Signal Chain Discussion Series: Impedance Matching

An RF Signal Chain is generally composed of a series of RF devices and components, each with their own unique behavior. In every case, however, each component and device within the signal chain presents an impedance at their input ports and output ports. Moreover, the interconnect between devices and components within a signal chain also presents it’s own impedance. Given that impedance at any port is a ratio of the current/voltage, the internal response of a component or device directly impacts the impedance at the ports of each element in a signal chain. 

At each node in an RF signal chain, if the impedances don’t exactly match, then a reflection is created. A common way of measuring this impedance mismatch is described by the resulting standing wave ratio (SWR) or voltage standing wave ratio (VSWR). This is why many common RF components and devices are designed for 50 Ohm or 75 Ohm port impedances, to minimize the concern for impedance mismatch and to alleviate the need for designers to use impedance matching between each element in a signal chain.

Impedance mismatch within a signal chain can produce a variety of undesirable effects. The immediate impact is that some of the signal strength meant to be carried along the signal chain is instead reflected by any impedance mismatch. This results in signal strength loss that compounds with any attenuation loss within the signal chain. Moreover, the reflected signals can “bounce” back and forth between two mismatched ports and develop a standing wave. This standing wave acts like a DC voltage at the ports, which may harm or change the behavior of some RF components and devices. Given that RF circuits are often nonlinear, reflected signals may also result in the development of spurs, harmonics, noise, and other undesirable signal degrading features that may appear in the passband of the signal chain. 

In extreme cases, an impedance mismatch may result in reflected signals that are strong enough to damage devices and components within the signal chain. For instance, if the output of a high power transmitter isn’t well matched with the antenna port, the reflected signal strength could be enough to damage the high power amplifier (HPA), which is typically an expensive and complex element to repair/replace.

In some cases, impedance mismatch within a signal chain is almost unavoidable. For instance, some filter types are reflective in nature. This means that for frequency content that is outside of the passband of a filter will see an impedance mismatch that induces a reflection of those signals outside of the passband. As filters are often placed at the output of mixers, oscillators, transmitters, and other active elements, this could result in degradation and damage of the filter, element, or signal quality. To overcome this, there are other filter types, such as absorptive filters, that are designed to absorb RF energy outside of the passband, or attenuators are placed in between reflective elements of a signal chain to absorb the reflected signal energy and prevent the development of a standing wave.

In cases where it is critical that there is minimal impedance mismatch induced reflections, maximum power transfer, or there are other constraints, such as minimizing noise content in an receive signal, impedance matching circuitry may be desirable. In essence, an impedance matching circuit performs an impedance transformation from one of its ports to another. If this impedance transformation is designed for maximum power transfer, then the impedance matching will transform the source and load ports into complex conjugates of each other. In the case of matching an antenna port to a receiver input, such as a low noise amplifier (LNA), then a different approach may be taken. If noise is the concern, then the impedance matching circuitry may not be the complex conjugate, and instead the impedance matching network is designed to present the noise figure optimized source resistance and bias point for the LNA.