Source Pull and Load Pull Tuner

Regrettably, most semiconductor components have neither an input nor an output impedance of 50W. But since most applications require a load of 50W, these components have to be equipped with additional circuits in order to adapt them to the necessary load impedances. Often, these are required at the input as well as the output of the component. The following illustration shows the affected impedances for an exemplary amplifier:

Amplifier chips with a HF output power of 1..5 Watts often have a high-impedance input with capacitive characteristics and an output impedance of only a few Ohms. Instead of conducting costly experiments to develop low-loss adaptive circuits, it is possible to simply produce the required source impedance (Z_source) or load impedance (Z_load) by means of a tuner. The tuner at the input is designated as a source pull tuner, and the tuner at the output as a load pull tuner. The basic principle of a mechanical tuner is shown in the illustration below:

The desired impedance for exactly one target frequency can be precisely adjusted with a sleigh. Consequently, it is not possible to modify the impedance for a harmonic. If it is necessary to set the impedance of a fundamental wave and a harmonic independently, a multi-harmonic tuner should be used. These have e. g. 2 sleighs for setting two different frequencies.

Apart from the ohmic loss, a mechanical tuner may be set to any required passive impedance. Active tuners even permit the creation of negative load impedances. The picture below shows our load pull expert, Mr. Remi Tuijtelaars (bsw TestSystems & Consulting bv), while conducting experiments with an amplifier DUT, which is being characterized with automatic tuners on the input as well as the output side.

Compared to cost-effective manual tuners, automated tuners have the additional advantage of allowing the complete characterization of a DUT by means of a measurement program. As an example, it would be necessary to conduct thousands of measurements in order to capture the HF power and the S parameters for a large number of possible combinations of complex input and output impedances.

Before they are used in a measurement application, the tuners have to be characterized with a network analyzer. For this, a table will be created, which is used to record the complex S parameters of the tuner for a useful selection of mechanical positions of the tuner sleigh. By means of interpolation, it is then possible to achieve an extremely precise specification of the S parameters (or impedances) for a specific mechanic constellation with a limited number of measurement points.