One of the newer requirements in electromagnetic compliance (EMC) testing demanded by carmakers is the pulsed radar immunity test. Though exact requirements vary from vendor to vendor, the test essentially requires the generation of a modulated microwave signal that emulates radar pulses and has a field strength of 600 V/m. Both General Motors and Ford require this test at 1.2 GHz to 1.4 GHz, while Ford also demands 600 V/m at 2.7 GHz to 3.1 GHz.
To produce the 600 V/m measurement at 1.2 GHz to 1.4 GHz requires a high-power microwave amplifier capable of delivering on the order of 500 W to 1000 W. The exact power level required to achieve the desired field strength will depend on the efficiency of the overall EMC test set up (reflections in the anechoic test chamber, the gain of the horn antenna, and cable losses).
Until fairly recently, these power levels and bandwidths were only available from traveling wave tube (TWT) amplifiers. For engineers performing EMC tests on automotive and other designs, the use of TWT amplifiers has meant reliance on bulky instrumentation with reliability well below what designers have come to expect from their solid-state test equipment. However, lately amplifier manufacturers have begun to exploit improvements in compound semiconductors to develop solid-state versions of their products that can be used to perform the radar pulse test.
A recent example is a 1 kW, 1 GHz to 2 GHz solid-state amplifier developed by MILMEGA. Whereas previously, this type of amplifier would consist of two 500 W TWTs, this solid-state version relies on a much smaller building block — a 10 W GaAs FET. Four of these are combined within a module that delivers about 40 W and then eight of those modules are combined into a rack-mount unit that delivers 290 W. To achieve the 1 kW rating, the outputs of four of the rack-mount units are combined in a 15-U-high rack assembly.
Migrating to a solid-state design provides significant performance benefits. In the TWT amplifier, P1dB is typically about 6 dB to 7 dB lower than PSAT, which for a 1 kW amplifier design results in only 250 W of linear power. However, in the solid-state amplifier, P1dB is typically within 0.5 dB of PSAT, resulting in 700 W to 800 W of linear power. In terms of reliability, a TWT is prone to hard failures such that if one tube fails, the whole amplifier fails. On the other hand, the solid-state amplifier will tend to experience soft failures in which a transistor or module will fail without disabling the entire amplifier.
Other vendors have developed solid-state microwave amplifiers with the power and bandwidth necessary for the radar pulse tests. For example, AR Worldwide recently introduced an 800 W solid-state amplifier that operates at 800 MHz to 3 GHz (800S1G3) and a 700 W amplifier that operates from 800 MHz to 4.2 GHz amplifier (700S1G4).
In addition to reliability and performance benefits, solid-state microwave amplifiers are much smaller and lighter than TWT amplifiers. In the case of the 1 kW MILMEGA amplifier, this solid-state unit is one-fourth the weight and half the size of an equivalent TWT amp.
The initial cost of a solid-state amplifier is a potential disadvantage. But that depends on whether you compare TWT vs. solid-state according to typical ratings (advantage TWT) or on the basis of minimum guaranteed saturated power levels (advantage solid-state at up to 2 GHz). Moreover, maintenance costs are much higher for TWT amplifiers where the tube may need replacement every two years. And in the long run, solid-state amplifiers will become less expensive as the transistors used to build them reach high-volume production. In addition, companies such as MILMEGA are developing amplifiers based on FET technologies such as GaN that will increase power density and operating frequencies, making solid-state amplifiers even more attractive for EMC testing.