Improvement of Load-Pull Characterization of Microwave Power Transistors: Measurements with Modulated Signals
Fig. 1: Typical measured gain distortion (l) and phase distortion (r) for a pre-amplifier used in the measurement setup.
Fig. 2: Load-pull measurement system with wideband modulated measurement capabilities.
One of the target applications for the GaN-HEMT transistors developed at FBH is power amplifiers (PAs) for telecommunication systems. Modern telecommunication systems use relatively wideband radio channels and complex modulation schemes to increase the data transfer rate. Therefore, the respective microwave PAs have to work over large bandwidth with high peak-to-average-power-ratio (PAPR) signals. Since the instantaneous power of such signals changes continuously and nevertheless the amplifier has to work in a linear region, the overall efficiency of the PA becomes very low, in the range of 5 - 15% for the modulated signal. Dedicated measurement setups, known as source- and load-pull setups, allow for analyzing and improving the trade-off between linearity and efficiency. This is accomplished by presenting a similar environment to the transistor as in a power amplifier application. Thus, the transistor behavior under multi-tone or modulated carrier excitation can be determined. At FBH, measurement capabilities were improved by this feature last year. The findings are reflected in improved circuit design and advances in device technology.
Modulated measurements are conducted using either simple two-tone modulation for intermodulation distortion (IMD) analysis or using more complex digital modulation schemes. Two-tone IMD is mainly used for technology comparison and improvement. With a complex modulation similar to the standards used in the applications, WCDMA, WLAN or Long-Term-Evolution (LTE), the transistor performance for a typical signal can be measured and compared to the specifications. Moreover, complex modulations provide deeper insight into the non-linear characteristics of the transistor. Gain and phase distortion from modulated signals represented as AM/AM and AM/PM graphs also provide information about so-called memory effects in the transistor, as shown in Fig. 1. Therefore, wideband complex signals can be used not only for benchmarking but also for technology improvement. The modulated measurements place specific demands on the measurement setup, which is shown in Fig. 2. All parts of the system have to be well characterized. Non-linear effects at the input of the measurement object, the device under test (DUT), have to be compensated using linearization. At FBH, this is accomplished using an in-house developed digital pre-distortion (DPD) software.
Publications:
O. Bengtsson, S. Shevchenko, R. Doerner, P. Kurpas, W. Heinrich, "Load-Pull Investigation of GaN-HEMT for Supply Modulated Applications", 2011 German Microwave Conference, Darmstadt, Germany, March 2011.
D. Gruner, R. Sorge, O. Bengtsson, A. Al Tanany, and G. Boeck, "<link /fileadmin/fbh-berlin/english/ver10/pub97.htm _blank internal-url-new-window>Analysis, Design, and Evaluation of LDMOS FETs for RF Power Applications up to 6 GHz", IEEE Trans. Microwave Theory Tech., vol. 58, no. 12, pp. 4022-4033, Dec. 2010.
D. Gruner, R. Sorge, O. Bengtsson, and G. Boeck, "<link /fileadmin/fbh-berlin/english/ver10/pub99.htm _blank internal-url-new-window>CMOS Compatible Medium Voltage LDMOS Transistors for Wireless Applications up to 5.8 GHz", EuMIC-2010, Paris, France, September 2010.
I. Jazvic, O. Bengtsson, W. Heinrich, and G. Boeck, "Signal Generator Non-linearity Analysis for DPD Applications", 2nd IEEE Germany Student Conference, Hamburg, Germany, May 2010
D. Gruner, R. Sorge, O. Bengtsson, A. Z. Marcos, and G. Boeck, "<link /fileadmin/fbh-berlin/english/ver10/pub39.htm _blank internal-url-new-window>A 1 W Si-LDMOS Power Amplifier with 40% Drain Efficiency for 6 GHz WLAN Applications", IEEE IMS2010, Anaheim, USA, May 2010.
FBH research: 20.04.2011