In cases where it may be necessary to connect 50 Ohm and 75 Ohm systems, there are a few methods to ensure impedance matching between these two impedance standards. The easiest and most cost effective, and smallest footprint, in most cases, is to use impedance matching pads based on resistive circuits. Resistive impedance matching pads allow for inline impedance matching between 50 Ohm and 75 Ohm systems and are available in common coaxial connector standards. Some impedance matching pads are even available that adapt between coaxial connector standards and impedance standards, which can help to simplify a BOM and reduce the footprint of the signal chain compared to using an adapter and impedance matching circuit.
Matching the impedance of the ports of two or more interconnected RF devices or components is essential for efficient, and often functional, RF circuits. Without proper impedance matching, like what can be achieved with Impedance Matching Pads, any signal passing through the node where two RF devices or components of disparate impedance induces reflections. These reflections can build up to substantial standing waves that may alter the behavior of, or even damage RF devices and components. Hence, impedance matching is typically vital between every RF device and component in a signal path.
This would generally require ensuring that every individual RF device and component is matched, and that the influence of each and every RF device and component would result in an overall matched impedance at every port. Beyond cumbersome and potentially impossible, this approach, though necessary sometimes, isn’t well suited to the use of modules or off-the-shelf components. This is why many RF devices and components in connectorized packages, surface mount packages, or other types of modules packaging are often available in standardized impedance values.
For most RF and microwave applications, this value is 50 ohm, or 75 ohms for some broadcast and other applications. Having RF devices and components already impedance matched to standardized values enables rapid design and assembly of standardized impedance packages and modules without the need for system or design engineers to account for impedance matching at every node.
This obviously isn’t the case with non-standardized impedance modules, drop-in style packages, or other RF devices which may not be feasible to offer in a standardized package. This includes some amplifiers for custom designs where the trade-off of ease-of-use and design/assembly simplicity is outweighed by other requirements.