Introduction
When working with modulated signals, i.e. for 4G, 5G and WLAN applications, 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.
Mixed-Signal Active Load Pull
Mixed-signal active load pull is an extension of open-loop active load pull, where a new signal is created to satisfy the equation gamma=a2/b2. However instead of looking at a single frequency (or a set of harmonic frequencies), we are looking at a wideband set of frequencies covering the modulation bandwidth of a realistic communication signal. By using wideband AWG and A/D, it is possible to control the impedance presented to the DUT over a bandwidth of up to 1000 MHz. As such, active load pull can be extended to modulated signals as well as single-tone and two-tone CW and pulsed-CW signals, and can compensate for phase-delay effects introduced into measurement systems that might degrade RF performance. The wideband signal generation and analysis can also be used with single-tone signals, where the signal is divided in time, and the record which would contain the wave modulation for a single modulated impedance can contain several wave magnitudes and phases representing many single-tone impedances. In this manner, it is possible to load pull 1000 impedance/power states per minute.
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