2 using a series arrangement and packaged devices. Broad tunable band­ width millimeter wave series connected microwave oscillators using IMPATT diodes in chip form for both active devices was reported by Denlinger, et.al.3 . Tunable bandwidths as high as 6 GHz with 28±8 mW were obtained from a single active device in the Ka-Band (26.5- 40 GHz) range. Over 8 GHz of electronic tuning with a power output of 6-26 mW was achieved but some mode jumping occurred over their 25 to 33 GHz tuning range. Sicking and Meinel4 constructed Ka-Band hybrid planar circuit oscillators by mounting their devices in antipar- allel and inserting a suspended substrate carrier between the two diode caps. The dc bias and rf filters were printed on both sides of the board. The circuit actually operated in the parallel resonant mode and lead to only 700 MHz of electronic tuning at 32.7 GHz. The analysis presented by Cawsey1 has been extended here to include the effects of packaged de­ vice parasitics in as much as hermetically sealed active elements are utilized in these os­ cillators. At millimeter frequencies both packaged devices present inductive reactances in series with resistive components5 / 6 . In this design a novel application of the radial mode geometry7 / 8 / 9 is employed in the series configuration in which the radial line is chosen to provide the essential capacitive reactive series component at resonance.Theory of operationThe equivalent circuit analyzed by Cawsey1 , for the parallel or constant voltage config­ uration, is shown in Figure 1. For this geometry, when the varactor quality factor Qv»l and the circuit is matched, the maximum fractional tuning bandwidth\(1)*Presently on leave of absence. Consultant, 139 East Goepp Street, Bethlehem, Pennsylvania 18018, USA.