Agilent 5992-0191EN Impedance Matching in the Laboratory - Application Note c20141017 [13] free download

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Keysight Technologies Impedance Matching in the Laboratory University Engineering Lab Series - Lab 4 Application Note Introduction The reception and transmission of small signals into amplifier stages and the transmission of large, powerful signals into loads both require careful attention to minimizing losses. At RF and microwave frequencies reflections of the propagating wave can drastically undermine the efficiency of transmit- ted power or the signal-to-noise ratio of transmitted information. Proper matching of impedances is needed to minimize these reflections and insure that any signal which reaches a load is actually ab- sorbed by that load as useful power. Proper impedance matching is a fundamental skill that is central to all RF and microwave engineering. The ability to design and execute proper impedance matches is a crucial and highly sought skill which can make or break a career as well as a specific design. The mathematical elements of impedance matching have already been investigated using SPICE and other tools such as the Smith chart. In this lab, some practical laboratory approaches to the problem of impedance matching will be examined. Network analyzers are the essential tool for assessing and tun- ing an impedance match. Impedance matching is often viewed as a difficult art because impedance matching involves an interplay of measurements and design calculations. However, there are devel- oped methods, and mastering these is essential to RF and microwave engineering. 03 | Keysight | mpedance Matching in the Laboratory, University Engineering Lab Series - Lab 4 - Application Note A quick overview Consider first the following problem. A load resistance of RL = 10 is to be fed by a Z0 = 50 transmission line. If the line were directly connected to the load, this would produce a reflection of = -0.667, which means that the load only absorbs ||2 = 0.555 of the incident power, and the other 1 - ||2 = 0.445 fraction gets reflected back to the generator. The voltage standing wave ratio (VSWR) for this situation is 5.00, indicating deep standing waves which will make the line impedance very sensitive to its length and potentially cause problems for the generating source. One approach to this problem is to add a series matching resistance of Rsm = 40 , which will bring the total of Rsm + RL equal to Z0 , as shown in figure 1. This indeed creates a perfect impedance match between the load and the line, with the reflection coefficient now reduced to = 0. However, the introduction of Rsm reduces the power that the load RL receives. Since these two resistances are in series, the load RL only receives 1/5 of the power leaving the transmission line. The matching network of Rsm thus introduces a huge insertion loss of 10 log (5.0) = 7.00 dB. Rsm = 40