As electronic circuits approach submillimeter wavelength frequencies (300 GHz) and higher, the traditional low-loss method of packaging electronic circuits in waveguide modules for guiding the signal requires more attention. The reasons are that circuits at higher frequencies have lower signal power levels due to limited gain and output power of semiconductor devices. As a result, the power lost by signals in the waveguide propagation environment becomes even more important at higher frequencies. In addition, previous efforts have based higher-frequency waveguide modules on existing lower-frequency module concepts and internal components.
Modifying lower-frequency package modules for higher frequency has limited the performance of the higher-frequency (smaller wavelength) modules due to usage of the same physically larger passive module components such as the bias board, connectors, and screw placement configuration. As a consequence of using larger parts, the resulting packaged electronics will have worse performance due to extra passive loss from longer package module waveguides and/or transitions.
This new approach focuses on the main concept of the minimization of passive package loss through minimizing all aspects of the physical length of the RF propagation path in the package module. Doing this requires design of the smallest bias circuitry, transitions, and waveguides, and use of the smallest available module components, such as screws and connectors, in developing a new module with the lowest loss.
To develop the lowest-loss package module, two practical considerations were to identify the smallest socket for supplying DC bias to the package, and identify the smallest screws that can provide sufficient force to adequately compress-close the split block package module. Once these basic fixed-size structures were chosen, two new critical features were introduced. The first was the technique of a DC bias board design that routes DC supply signals around required package structures such as screws; that is, design the smallest bias board for the required bias levels with holes to allow for module screws to go through the board to reduce the module length. Second, the module length in the new design approach will be shorter than the threaded parts of screws for mating RF flanges.
As a result, a new scheme for allowing modules to be bolted in series (which is often the case) was also devised. Four threaded screw holes that lack 180 degrees of rotational symmetry are placed on the faces of the package with the RF wave ports. These four screw holes allow one module to be bolted to another. In certain instances, additional-length screws are required to interface the new module with standard waveguide flanges. Finally, for the cases where the circuit does not have integrated circuit-to-waveguide transitions, the shortest microstrip-to-waveguide transitions were developed that can still be manually mounted into the module by hand. They are made of low-loss substrate materials to also minimize loss in this portion of the signal propagation path.
The new WR3 (220-325 GHz) module (4.5 × 20 × 26 mm) has a measured input and output waveguide loss of about 0.2 dB (S11,22/2) at 220 GHz, and 0.15 dB at 320 GHz, while the prior module evolved from the traditional lower-frequency method has a loss of about 0.70 dB at 220 GHz, and 0.5 dB at 320 GHz. In addition, the length of the transition probe in the new module was reduced to 390 μm from 850 μm in the prior module, which reduced the total loss from the waveguide flange to the end of the probe microstrip line at the device under test cavity by more than 0.95 dB (to a net loss of less than 1.1 dB) at 220 GHz, and 0.7 dB (to a net loss of less than 1.7 dB) at 325 GHz for each of the input and output waveguides and probe transitions of the package module.
This work was done by King Man (Andy) Fung, Lorene A. Samoska, Robert H. Lin, Choonsup Lee, Alejandro Peralta, Sharmila Padmanabhan, Mary M. Soria, and Pekka P. Kangaslahti of Caltech for NASA’s Jet Propulsion Laboratory. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Technology Transfer at JPL
Mail Stop 321-123
4800 Oak Grove Drive
Pasadena, CA 91109-8099
Refer to NPO-49930