Active Load Pull - Open Loop

The open loop active load pull technique replaces passive tuners with RF signal sources. By varying the amplitude and phase of the sources, any impedance can be created at the DUT interface with no limitation on Smith chart coverage. With advances made in the test and measurement arena, these signals are very stable, repeatable and can be changed very quickly.

In an open-loop active load pull system the output of the device (b2) is terminated and rather than relying upon a reflection of a passive load, sources are used to inject  signal(s) into the output of the DUT, thereby creating the reflected signal a2. The simple active tuning chain consists of the signal source, a variable phase shifter, and a variable gain stage, which are normally integrated within commercially available signal generators.

Open Loop Active
With active load-pull techniques, harmonic load-pull tuning is simplified since a multiplexer can be used to merge multiple active tuning paths, one per frequency. Any losses inherent within the multiplexers or other parts of the measurement network can be easily overcome by the amplifiers used within the active tuning chain.
4 Channel Receiver
Unlike closed-loop system active systems, these amplifiers do not have to be linear since the user specified reflection coefficient is reached using an iterative step process within the control software. Since iterations are required to set the desired reflection coefficient, measurement speed can be an issue, however, advances in both the COTS equipment utilized and through optimization of the iteration algorithms the impact of this has been reduced.
 
The cost of system amplifiers is another hurdle, although this can usually be overcome through the use of broad-band impedance transformers.
 
Open Loop Active Load Pull measurement Advantages

  • Ability to add couplers to sample the signals directly at the device output allowing for waveform measurements, which have proved key in delivering very high efficiency amplifier modes of operation.
  • Unconditional stability
  • There is no system feedback path, therefore, no chance of tuning-loop oscillation.
  • Complete control of all load/source impedances, inside or outside the Smith Chart
  • Any impedance within the power budget of the system can be defined within the system and presented to the DUT
  • Allows for source/load pull at high power levels
  • The Mesuro architecture can be configured to cope with power levels of up to 150W CW. Pulsed capability can be added to allow for higher power devices to be tested.
  • All electronic solution ideal for on-wafer measurements
  • The system has no moving parts, therefore, is ideal for on-wafer measurements as there are no vibrations
  • High measurement accuracy for source/load reflection coefficients
  • Suitable for generation and measurement of modulated and pulse signals
  • Simple calibration process based on traditional reference standards
  • Standard SOLT or TRL/TRM calibration processes are employed ensuring the system is quick and simple to calibrate and that the process if familiar to users.
  • Fast calibration process - As the impedance is continually measured, no need for time consuming pre-calibration of tuners
  • Capability can be expended to baseband frequencies
  • Default broadband 50ohm impedance maximizes device stability
  • Fast Tuning