Hybrid Load Pull Systems
When mechanical tuners are used, this may lead to limitations regarding the realization of load impedances at the edge of the Smith chart, i.e. close to the case of total reflection. These limitations are reinforced when measurements are conducted on wafer probers, since in this case the losses created by the tuner are increased by the ohmic losses of the measurement probe and the HF cables. One example for such a measurement setup is shown in Figure 1. It shows that a large proportion of the possible impedances in the Smith charts are no longer available as load impedances for the device under test.
The following options are available if it is necessary to reach the edge of the Smith chart, or even to setup negative impedances (negative impedances represent reflection amplifiers, which in this case allow a free choice of the phase position):
- Hybrid Load Pull methods
Figure 2 shows the principle behind the hybrid load pull process for the simplest example, where the load impedance is tuned for one frequency. First, the mechanical tuner is set to a fixed impedance value. In case the amplifier and the synthesizer on the right-hand end of the tuner are switched off, representing a load of 50Ω, the DUT will now detect the load impedance that has been set with the tuner. It is now possible to generate an HF signal with the synthesizer and the amplifier, which may be "fed back" through the tuner into the DUT output. Similar to active load pull systems, this allows an electronic adjustment of the load impedance. The prerequisite is that the synthesizer and amplifier can be synchronized to the HF signal at the input of the DUT, and that the phase can be precisely adjusted. This is achieved by calibrating the S parameter and the power by means of a bidirectional coupler between DUT and tuner.
An appropriate external device or an appropriately equipped network analyzer may be used as the synthesizer. The tuner control software supports efficient calibration, control, caption of measurement values and data representation of the entire measurement station, including all devices. The user is comfortably guided through the entire complex calibration procedure by means of clearly understandable menus. This allows the quick creation of precise measurement sequences without any detailed understanding of the procedure.
Figures 3 and 4 show how the procedure can be expanded to include several frequencies. As an example, in illustration 3 a triplexer is used for tuning 3 active frequencies: a mechanical tuner for the base frequency (f0) and two active sources for the first harmonic (2·f0) and the second harmonic (3·f0). Appropriate triplexers (or duplexers for 2 frequencies) can be found on the following website: http://www.bsw-ag.com/de/komponenten-diplexer-duplexer-triplexer.html.
In this example, the triplexer was installed between the measurement coupler and the tuner. The advantage of this setup is that the impedances of the harmonics are largely independent of each other and may be adjusted based on the basic wave. However, a disadvantage of this setup is the fact that triplexers with high selectivity are only available for relatively low bandwidths. In order to cover a broad frequency bandwidth, it is therefore necessary to use several triplexers, which have to be exchanged accordingly
Broader bandwidth coverage may be achieved with a power splitter, as shown in Figure 4. There are power splitters available which cover bandwidths of up to several octaves. In this case, it is however necessary to conduct the HF signals of the synthesizer through the tuner to the DUT output; due to the adaption losses of the tuner, this would require a higher output power of the amplifier.
There are many possible combinations for setting up hybrid load pull systems. The deciding factors are the number of required frequencies and HF power requirements. Compared to the new procurement of an active load pull system, a commercial advantage may be achieved by integrating available mechanical tuners and synthesizers. If you are interested, we will gladly prepare a technical and commercial proposal for your application which takes these factors into account.