RF Test Equipment Series: Load Pull Testing Primer

Load Pull measurements is a class of non-50 Ohm measurements that is used during the design, model extraction, and performance testing of a variety of RF/Microwave devices. A main topic of discussion with load-pull testing is the evaluation of amplifier designs in a large signal sense. Load-pull testing essential consists of taking signal measurements from the reflections of a device while a device is connected to a varying load. This type of testing reveals the behavior of a device under varying load conditions, which is often essential for designs that may face a wide range of load impedances in practice.

The results of load pull testing are several sets of data, consisting of measured a- and b-waves, as well as other parameters that can be calculated from the results. These parameters include gain, power, and efficiency. The large-signal input impedance of the DUT, power sweep curves at various impedance states, and the iso-contours of the output power can also be plotted in real-time during the measurements.

Typical Load Pull testing involves a signal source, some type of signal measurement equipment, and a variable load/tuner that can handle the power of the operating power of the device under test (DUT). Several types of signal measurement equipment can be used, such as spectrum analyzer, RF power meters, vector receivers, and etc. The type of source, tuner, and receiver determines the methods of testing, of which, each has their advantages and disadvantages.

Types of Load Pull

Basic, Traditional, and/or Scalar Load Pull

• • • Involves the use of a signal source, input tuner, DUT, output tuner, and RF power meters
  • • A relatively simple and potentially inexpensive setup that is highly modular and can readily be adapted to high precision measurements
  • • The instrumentation is generally all scalar, and a network analyzer is required to yield vector-based parameters
  • • The actual delivered power is unknown due to source tuner and DUT mismatch
  • •De-embedding to the DUT plane is required

• Vector-Receiver Load Pull (Real-time Load Pull)

• • •Requires a vector-receiver, signal source, source tuner, and input tuner
  • •Measurements are referenced to the DUT plane and no de-embedding is required.
  • •Available and actual power delivered is measurable
  • •Vector measurements, such as AM/PM can be made
  • •Avoids inaccuracies introduced by impedance tuners
  • •Harmonic and intermodulation parameters can be measures
  • •Unable to measure modulated signals
  • •Use of narrow-band ADCs limits wideband signal decoding capability
  • •Couplers are required between the signal source and DUT reduce achievable VSWR to the DUT

• Open-Loop Active Load Pull

• • •Much like Vector-receiver load pull with the difference that mechanical impedance tuners are replaced with active tuning chains made up of signal sources with magnitude and phase control circuitry.
  • •Can be modified to enable harmonic load pull with addition of a multiplexer, passives, coupler, and combiners.
  • •Can reach Gamma levels greater than one for very large VSWR measurements.
  • •Speed advantages compared to mechanical tuning
  • ••Design may be more readily adapted to probe station
  • Relatively expensive compared to other load pull methods
  • •Cannot measure modulated signals

• Hybrid-Active Load Pull•Includes both active and passive load pull.

• • •Combinations can mitigate some of the disadvantages of each distinct method.
  • •Leads to a mixed-cost and mixed-complexity design

• Mixed-Signal Active Load Pull

• • •A form of active load pull that uses wideband ADCs and DACs along with upconverters/downconverters to enable wider bandwidth operation.
  • •Enables faster measurement speeds
  • •Wideband impedance control
  • •Modulated signals with bandwidths above 100 MHz can be generated and measured

Useful Load-pull Testing Links

• •Pasternack VNA Calibration Kits
• •Pasternack Phase Stable (VNA) Coaxial Cables
• •Pasternack Cable Generic Test Cable Assemblies
• •Pasternack Skew Matched Pair Coaxial Cable
• •Pasternack Precision Coaxial Adapters
• •Pasternack In-series and Between-series Adapters
• •Pasternack RF Coaxial Probe
• •Pasternack Coaxial Probe Leads
• •Pasternack Antennas