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Tải xuống
hoặc thậm chí là 0,75 trong dải tần số cao hơn lò vi sóng. Đây là một vấn đề thực sự, đặc biệt là khi mô tả các bóng bán dẫn, năng lượng cao có tối ưu trở kháng đầu vào và đầu ra là rất thấp và nằm gần với các cạnh của bảng xếp hạng Smith. | 68 NONLINEAR MEASUREMENTS Figure 2.7 A typical set-up for scalar passive source load pull with individual control of the harmonic impedances or even 0.75 in the higher microwave frequency range. This is a real problem especially when characterising high-power transistors whose optimum input and output impedances are very low and lie close to the edge of the Smith Chart. A simple solution includes pre-matching circuits that is impedance transformers between the DUT and the tuners this solution allows for a better accuracy within the transformed region. The pre-matching can also be included in the tuners. Obviously the pre-matching circuits must be characterised in terms of S-parameters and replaced by different ones whenever the region of interest changes. A radical solution to the losses problem is the active-load approach. A possible scheme is the two-path technique 11 the signal source is split and fed both to the input of the DUT and to the output of the DUT after amplification and phase shift Figure 2.8 . The output reflection coefficient seen by the DUT is the ratio of incident to reflected wave at its output port aL _ rL 7T 2-1 bL Now bL is the wave coming out of the DUT while aL is the wave injected from the output path the amplitude and phase of the latter are easily set by means of the variable attenuator and phase shifter in the output path. In this way any ratio can be synthesised even greater than one in amplitude since the amplifier in the output path overcomes all the losses. The value of the output reflection coefficient is checked on-site by means of the output directional couplers and a VNA. The two-path technique is an easy and stable technique for active-load synthesis. However when simulating an actual power amplifier the value of the output load must be kept constant for increasing input power levels and also for the associated increasing DUT temperature. This implies that while bL changes because of the above aL must LOAD SOURCE PULL Source .
