12-2018

RF & Wireless Testing

RF & Wireless Testing High Power GaN Amplifiers for Radar Signals using Peak Power Meters GaN technology has become a staple for high power amplifiers (PAs) used in radar applications. Moreover, the high power and/or the radar application often require the signals to be pulsed. Measuring and characterizing pulsed RF signals used in radar applications present unique challenges. This article explains why the peak power meter is a must have test instrument for characterizing the behavior of pulsed RF PAs used in radar systems. Pulsed radar signals are “on” for a short time followed by a long “off” period. During “on” time, the system transmits anywhere from kilowatts to megawatts of power. The high-power pulsing can stress the power amplifier (PA) in a number of ways both during the “on/off” transitions and during prolonged “on” periods. As higher demands are being placed on these PAs in areas such as output power level, linearity, and efficiency, the behavior of the amplifier needs to be thoroughly tested and evaluated. The time domain nature of the pulsed RF signal necessitates time domain signal analysis. This article explains why a peak power meter is the ideal test instrument for characterizing the behavior of GaN power amplifiers used in radar systems. GaN Radar Power Amplifier Technology Overview For many years, radar systems employed vacuum electron devices (VEDs), commonly referred to as tubes, for high power applications. Most commonly used VED technologies in radar applications today are Traveling Wave Tubes (TWT), Klystrons, Magnetrons and Gyrotrons. TWT amplifiers (TWTAs) provide multi-octave bandwidths, multi-kilowatt peak power output, support of high frequencies, as well as ruggedness. However, TWTAs are relatively costly, large vacuum tube structures that require significant size and mass. VEDs and their associated high-voltage power supplies often suffer from short lifetimes, varying from as few as several hundred hours to ten thousand hours for relatively benign environments. To address these shortcomings, engineers have researched alternatives. About three decades ago semiconductor-based PA solutions called solid state power amplifiers (SSPA) started making modest inroads as an alternative technology for certain radar applications. Silicon-based laterally diffused metal oxide (LDMOS) offered a few hundred watts of output power, ruggedness and reliability. Its upper frequency limit of about 3 GHz (S-Band) at high output powers curbed its applicability in radar. Gallium arsenide (GaAs), a wide-bandgap semiconductor, overcame the high frequency deficiency of LDMOS, reaching above 100 GHz, yet its lower operating voltage limits its output power capability. High power GaAs amplifiers often require paralleling of multiple devices to reach desired power levels at the expense of loss in efficiency. GaAs found common use in radar but still was Author: Walt Strickler, VP GM Boonton Application Note 0918/EN Wireless Telecom Group WTGinnovation www.boonton.com Figure 1: Peak Power Meter Block Diagram 66 hf-praxis 12/2018

RF & Wireless Volts Volts Time Wide Bandwidth Detector Figure 3a: A time domain diagram of the conventional sampling and interpolation method Volts not a viable alternative in most high-power radar applications. Time Figure 2: Impact of risetime and bandwidth capability of the sensor for accurately measuring and displaying the pulse RF signal Figure 3b: The RIS method Over the last 20 years, gallium nitride (GaN) has gained popularity for use in SSPAs. GaN offers significantly higher power density, greater efficiency, and higher electron mobility (enabling it to be used at higher frequencies) relative to GaAs devices. Initially, these advantages came at the expense of lower reliability and higher cost. The lower reliability was primarily due to lack of a suitable substrate to remove heat from high power applications out of the die. This is now being addressed by placing GaN on a siliconcarbide (SiC) wafer substrate. SiC provides three times better thermal conductivity. The enhanced thermal performance improved the reliability and ruggedness. Through advances in manufacturing processes as well as increased volume, the cost of GaN high power amplifiers has decreased significantly. Today gallium nitride (GaN) transistors are widely used in many commercial and defense applications. Many consider it the technology of choice for high-power applications in electronic warfare (EW), radar, satellites, cable TV, and mobile communications. Because of the higher powers involved, many radar signals are pulsed. Measuring and characterizing pulsed RF signals used in radar applications present unique challenges. Given the time domain nature of the pulsed RF signal, the best way to observe the performance of the amplifier is through time domain signal analysis. Peak Power Meter for Pulsed Radar Measurements The most critical analysis of the pulsed RF signal takes place in Figure 3c: These four screenshots show how a waveform is built through repetitive sampling techniques. The first sweep (left) shows an initial set of three data points equally 20 ns apart. The remaining three show 10, 50 and 200 sets of additional data. This method achieves the highest resolutions, allowing zoom in to fast signals hf-praxis 12/2018 67