Growing use of millimeter-wave (mmWave) frequencies for wireless communications is a highly touted part of 5G wireless communications systems, notably at 28, 60, and 77 GHz. But at even higher frequencies, mmWave and sub-mmWave remote sensing techniques are vital to atmospheric measurements due to the water-vapor absorption line around 183 GHz. This makes the G-band range from 160 to 210 GHz of interest to scientists and researchers with an eye to the weather.
In support of those scientists, circuit/device designers at NASA Goddard Space Flight Center (Greenbelt, Md.) developed a waveguide mmWave noise source for use at G-band frequencies. The noise source, with a center frequency of 183 GHz, was designed onto a quartz substrate implemented by means of microstrip and waveguide transmission-line technologies. It combines a Schottky-diode chip noise source within a waveguide package. It features a lowpass filter as part of the biasing of diodes without high-frequency interference, a back-short designed at the center frequency of 183 GHz with viaholes, and a longitudinal chip/waveguide probe.
The back-short structure is included in the microstrip circuitry because of the nature of how Schottky diodes generate noise. When reverse-biased, a Schottky diode will generate noise from both terminals and in both directions of the connected microstrip circuitry, thus requiring something to direct all of the noise in the desired direction.
The back-short structure accomplishes this by reflecting all noise in the unwanted direction and adding it in phase with the outgoing noise to achieve a higher output noise level from the noise source. For this noise source design, the RF back-short consists of a capacitive delay transmission line with phase delays of 180 deg. at 184 GHz, 160 deg. at 191 GHz, and −160 deg. at 176 GHz. The back-short is constructed with multiple viaholes to support heat flow away from the diodes, so that continuous operation of the diodes (and not just pulsed operation) is possible.
Measurements were made in a 50-Ω test environment, comparing results with the noise diode powered on and looking at a room-temperature target as well as a cold target, and with the noise diode turned off with the same parameters. The calculated excess noise ratio (ENR) of the noise diode was found to be about 10 dB at 200 GHz, although with proper impedance matching, it was felt that the noise output could be improved by another 3 dB.
The technology has obvious merit for generating usable noise outputs at mmWave and sub-mmWave frequencies for deep-space exploration. NASA is seeking licensees to commercialize this technology. For more information, contact the Goddard Strategic Partnerships Office via e-mail or by calling (301) 286-5810.
See: “A Robust Waveguide Millimeter-Wave Source,” Tech Briefs, November 2018, Vol. 42, No. 11, p. 44.