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42 PCB007 MAGAZINE I SEPTEMBER 2023 Time-domain reflectometry (TDR) is a mea- surement of impedance in a transmission line. is becomes very important in high speed or RF printed circuits. As a long-time ama- teur radio operator, I have had a lot of experi- ence with characteristic impedance regarding power transmission. For example, this is very important with transmission lines leading to an antenna. In the circuit board world, this could be cellphones or Wi-Fi devices. ese devices transmit and receive RF signals at high frequencies, and mismatches from the transmission line to the antenna can have adverse effects on perfor- mance. So, in amateur radio, standard trans- mission cables are used with known imped- ance values. One example is coaxial cables, such as RG-8 or RG-58. ese cables have a characteristic impedance of 50 ohms. is is considered a "constant" so that changes to only the antenna affect the resonance of the output. In RF applications, a mismatch of the antenna length at the desired frequency can have sig- nificant effects on the effective radiated power (ERP) or reception functionality. When this happens there is a change in the standing wave ratio (SWR). On a matched transmission line to load (antenna), the SWR will be extremely low, and a perfect match will be a ratio of 1:1. is means full transmitted power is being radiated or passed to the load. As SWR increases, a portion of the power of the circuit is reflected toward the source or transmit- ting circuit. e higher the SWR, the more power is reflected. is is dangerous to the source as heat is produced and the high reflected signal can damage the output cir- cuit causing failure. Let's look at a Wi-Fi router. It is typically transmitting in the 2.4 GHz–5 GHz bands. ese rout- ers usually only transmit milli- watts, so it is very important that the best ERP can be achieved (yes, there are antennas in these devices). Depending on the design of the router, the length of the antenna may vary. Since we are in such a high frequency, the antenna will be small. For exam- Why TDR? Testing Todd by Todd Kolmodin, GARDIEN SERVICES USA

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