13 April 2010
System pulls its weight
Making accurate nonlinear measurements in power amplifier test.
With pressure for 3G coverage increasing and the imminent introduction of 4G systems, design activity for digital communication rf components has never been greater. At the forefront of this activity is power amplifier (PA) development and top of the list here is power added efficiency (PAE). High PAE brings longer battery life for handsets and better coverage for operators, as well as lower electricity bills.
Digital modulation schemes result in rf carriers with high peak to average ratios (PAR). PA designers must consider the consequences of amplifying a communication signal with a high PAR, while maintaining linearity and an acceptable error vector magnitude. A linear vector network analyser (VNA) provides essential information on the performance of a PA operating under linear conditions. However, when the PA is designed for compressed nonlinear operation at higher power levels, the VNA must provide additional data.
A nonlinear VNA should be able to measure harmonic content, as well as the fundamental signal, and provide the performance data needed to optimise nonlinear PA design.
A critical element of nonlinear analysis is load pull measurement, especially harmonic load pull. Load pull analysis is necessary to maximise active device performance. In order to optimise nonlinear device performance, the match presented to the device must be optimised for both fundamental and harmonic frequencies.
Design of a nonlinear PA begins at the wafer level. The on wafer device will ultimately be embedded into a 50? system, but on wafer active devices do not have a 50? input impedance and the output impedance of most high power devices is normally 1 to 2?. So a matching network is necessary to transition input and output to a 50? system. Since the device is rich in harmonics, the matching network must include the optimum match for fundamental and harmonic components.
The primary objective of a nonlinear VNA measurement system is to provide information on device performance relative to the source and load impedances, so the load pull process is at the heart of nonlinear measurement.
In a typical load pull system, the coupler is located outside the tuner and monitors the power output of the device as tuner impedance is varied.
This requires the tuner to be precalibrated and measurement accuracy is determined by the repeatability of the tuner, cables and connectors after calibration. In this configuration, the tuner is controlled by vendor software for both calibration and control, creating a complex relationship between the tuner software and nonlinear VNA software. Since many devices need to be tuned over a large area of the Smith Chart, calibration takes hours.
A system developed by Anritsu and High Frequency Engineering (HFE) inserts an ultra low loss coupler between the device under test (DUT) and tuner. Because the VectorStar system locates the coupler next to the DUT, high measurement accuracy of the source and load impedance at the DUT is achieved. In addition, a method of simultaneously monitoring the impedance and DUT performance in real time is created, supporting real time tuning and eliminating precalibration.
Because the optimum PAE and minimum PAE can often be quite close, harmonic load pull is critical to optimum design: it not only finds the location for best PAE, gain and max power, it also identifies areas to avoid.
One of the design requirements of nonlinear classes of operation is the need to fully reflect harmonics back to the device to minimise harmonic content at the output and maximise PAE performance. This means that, during the analysis portion of the load pull process, the harmonics must see a full reflection at the device output. When the load pull tuner provides maximum reflection, any insertion loss between the tuner and the device will result in lower gammas at the device.
These measurements are taken on wafer using probe stations and probe tips, so the insertion losses, with long rf cables and lossy probe tips, can be substantial, resulting in far from ideal gammas at the device.
With traditional load pull systems, the monitoring coupler is located after the tuner and susceptible to mismatch loss errors that degrade power output measurement accuracy. At high gammas, the mismatch loss error in the traditional system is substantial; in the Anritsu/HFE system, mismatch loss errors are minimised since the coupler is located next to the DUT and monitors power output of the device before the tuner.
The Anritsu/HFE nonlinear system can be installed using passive or active tuners. When configured with the Anritsu/HFE active loop tuner, gammas up to 1 at the DUT port can be delivered.
Load pull analysis provides measured data of an operating device under real conditions. Assuming the device represents typical performance for all devices on the same wafer (and subsequent wafers), the data can be used to design the optimum network. Alternatively, data can be exported to an eda program to create, run or improve models.
Because of the large quantity of data acquired during nonlinear measurements, it should be formatted in a way that is easy to store, convenient to open and view, organised in a flexible manner and easy to share.
These goals are behind the effort to create a nonlinear open standard data format. The OpenWave Forum is an alliance of rf and microwave firms designed to create and promote a unified and transparent data exchange format for large signal nonlinear simulations, measurements and models.
MMSNT_LP software is structured as a multidocument and multiview application. Each document represents a particular measurement taskand the software can display power sweep curves, time domain waveforms, load pull contour maps, eye diagrams, and I/V parameters simultaneously on the same screen. Each document controls the presentation of the main application menu and toolbars and the respective measurement is triggered and updated by the corresponding window.
Calibration of the nonlinear system is controlled by the MMSNT_LP software. Based on a computer analysis of the topology conditions, the software generates an optimised standard sequence that leads the user through the procedure. A range of options provides the opportunity to select the best possible accuracy for a given set up.
Another advantage of active loop modules is the ability to upgrade the system for load pull measurements of differential devices. Since it is possible to monitor actively the real time impedance presented to the device, it is also possible to actively and independently tune the input of the device to specific phase positions. For a differential device, this means it is possible to actively control the differential input and tune the device under varying source and load conditions.
Author profile:
Steve Reyes is a product marketing manager with Anritsu.
Author
Steve Reyes
Supporting Information
Websites
http://www.anritsu.co.uk
Companies
Anritsu Ltd
This material is protected by Findlay Media copyright
See Terms
and Conditions.
One-off usage is permitted but bulk copying is not.
For multiple copies contact the
sales team.