Load-pull tuning systems provide tremendous insight into high-frequency devices under variable-impedance conditions. In terms of device and component optimization, for example, a loadpull tuner makes it possible to find the impedance-matching conditions for optimum transistor or amplifier output power or efficiency, of particular importance in the increasingly competitive wireless equipment market. The new, patent-pending High-Gamma-Tuner™ (HGT™) system from Maury Microwave (www.maurymw.com) brings a new dimension to load-pull testing by allowing for the first time direct on-wafer GSM/EDGE load-pull measurements at RF output power levels to +35 dBm. Coupled with the tuners' wide VSWR (reflection) range and low loss, the automated tuner systems help reduce modeling and process uncertainties and cut process development time for RF power transistors, amplifiers, and modules at frequencies from 825 MHz to 6 GHz.
The new HGT™ load-pull systems (Fig. 1) make it possible to make on-wafer measurements across an entire semiconductor wafer under true large-signal conditions. This makes it easier to understand device trends for wireless applications, such as for GSM/EDGE systems, including linear power, power-added efficiency (PAE), adjacent-channel power ratio (ACPR), adjacent-channel leakage ratio (ACLR), error vector magnitude (EVM), and CDP. Availability of the new tuner systems means that process development time associated with load-pull testing can be reduced by nearly 10X compared to multiple-probe and prematched tuner systems. The HGT™ Series of automated tuner systems are designed to achieve high VSWR with extremely low vibration and stable operation for on-wafer measurements. They can achieve VSWRs as high as 150:1 for on-wafer and in-fixture load-pull measurements on devices with impedances of 1 Ω or less. The automated tuners feature a standard Universal Serial Bus (USB) interface for connection to an external computer.
The HGT™ Series of load-pull tuners covers frequencies from 825 MHz to 6 GHz. Model MT981HUx1 operates from 825 MHz to 5.5 GHz while model MT981HUx2 operates from 1.8 to 6.0 GHz. Both systems are available with a variety of coaxial connector types, including 3.5-mm, 7-mm, 14-mm, Type N, and EIA 7/16 type connectors. Both systems are designed to handle power levels to 200 W CW and 2.5 kW peak envelope power (PEP). The typical repeatability is better than –40 dB across the entire Smith Chart, with maximum VSWR of 1.05:1 and maximum insertion loss of 0.3 dB.
An example may serve to demonstrate the effectiveness of the new HGT™ load-pull systems. Measurements were performed on devices from an advanced heterojunction-bipolar-transistor (HBT) process designed for GSM and EDGE applications. It is important to note that a GSM load-pull measurement at +3.2 VDC requires a loadline of about 1 Ω or less. After backing out losses due to the probes, this translates into a VSWR requirements of 150.0:1 for the load-pull system. Figure 2 shows a typical 900-MHz tuner domain at the probe tip, illustrating that the HGT™ tuner systems are more than capable of generating these demanding low impedance conditions for GSM and EDGE load-pull testing.
Using an advanced HBT wafer with on-chip bias provided by RF MicroDevices (www.rfmd.com), on-wafer load-pull measurements were performed at 900 MHz. Figure 3 shows a close up of the load HGT™ used for the measurements, including Agilent Connected Solutions from Agilent Technologies (www.agilent.com) for signal synthesis and signal quality measurements (see sidebar) and probe solutions from Cascade Microtech (www.cascademicrotech.com). Cascade offers two series of low-loss, high-power probes for use with the HGT™ systems: the ACP-L probes which are optimized for GaAs and gallium-nitride (GaN) wafer testing and the InfinityHC probes, which are optimized for silicon-based devices, such as LDMOS power transistors. Both probe families feature low 0.1-dB insertion loss and are designed for RF power levels in excess of +40 dBm CW.
Optimum operating conditions were determined using the Maury ATS 4.00 Power Environment's Advanced Sweep Plan, which can automatically determine the optimum source and load impedances and bias conditions without user intervention. Based on the Advanced Sweep Plan, optimum power and PAE trade-off conditions were achieved at collector voltage, Vcc = 3.2 V, collector current, Iceq = 300 mA, source impedance, Zsource = 8.6 + j5.4 Ω, and load impedance, Zload = 1.3 + j2.7 Ω. Figures 4a and 4b show transducer gain and PAE versus load power, respectively. The Maury HGT™ has terminated the transistor to deliver over +36 dBm CW power in an on-wafer environment. As an added bonus, the high PAE of 80 percent was achieved without the aid of harmonic loadpull tuning. Given the ever-increasing of transition frequencies in the transistor technologies used for wireless applications, the importance of harmonic loadpull tuning will decrease as high-efficiency designs, such as Class E and Class PFS, become more commonplace.
Using the same HBT wafer, EDGE load-pull measurements were performed at 900 MHz using the Maury ATS Advanced Sweep Plan, yielding Vcc = 3.4 V, Iceq = 300 mA, Zsource = 8.1 + j13 Ω, and Zload = 1.3 + j4.4 W as the optimum conditions. Special care was taken, using advanced bias network design methods, to provide constant group delay at the EDGE envelope to minimize ACPR asymmetry. These measurements obtained +29 dBm linear power at the critical 400-kHz ACPR offset target of –60 dBc, with an associated PAE of 40 percent. The EVM under these conditions was 0.3 percent less than the target of 3.0 percent.
The development of the next generation of wireless products requires a complete reassessment of the entire development pipeline, from processing back-end to supply chain optimization. Back-end process development, in particular, is amenable to improvement since it consumes significant time and resources for wafer evaluation. New product launches, which are critically dependent on optimized process performance and the associated demanding launch schedules, clearly benefit from a reduction in backend process development cycle time. The Maury HGT™ fills this critical unmet need by enabling a 10 X reduction in wafer evaluation effort by providing direct onwafer +35-dBm GSM/EDGE load-pull testing. The Maury HGT™ systems, building on the company's repeatable, accurate, and fast device characterization tools, is an evolution of load-pull tuner technology well beyond multipleprobe, prematching tuner designs.