Document 1077-1 Revised 01/26/21 Wideband Transformers and Inductors Using Baluns and RF Components for Impedance Matching Impedance Matching Impedance mismatch in a circuit results in energy be- ing reflected back to the source, reducing the amount of power available to the load and possibly causing damage to the power source. Matching the output impedance of the power source to the input impedance of an electrical load maximizes power transfer from source to load. By matching the impedance of the source and load, circuit designers protect the power source from reflected energy and optimize circuit efficiency. A symmetrical balanced line has two conductors having equal current in opposite (differential) directions. Twisted pair cable and ribbon cable are examples of balanced line cables. An asymmetrical unbalanced (single-ended) line has one conductor connected to ground, such as coaxial cable. When a balanced source is to be isolated from an un- balanced load, the impedance matching transformer is referred to as a balun. If the impedance of the load matches that of the source, impedance matching is not required, and the balun has a 1 : 1 impedance ratio. When the load impedance is mismatched to the source in a 1 : N imped- ance ratio, a 1 : N impedance ratio transformer is required. A classic example of an impedance mismatched circuit is a 300 Ohm ribbon cable (balanced) from an antenna attached to 75 Ohm coaxial (unbalanced) receiver input. Without a balun to match the impedance of the load to the source, energy is reflected back from the relatively low-power input signal of the antenna, providing a reduced signal to the receiver. 300 Ohms 75 Ohms Balanced (Differential) 4 1 Impedance Ratio : Unbalanced (Single-ended) 300 Ohm to 75 Ohm Balun Baluns provide a 180° phase shift and ideally equal bal- anced impedances. Wirewound wideband RF transformers make excellent broadband baluns, providing two 90° phas- ing lines to create the 180° shift. Baluns are used with antennas, mixers, and push-pull amplifiers to create the correct phase relationships and to match impedances. Wirewound RF transformers are typi- cally used in push-pull amplifier applications at frequencies ranging from a few kHz to around 2 GHz. Alternative con- structions may include center taps for biasing or grounding. Center Tapped Ruthroff Balun Ruthroff style baluns provide isolation as well as imped- ance matching. If isolation is not required, bandwidth can be extended to frequencies as high as 3.5 GHz when connected as a Guanella (transmission line) transformer. Pri Sec 1:1 Guanella Balun Coilcraft wideband RF Transformers provide a wide range of impedance ratios and frequency bandwidths for use as baluns and for impedance matching. The complete selection of wideband transformers is shown at https://www.coilcraft.com/en-us/products/transform- ers/wideband-rf-transformers/#/. These transformers are intended for lower power (~1/4 Watt or less, 250 mA or less) applications. Simple Balun Connection Example When attaching a 200 Ohm balanced antenna to a 50 Ohm unbalanced (coaxial) cable, connect the high impedance (200 Ohm) balanced antenna to the high impedance side of the transformer, and the low impedance (50 Ohm) Document 1077-2 Revised 01/26/21 unbalanced end to the low impedance side of the trans- former. This connection steps down the high antenna impedance to match that of the 50 Ohm cable. 200 Ohms 50 Ohms Balanced (Differential) 4 1 Impedance Ratio : Unbalanced (Single-ended) 200 Ohm to 50 Ohm Balun High-Frequency Narrow-Band Baluns For high-frequency narrow band applications that do not require isolation, lumped element designs may be imple- mented. The availability of tight-tolerance surface-mount chip inductors and capacitors allows for minimal variation in production. Designers can employ circuit simulation to optimize the element values to achieve the required im- pedance matching network using LC components. Initial design values can be estimated using online calculators such as these: LC Balun Designer: http://leleivre.com/rf_lcbalun.html LC Matching Network Designer: http://leleivre.com/rf_lcmatch.html At GHz frequencies, accurate inductor models that in- clude parasitic elements are necessary for getting closer to a reality-based design. Coilcraft’s models for air core and chip inductors are measurement based in order to provide accurate simulation of your LC balun or matching network. These models can be found on our website at http://www.coilcraft.com/models.cfm References Cripps, Steven C., RF Power Amplifiers for Wireless Communications -2nd ed. (Artech House, 2006)