Load pull alters the load impedance of an RF device to measure performance under extreme conditions. RF amplifiers are designed for a range of frequencies, and load pull can measure power transfer characteristics, including transmission efficiency and harmonic output. Impedance matching involves compensating for capacitive effects with inductive elements, aided by a Smith chart. Load pull measurements ensure acceptable circuit performance.
A load pull is the alteration of the load impedance of a radio frequency (RF) load for the purpose 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 a typical impedance of 50 ohms, which is the nominal line impedance. Load pull measurements allow you to observe circuit characteristics that can help you improve a circuit design for better performance under extreme signal conditions and operating conditions.
In radio electronics, an RF power amplifier is ideally classified as purely resistive at its center frequency. An RF amplifier is designed to operate at a certain range of frequencies, so performance measurements at frequencies other than the center frequency will be needed. Usually, there is a reduction in performance at the extremes of the frequency range. Extreme low and high frequencies for the range can result in amplifier gain that is half that in the center frequency.
Load pull changes the impedance of the load for testing power amplifiers, while source pull changes the output impedance of the signal source. For example, the output impedance of a power amplifier could be changed to measure the resulting power transfer characteristics. This could include measuring the transmission efficiency, determining the ratio of the actual power reaching the load to the actual power sent by the transmitter. Harmonic Load Traction takes note of the output impedance and line impedance to harmonics, which are frequency multiples of the operating frequency. For example, twice 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 involving the capacitive and inductive characteristics of both the radio transmitter output and the transmitter. Capacitive reactance in a circuit is caused by the proximity of the circuit nodes causing the production of an electrostatic field from the difference in voltages. The result is a tendency for the voltage to lag the flow of current. This mechanism causes the need to compensate for capacitive effects with inductive elements in the circuit. The inductive element can be a lumped inductor or it can be a distributed inductance due to the lengths of the loop wires or copper traces.
A tool called a Smith chart aids in the impedance matching process. Smith’s graph indicates the purely resistive circuit as well as the two cases where a reactance dominates. A circuit can be capacitive or inductive if it is not purely resistive. In a purely resistive circuit, the load draws all of the input power. Load pull measurements can ensure that circuit performance at small and large signal levels is acceptable by considering criteria such as transmission efficiency and harmonic output.
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