Challenges of Passive Load Pull and Wideband Modulated Signals

When working with modulated signals, for a well controlled linearity behavior of the DUT, the reflection coefficients offered to the DUT should ideally be constant (not vary versus frequency) within the modulation bandwidth at the fundamental, as well as in all related frequency bands at baseband and harmonic frequencies. This situation is approximated in real circuit implementations, where the matching networks are placed directly at the reference planes of the active device.

In conventional load-pull setups, however, the actual physical impedance is always located at some distance from the DUT, which is much larger than for any practical matching network. This distance, as well as any physical length within the tuning element itself (such as the position of the probe in mechanical tuners), yields very large electrical delays causing rapid phase changes of the reflection coefficients versus frequency.

It is clear that these large phase deviations represent nonrealistic circuit conditions and will cause measurement errors such as IM3 asymmetry, spectral re-growth and EVM degradation. In general, maintaining the reflection coefficients constant over frequency is getting more and more difficult with the increase in modulation bandwidth of communication signals, not only in practical circuits, but definitely in load pull measurement setups.

Wideband Impedance Control

The MT2000 is the best commercially-proven solution capable of wideband impedance control of up to 500 MHz bandwidth at the fundamental, harmonic and baseband frequencies and is ideal for:

• Using ACPR and EVM measurement data in the design of wideband PA circuits
• Improving PA linearity based on controlled baseband terminations
• Evaluating the performance of a DUT under realistic antenna load conditions
• Evaluating the performance of DUT under different matching network topologies

The MT2000's wideband impedance control capabilities include:

• Library of standard commercially available modulated signals
• Utility to define custom modulated signals
• Automatic signal pre-distortion to create a clean modulated signal at the DUT reference plane
• Wideband impedance control as follows:
• Ability to set all impedance over the modulated bandwidth at a single impedance point (i.e. all frequency components of an 80 MHz 5G signal should be tuned to 5Ω)
• Ability to set user-defined phase delay of impedance vs frequency over the modulated bandwidth (i.e. a 0.1 degree/MHz phase delay resulting in an overall phase shift of 8 degrees on the Smith Chart for an 80 MHz 5G signal)
• Ability to load S1P file (user-created, from circuit simulator…) defining impedance vs frequency over the modulated bandwidth. Ideal for evaluating realistic matching network designs (i.e. stub vs transmission line) and evaluating DUT performance under realistic antenna load response
• Vector signal analysis of modulated signals
• Adaptive averaging enhances measurement speed without sacrificing accuracy
• Import and export I and Q baseband waveforms for offline digital pre-distortion load pull (DPD)
• Standard measurement parameters include ACPR, EVM, spectral mask; custom parameters

Fundamental-frequency modulated load pull of 20 impedance states and power sweep at 26 power levels, where all impedances over the modulated bandwidth of 200 MHz are set to a single impedance state.

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