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What Is a Load Pull?

Geisha A. Legazpi
Geisha A. Legazpi

A load pull is the alteration of the load impedance of a radio frequency (RF) load for purposes of measuring the resulting performance of RF power devices for large signals and extreme conditions. The device under test could be an RF power amplifier with typical 50-ohm impedance, which is the nominal line impedance. Load pull measurements make it possible to observe circuit characteristics useful in improving the design of a circuit for better performance under extreme signal conditions and operating conditions.

In radio electronics, an RF power amplifier is ideally rated as purely resistive at its center frequency. An RF amplifier is designed to operate at a certain range of frequencies, thus there will be performance measurements needed at frequencies other than the center frequency. Usually, there is diminished performance on the extremes of the frequency range. The extreme lowest and highest frequencies for the range may result in an amplifier gain that is half of that in the center frequency.

Man with a drill
Man with a drill

Load pull changes the impedance of the load for testing power amplifiers, while source pull changes the output impedance of the signal source. For instance, the output impedance of a power amplifier could be modified to measure the resulting power transfer characteristics. This could include the measurement of transmission efficiency, determining the ratio of the actual power that reaches the load to the actual power that was sent from the transmitter. Harmonic load pull takes note of the output impedance and line impedance at harmonics, which are frequencies that are multiples of the operating frequency. For instance, double the operating frequency is the second harmonic, while triple the operating frequency is the third harmonic.

Impedance matching between the radio transmitter and the transmission line requires electrical conditions that involve the capacitive and inductive characteristics of both the radio transmitter output and the transmitter. The capacitive reactance in a circuit is caused by the proximity of circuit nodes that cause an electrostatic field to be produced by the difference in voltages. The result is a tendency for the voltage to lag the current flow. This mechanism causes a need to compensate the capacitive effects with inductive elements in the circuit. The inductive element may be lumped inductors or may be distributed inductance due to the lengths of the circuit wires or copper traces.

A tool called a Smith chart aids the process of impedance matching. The Smith chart indicates the purely resistive circuit as well as the two cases where a reactance dominates. A circuit may be capacitive or inductive if it is not purely resistive. In a purely resistive circuit, the load absorbs all input power. Load pull measurements can assure that the performance of the circuit at small and large signal levels is acceptable, considering criteria such as transmission efficiency and harmonic output.

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