Load Pull with X-Parameters Produces Instant Large Signal Models Preliminary Application Note October 8, 2008 Abstract Maury Load Pull with the Agilent NVNA produces an instant large signal model, which can be loaded easily (drag and drop) into Agilent ADS for circuit simulation. This process is a breakthrough for the industry. The accuracy of the model is derived from actual large signal measurements. The range of validity is based on the sweep plan selected by the user, which can cover a range of impedance, input power, bias, and frequency. The model is independent of the specific device technology, and can be used in simulation for power amplifier design, including multi-stage, Doherty, or other complex circuits. Introduction Much work has been devoted to large signal model development at microwave frequencies, because accurate simulation of a power device with varying drive and impedance conditions would allow design of complex or multi-stage power amplifier (PA) circuits. Analytical or compact models are commonly used, and consist of circuit models where some elements are non-linear. The parameters of this type model are generally determined by a series of small signal and DC measurements. But this is generally a time consuming process, and often is accurate only over a limited range of operation. The PHD large-signal model solves the extrapolation problem because it is based on device xparameter measurements under actual large-signal operation. X-parameters are the mathematically correct superset of s-parameters, valid for nonlinear (and linear) components under large-signal (and small-signal) conditions. X-parameters include the complex magnitude and phase relationship between harmonics and intermodulation frequency components, and also the relationships between all those frequencies for a given drive amplitude and frequency. Load pull measurements have become widely used because they allow direct measurement of the device under test (DUT) at actual operating conditions. But in the past, non-linear circuit simulators could not easily use load pull data to automatically analyze or optimize power amplifier circuits. Load Pull with X-Parameters Combining load pull with x-parameters now provides a method to instantly use load pull data as a large signal model. The setup uses the Maury load pull system with the Agilent NVNA. It provides automated measurements over a range of impedance, input power, bias, and frequency. At each measurement point, the x-parameters are measured along with any desired scalar measurements. When all the measurements are complete, they are saved into a single xparameter data file. The x-parameter file generated by the Maury load pull system is then loaded directly into Agilent ADS as a PHD model, with drag and drop simplicity. ADS can then simulate a circuit over the 1 Load Pull with X-Parameters Produces Instant Large Signal Models Preliminary Application Note October 8, 2008 measured range using the PHD model to represent the power device. This allows design of complex PA circuits, such as multi stage amplifiers or Doherty amplifiers. Figure 1 – Simulated vs. measured power contours (left) and PAE contours (right) 20 0.30 15 0.25 10 0.20 5 0.15 0 0.10 -5 0.0 0.2 0.4 0.6 0.8 0.05 1.0 S im u la te d C u r re n t M e a s u r e d C u r re n t S im u la te d V o lta g e M e a s u r e d V o lta g e The PHD model accuracy comes because it is based on actual large signal measurements. The model is independent of the specific device technology, and the range of validity is determined by the user when the measurement sweep plan is set up. The design accuracy should therefore be as accurate as the load pull measurements. Figure 1 shows an overlay of simulated vs measured output power and power-added efficiency (PAE), and Figure 2 shows an overlay of simulated vs. measured time domain-waveforms. Figure 2 – Simulated vs. measured time domain waveforms Equipment required for Load Pull with X-Parameters Maury Microwave Tuners – select from: Model MT981BU, 0.4 to 4 GHz Model MT982EU30, 0.8 to 8 GHz Model MT983A01, 4 to 26.5 GHz MT993A Power Measurement Software MT993D03, X-Parameter Measurement Option Other options are available, contact Maury Sales for more details Agilent Technologies Model N5242A, Performance Network Analyzer (PNA-X) Option N5242A-400, 2 ports, dual source Option N5242A-419, Add extended power range and bias-tees to 4 port analyzer Option N5242A-423, Add internal combiner and mechanical switches to 4 port analyzer Option N5242A-080, Frequency offset measurements Option N5242A-510, Non-linear component characterization Option N5242A-514, Non-linear X-Parameters Model U9193C (2 each required), 26.5 GHz Comb Generator Model U2002, 26.5 GHz USB power sensor (or other Agilent power meter) Other options are available, contact Agilent Sales for more details 2