Research

Armadillos and Toasters

(The easy way to generate millimeter wavelength radiation)

This section describes the method we use to generate millimeter and sub-millimeter wavelength radiation (mm/submm-waves).  For a discussion of the frequency regions we use and why (click here).  In our lab, we can currently cover the region from 50-330 GHz.  There are other ways to do this, but this is the simplest and most flexible way.

The Source:

The “millimeter-wave source modules”, depicted above, are from Agilent Technologies (Models: 83557A and 83558A covering 50–75 GHz and 75–110 GHz, respectively); similar systems are also available from Anritsu Corporation. Because of their appearance, the Agilent source modules have been nicknamed “Armadillos” by network analysis engineers. Like the earlier cat-whisker Schottky diodes, these new sources are also microwave multipliers. However, instead of the Schottky diode junction being formed between a sharpened wire cat whisker and crystal, the new commercial multipliers are made from planar solid-state Schottky diode arrays.  Not only are they much more robust than the homemade diodes, but the new sources are also several orders of magnitude more powerful, providing power output levels on the order of 1 mW. In addition, the power and frequency stabilization electronics are seamlessly integrated with standard microwave synthesizers. The system requires little or no user intervention and, when connected to a highend microwave synthesizer such as the Agilent HP83623B (with option 008; 1 Hz resolution, may be operated in CW mode, modulated AM or FM, swept, or pulsed.

 

Various third party millimeter-wave amplifiers (from Spacek or Terabeam) and multipliers (from Virginia Diodes) may in turn be added to the armadillos, further pushing their outputs well into the submm-wave region. In our lab, sets of commercial solid-state millimeter-wave amplifiers and multipliers are used to extend the range to 330 GHz.  With the exception of small gaps between 130–150 and 220–225 GHz, output power levels across the entire range on the order of 1 mW are achieved. Multipliers up to 1.7 THz are now commercially available.

The Detector:

The transient mm/submm-wave absorption signal is detected with a liquid-helium-cooled InSb hot-electron bolometer chip made by QMC Instruments Ltd. (Model: QFI/X) and housed in a liquid-nitrogen/helium Dewar (Infrared Laboratories Inc. Model: HDL-5) with a 1 in. diameter entrance window and internally mounted millimeter-wave feedhorn, known as a Winston cone. For increased sensitivity, the active area of the 5×5 mm2 InSb detector chip has a serpentine layout, earning it the nickname “toaster” by millimeter-wave engineers.  This type of bolometer is sensitive to frequencies in the range of about 60–900 GHz, and has a measured system optical NEP of 7×10−13 WHz−1/2 at about 1.5 K, a responsivity of 5 kV/W, and a response time on the order of a third of a microsecond (i.e., 3 MHz). An ultra-low noise preamplifier (Infrared Laboratories Inc. Model: UNL-6) is used to amplify the transient signals without adding noise.