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THEORY OF LOAD AND SOURCE PULL MEASUREMENT

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Load pull consists of varying or “pulling” the load impedance seen by a device-under-test (DUT) while measuring the performance of the DUT. Source pull is the same as load pull except that the source impedance is changed instead of the load impedance.

Load and source pull is used to measure a DUT in actual operating conditions. This method is important for largesignal, nonlinear devices where the operating point may change with power level or tuning. Load or source pull is not usually needed for linear devices, where performance with any load can be predicted from small signal x-parameters.

Calibrating to measure output power and gain consists of measuring the available input power at the power source reference plane and the coupling value of the directional coupler. If the coupler had perfect directivity, then coupling could be measured with only a short at the source power reference plane. However, finite directivity causes the apparent reflection to vary with reflection-phase, so a more accurate coupling value is found by taking the average of both short and open measurements. This minimizes directivity errors, although good coupler directivity is still important for the best accuracy.

Once the available input power and coupling are known, the output power, transducer gain, and power gain can all be measured with any combination of source or load impedance. Output power is the power delivered to the load. Transducer gain is the ratio of delivered output power to available input power. Power gain is the ratio of delivered output power to delivered input power.

The objective of the measurement is to get the power and gain values at the DUT reference planes. Although the tuners are very low loss, bias tees and other components may be included as part of the “ stub tuner” characterization, so the loss must be considered. To get the output power at the DUT reference plane, the dissipative loss of the load tuner is added to the raw measured output power. To get the available input power at the DUT reference plane, the dissipative loss of the source tuner is subtracted from the calibrated available input power. To get the delivered input power at the DUT input reference plane, the reflected power at the source is subtracted from the calibrated available power at the source. The dissipative loss of the source tuner is then subtracted from the result to shift from the source power reference plane to the DUT input reference plane.

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