A prototype mixer designed for operation at input frequencies near 2.5 THz incorporates a planar Schottky-barrier diode and a radio-frequency (RF) filter that are parts of an integrated circuit on a GaAs membrane strip, plus several other unique features for which there are numerous potential applications. This mixer is intended to provide a less expensive, more reliable alternative to older whisker-contact-diode mixers. The components of the mixer are integrated into a robust package with an overall volume <1 in.3 (<16 cm3). The design can readily be scaled to higher and lower frequencies.

Figure 1. Integrated Circuitry is formed on top of the monolithic GaAs structure. The planar Schottky diode, shown at the highest magnification, includes an air-bridge contact finger and stress-relief air bridges at both ends.

The GaAs structure is monolithic and is patterned to form circuit elements by use of photolithographic techniques. The GaAs structure includes the membrane strip suspended over a rectangular hole in a rectangular frame (see Figure 1). The membrane strip serves as part of a low-loss transmission line and RF-coupling structure as well as a support and as an integral part of the diode and other circuit elements. The strip is 3 *m thick by 40 *m wide by 600 *m long; other dimensions could be chosen according to the intended operating frequency and requirement for mechanical rigidity. Beam leads for electrical contact with external circuitry are formed on the GaAs structure.

Figure 2. The Mixer Is Packaged in two half blocks of brass. Shown here is the lower half block containing the button and the transformer.

The GaAs structure is glued into a copper button with a diameter of 0.175 in. (4.4 mm) and a thickness of 0.075 in. (1.9 mm). The button contains a 2.5-THz rectangular waveguide, a rectangular-to-circular waveguide transition, and an integrated dual-mode conical feed horn (see Figure 2.) The copper button is fabricated by electroforming plus conventional machining. The desired alignment of the GaAs structure with respect to other circuit elements is enforced by use of a reference surface milled into the button. The button is pressed into a larger block (see Figure 2), made of brass, that holds a fixed (but replaceable) waveguide tuner section containing a backshort, a separate intermediate-frequency (IF) quartz suspended stripline impedance transformer, and a coaxial connector to couple with such external circuitry as an IF amplifier or a splitter.

In operation, the local oscillator and the RF signal of interest enter the mixer via the feed horn. The input signals are combined at the diode and the beat-frequency output signal (that is, the IF signal) is removed via the RF filter on the membrane, the quartz impedance transformer, and the coaxial connector. There is no tuning during operation except what can be achieved by adjustment of the dc bias on the diode. The optimal backshort setting is determined in a trial-and-error procedure in which the mixer is operated with tuner sections of various waveguide lengths inserted in the block.

This work was done by Peter H. Siegel, R. Peter Smith, Suzanne Martin, Peter Bruneau, and Michael Gaidis 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

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Refer to NP0-20397, volume and number of this NASA Tech Briefs issue, and the page number.