The ‘standalone’ performance of ET PAs cannot be measured unless the shaping table is first defined. This requires measurement of the PA’s fundamental characteristics (Pout, Efficiency, Gain, Phase) over the full range of supply voltage and input power. In principle this characterization could be carried out using a CW network analyzer and a variable DC supply, but results are typically poor due to thermal effects, ranging errors and drift in phase measurements. It is also far too slow to allow load pull techniques to be used. An alternative approach is to use a pulse characterization using ATE controlled standard test equipment. This avoids the need for a high bandwidth low impedance supply and is sufficiently fast for Load Pull to be viable, but has the drawback that it is difficult to make accurate phase measurements. The last approach is to use real waveforms and to vary the shaping table to allow all combinations of input power and supply voltage to be measured. This requires a supply modulator, but is very fast, allows accurate phase information to be gathered and can also be used to characterize memory effects.
Figure 8: ET PA Characterization bench
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A ‘basic’ ET PA characterization can be used to create a quasi-static (i.e. memoryless) data model of the PA having output power, phase and efficiency as outputs and input power and supply voltage as inputs. Once the shaping table is defined, the model can be used to predict PA system performance parameters such as ACPR, EVM and efficiency for standard test waveforms.
In addition to being used for PA device level characterization, the same hardware can be used for direct verification of PA system performance using the defined shaping table (see Figure 9).
Figure 9: ET PA AM/AM and AM/PM after shaping table definition
For higher bandwidth waveforms, PA memory effects can be a significant source of non linearity. The PA output parameters (AM, PM, efficiency) now depend on time (i.e. signal history) in addition to instantaneous input power and supply voltage. Memory effects show up in the PA characterization as a ‘broadening’ of the AM/AM and AM/PM characteristics and can result from electrical time constants in input or output bias circuits, thermal time constants associated with local die heating, or technology specific ‘charge storage’ effects.