Progress often means leaving something behind. For the RF/microwave industry, technologies are rarely abandoned for the sake of progress, but may continue with changed or diminished roles (vacuum tubes in radio receivers come to mind). In this industry, it is rare to find a technology that, once adopted, is fully abandoned in favor of some other technology. In looking back over the history of this magazine, some technologies surface as ideal solutions when they were first introduced, only to eventually become secondary choices for some applications as alternatives were developed. This short survey will review some of the high-frequency technologies that may be forgotten, but are not gone.
Vacuum tubes may have all but vanished in table radios. But at microwave frequencies, traveling-wave tubes (TWTs) and klystrons are still widely used. Electron tubes are capable of high power levels in an amplifier compared to solid-state devices. The outputs of multiple power transistors must be combined to achieve high power levels. The losses associated with the components needed to combine those transistor outputstypically power combinersdissipate some of the transistor power as heat, resulting in lower overall amplifier efficiency. Electron tubes, on the other hand, can achieve high power levels with high efficiency, often resulting in lower associated power-supply costs over time.
While it is not unusual to see TWTs and klystrons in high-power microwave systems, some vacuum tubes are less familiar such as gyrotrons. Although developed around 1958 as a power source for millimeter-wave frequencies, work on higher devices in the United States lagged behind research performed in the Soviet Union, which developed a strong reputation for higher-power, millimeter-wave vacuum tubes. Some of that technology was made available in the US due to the efforts of Harry Rutstein, Founder of Dorado International Corp., to import soviet high-frequency technology, including high-power gyrotrons.
A leading supplier of modern-day gyrotrons is CPI, which has supplied the devices for applications as diverse as plasma heating, particle acceleration, high-resolution radar systems, and nonlethal weapons. For example, CPI's model VGB-8095 95-GHz, 100-kW gyrotron is part of the US Air Force's advanced nonlethal vehicle-mounted Active Denial System. Since 1977, the firm has produced units from 8 to 250 GHz with CW power levels to 900 kW and pulsed power levels to 1.3 MW. The company has produced gyrotron oscillators at a variety of frequencies, including 28 GHz. By employing diamond output windows formed by chemical vapor deposition (CVD), the company has produced long-pulse 110-GHz gyrotrons capable of power levels to 1.3 mW (model gt-8115).
Another leading supplier of gyrotrons is ToshibaElectron tubes and Devices Ltd. The firm has developed advanced collector technology to achieve extremely high efficiency (about 50%) in its gyrotrons. Used for such applications as plasma and industrial heating, toshiba's industrial gyrotrons include the model E3980, which employs a superconducting magnet structure to deliver 500 kW output power at 5 168 GHz.
Superconductivity, or more specifically, high-temperature-superconductor (HTS) technology, was at one time viewed as a widespread signal-processing solution for high-frequency systems. Superconductivity is based on the concept that the electrical resistance of certain conductors would approach zero as the temperature approached absolute zero. The properties of superconductors have been known since the latter part of the 19th century. In 1962, the first superconducting wire, made from a niobium-titanium combination, was developed at Westinghouse. That same year, Brian Josephson predicted that a superconducting current would flow between two superconductors separated by a thin insulator layersomething later to be known as a Josephson Junction (JJ).
Of course, maintaining a circuit at temperatures close to absolute zero is not trivial. Early superconductors required liquid helium for the extremely cold temperatures. It was not until the mid-1980s that materials capable of exhibiting superconductive characteristics at higher temperatures were discovered, allowing the use of liquid nitrogen at 77 K as a coolant.
To this day, HYPRES, Inc. has remained an innovator of LTS components and systems, using niobium-based integrated circuits with JJs to fabricate high-speed analog-to-digital converters and advanced LTS-based receiver systems (see June/July 2011 Defense Electronics, p. S37). The company's Digital-RF product line includes some of the world's fastest digital circuits, working at clock speeds of 20 to 30 GHz. Some of the ICs may incorporate more than 12,000 JJs per chip. The firm offers complete LTS systems (see figure), packaged with cryocoolers within a rack-mount enclosure.
A forgotten test instrument is the scalar network analyzer (SNA), largely replaced by the vector network analyzer (VNA). While a microwave SNA detects and displays variations of amplitude as a function of frequency for a device under test (DUT), it has been superseded by the VNA; the latter can show both amplitude and phase as functions of frequency and, in so doing, show all four scattering (S) parameters for a DUT. But in the 1970s and 1980s, the SNA was the microwave test system of choice, with new models touting advances in measurement precision and upper frequency limit. SNA suppliers included trusted names in test, such as Pacific Measurements, Inc. (PMI), Hewlett- Packard Co. (now Agilent Technologies), and Wiltron Co. (now Anritsu).
As an example of an SNA, the model 8757D from Agilent (discontinued) provided four measurement channels to show forward and reverse transmission over a dynamic range of -60 to +16 dBm. The measurement bandwidth was dependent upon the choice of precision detector, with standard models covering 10 MHz to 18 GHz and 10 MHz to 26.5 GHz.