ADVANCES IN QUASI-OPTICAL MM-WAVE SOLID-STATE ELECTRONICS

The goal of the research effort is to develop low-cost millimeter-wave and submillimeter-wave front-end electronics. The idea rests on the near total integration of the front-end, using high-efficiency planar antennas, active and passive MMICs and grid-based systems. The application areas are automotive collision avoidance systems and mm-wave communication systems. Furthermore, the grid concept allows the integration of thousands of devices (transistors, varactor diodes, etc.) in a relatively small area, and is a strong candidate for successful generation of RF power at submillimeter-wave frequencies.

I. Monolithic 250 GHz AND 90 GHz Receivers

A monolithic 250 GHz and Schottky-diode receiver has been developed using a cpw-fed double slot antenna on a silicon dielectric lens [1]. A planar 1.2 um Schottky-diode is monolithically integrated in series between the two slot antennas. The lengths and widths of the slot antennas are optimized to result in a conjugate-match impedance at the diode terminals. This results in an extremely compact receiver, measuring only 300 μm X 600 μm, and requiring no matching network. The local oscillator is injected quasi-optically using a Fabry-Perot diplexer. The measured 250 GHz receiver DSB conversion loss and noise temperatures are 7.6 dB and 1600 K, respectively, and is within 3-dB of the best waveguide planar Schottky-diode performance. A similar design was also performed at 90 GHz [2] and resulted in a DSB conversion loss and noise temperature of 5.5-6.0 dB and 500 +- 50K, respectively (again within 3 dB of the best waveguide receivers). This receiver is ideal for linear 1 X N (or 2 X N) imaging arrays, and for submillimeter receiver designs at 500-2.5 THz.

II. 155 GHz and 215 GHz HEMT Slot-Oscillators

The circuit is based on the cpw-fed quasi-optical slot oscillators [3], and follows the reflection amplifier approach. The slot-antenna impedance is used as a parameter in the oscillator design which results in a circuit which is much smaller than a wavelength. The 0.05 μm InP HEMT is a delta-doped, strained layer structure grown at Hughes Malibu Research Laboratories with a Indium channel concentration of 80\%. The extrinsic short-circuit current gain extrapolated cutoff frequency is 340 GHz and the extrapolated maximum oscillation frequency is 740 GHz. The quasi-optical oscillator generated an output power of 10 μW at 150 GHz and 1 uW at 215 GHz (separate designs). The corresponding DC-RF efficiency is 0.13 % at 155 GHz. This is not surprising since the transistor is very small with only 10 μm gate width. It is possible to power combine many of these oscillators on a single chip to generate more output power at terahertz frequencies. To our knowledge, these results are the highest frequency three-terminal oscillators to date.

III. 80/160 GHz Quasi-Optical Subharmonic Mixers

We have developed a new quasi-optical subharmonic mixer topology based on a dual double slot antenna and planar surface-channel Schottky diodes [4]. RF and LO signals are coupled to the mixer quasi-optically using external lenses and a wire-grid diplexer. The mixer is comprised of a ring of four Schottky diodes placed at the center of the double slot antennas. During each LO cycle, alternate pair of the diodes are driven into forward conduction, resulting in a time-dependent conductance waveform at twice the LO frequency. The diodes are coupled to the double slot antennas through coplanar transmission line feeds. The slot antennas are designed to operate at their second resonance and arranged to receive orthogonally polarized RF and LO signals. This allows the mixer to be operated over a broader bandwidth and gives a large degree of isolation between the RF and LO ports. In a proof-of-principle demonstration, a hybrid mixer using SC2T6 diodes fabricated at the University of Virginia and operating at an RF frequency of 161.8 GHz (LO of 80 GHz) has given a single-side band (SSB) conversion loss of 8 dB.

IV. Submillimeter-Wave Sideband Generators

Quasi-optical techniques offer the possibility and means of extending conventional low frequency circuit concepts to the millimeter and submillimeter range. Here we report a sideband generator designed to operate at 1.6 THz. The sideband generator consists of a 6 X 6 array of Schottky varactor diodes monolithically integrated into an array of bowtie antennas. The array is fabricated on a semi-insulating GaAs substrate which is subsequently mounted on quartz and removed. This leaves the diode array lying on quartz, which is preferable because of the lower dielectric constant. A coplanar transmission line feeds the array from the side, permitting a 0--20 GHz signal to be injected. This signal generates tunable sidebands on a 1.6 THz source illuminating the array, which can be filtered out using a Fabry-Perot etalon.

REFERENCES

[1] S. S. Gearhart and G. M. Rebeiz, ``A monolithic 250 GHz Schottky diode receiver,'' IEEE Trans. Microwave Theory Tech., vol. MTT-42, pp. 2504--2511, Dec. 1994.
[2] G. Gauthier, W.Y. Ali-Ahmad, T.P. Budka, D.F. Filipovic and G.M. Rebeiz, "A uniplanar 90 GHz low-cost millimeter-wave receiver,'' IEEE Trans. Microwave Theory Tech., vol. MTT-43, pp. 1669-1672, July 1995.
[3] S.E. Rosenbaum, B.K. Kormanyos, L.M. Jelloian, M. Matloubian, A.S. Brown, L.E. Larson, L. Nguyen, L.P. Katehi and G.M. Rebeiz, ``155 GHz and 213 GHz AlInAs/GaInAs/InP HEMT MMIC oscillators,'' IEEE Trans. Microwave Theory Tech., vol. MTT-43, pp. 927-932, Apr. 1995.
[4] J.P. DeLap, T.M. Cunningham, T.W. Crowe, R.M. Weikle, II, ``A Quasi-Optical Subharmonically Pumped Double Slot Mixer," to be published in the Proceedings of the Seventh Internat. Symposium on Space Terahertz Tech., Charlottesville, VA, March 1996.