A large category of scientific investigation takes advantage of the interactions of signals in the frequency range from 300 to 1,000 GHz and higher. This includes astronomy and atmospheric science, where spectral observations in this frequency range give information about molecular abundances, pressures, and temperatures of small-sized molecules such as water. Additionally, there is a minimum in the atmospheric absorption at around 670 GHz that makes this frequency useful for terrestrial imaging, radar, and possibly communications purposes. This is because 670 GHz is a good compromise for imaging and radar applications between spatial resolution (for a given antenna size) that favors higher frequencies, and atmospheric losses that favor lower frequencies. A similar trade-off applies to communications link budgets: higher frequencies allow smaller antennas, but incur a higher loss.

All of these applications usually require converting the RF (radio frequency) signal at 670 GHz to a lower IF (intermediate frequency) for processing. Further, transmitting for communication and radar generally requires up-conversion from IF to the RF. The current state-of-the-art device for performing the frequency conversion is based on Schottky diode mixers for both up and down conversion in this frequency range for room-temperature operation. Devices that can operate at room temperature are generally required for terrestrial, military, and planetary applications that cannot tolerate the mass, bulk, and power consumption of cryogenic cooling.

The technology has recently advanced to the point that amplifiers in the region up to nearly 1,000 GHz are feasible. Almost all of these have been based on indium phosphide pseudomorphic high-electron mobility transistors (pHEMTs), in the form of monolithic microwave integrated circuits (MMICs). Since the processing of HEMT amplifiers is quite different from that of Schottky diodes, use of Schottky mixers requires separate MMICs for the mixers and amplifiers. Fabrication of all the down-/up-conversion circuitry on single MMICs, using all-HEMT circuits, would constitute a major advance in circuit simplicity.

Three pHEMT-based subharmonic 670-GHz mixers were developed that are all subharmonically pumped at about 300 GHz, which greatly simplifies the local oscillator (LO) source, compared to a fundamentally pumped mixer requiring a 600-GHz source. The mixers use an active topology. Fundamentally, they are configured as a single-stage, grounded-source amplifier with a drain load controlled by the LO. The drain load is an additional transistor, or pair of transistors, switched by the LO signal. This effectively samples the signal from the amplifier at the LO frequency, and passes the beat note on to the output terminal of the mixer.

In the down-converting mixer, the 670-GHz RF input is connected to the gate of the grounded source stage, whose drain is directly connected to the source or sources of the LO FETs (field-effect transistors). One version has only a single transistor in the drain load, and relies on the non-linearity of the FET plus the output tuning circuitry to block the RF and LO signals and passes only the IF to the output terminal.

The second down-converting mixer replaces the single LO FET with a pair having sources and drains connected together. The LO signal is fed to the two gates through a network that gives a 180° phase shift to one FET. Hence, the two FETs are switched on for alternating half-cycles of the 300-GHz LO, and the drain FET pair acts like a sampler at twice the LO frequency. Simulations indicate about 6 dB of improvement in the conversion gain, from –6 dB for the two-FET design to around 0 dB for the three-FET design.

For the up-converting mixer, the circuit is similar to the three-FET downconverter, but with the IF input going to the gate of the grounded source stage, and the RF output taken from the drains of the LO transistors. The RF and IF matching networks are also modified to the correct frequency ranges. Simulations indicate a conversion gain of about 3 dB.

This work was done by Erich T. Schlecht, Goutam Chattopadhyay, Robert H. Lin, and Seth Sin of Caltech; and William Deal, Bryan Rodriguez, Brian Bayuk, Kevin Leong, and Gerry Mei of Northrup Grumman for NASA’s Jet Propulsion Laboratory. NPO-48204

This Brief includes a Technical Support Package (TSP).
670-GHz Down- and Up-Converting HEMT-Based Mixers

(reference NPO-48204) is currently available for download from the TSP library.

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This article first appeared in the December, 2012 issue of NASA Tech Briefs Magazine.

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