What are Load Pull Measurements?

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- everything RF

Jun 5, 2021

Load Pull is a technique that measures the characteristics of a device while modifying the load impedance using an impedance tuning system. The objective of carrying out load pull measurements is to identify the optimum operating point for an impedance match that ensures maximum power transfer between network stages while operating under large-signal conditions. This increases the overall efficiency of an RF system along with minimizing thermal issues, dissipation, and signal reflections.

A traditional Load Pull system measures the power injected (available) into the input of the DUT (Usually a Power Amplifier or Transistor), the power extracted (delivered) at its output and the transducer gain at different impedances. This data is used to create load pull contours on a smith chart that show device behavior as the load impedance varies. 

This also allows for the development of matching networks that maximizes power transfer while minimizing reflections and standing waves.

What is Load Pull Contour Mapping?

Load pull analysis is used to map a set of contours (typically on a Smith Chart), which determine the maximum power output that can be achieved with a given load. These contours are very useful in calculating the actual impedance a device should see when it is being used.

Load Pull Contour

How are the contours constructed on a Smith Chart?

The device under test (DUT) operates between two impedances, one at the input and one at the output. Between these impedances sit two impedance tuners, an input tuner, and an output tuner. The function of the input tuner is to adjust the impedance matching such that the large signal input power is always constant, even when the output impedance and the output tuner are being adjusted.

Initially, the closest match at the output is found for optimum output impedance. This is done by adjusting both the input and the output tuners to get the maximum constant output power. This forms the center of the load pull loci and is a single impedance.

Then the output impedances are changed, and the input tuner is adjusted to provide conjugate matching and thus constant input power. This is repeated as many times as possible and for each constant output power point, a set of loci are generated which provide the impedances which provide that power. These loci are shown on Smith Chart above. Each contour represents the maximum output power achievable with a given load impedance.

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