Both RF limiters and attenuators reduce the power of an RF signal. However, they do so in different ways that are key to understanding their use. These components are often used to protect sensitive RF components and devices. There is also a wide range of RF limiters and attenuators that operate in various ways that may be advantageous for some systems and not others.
RF limiters
RF limiters are devices that reduce an incoming signal if its power level exceeds a threshold of the limiter. It is common to use RF limiters to protect sensitive receivers and signal conditioning circuitry from high incoming signals that would otherwise desensitise the receiver or damage the signal conditioning components. For instance, a low-noise amplifier (LNA) is often a sensitive device where incident power beyond a certain threshold may result in derating or damage to the LNA.
RF limiters may be used anywhere in a signal chain that may experience unintended or unavoidable high signal energy that could damage components or result in undesirable operation. This is the case in portions of the signal chain, such as the input ports of a mixer or gain block amplifier.
PIN diodes are commonly used to implement RF limiters as incident-power controlled, variable resistors. In this implementation the resistance, or attenuation value of the RF limiter, is a function of the incident signal power. Hence, a greater incident signal energy will result in higher RF limiter attenuation and a proportionally reduced output signal. RF limiters are typically connectorised with coaxial or waveguide interfaces, though there are surface mount RF limiters.
RF attenuators
RF attenuators can be passive or active devices with variable attenuators depending on the design. The basic passive attenuator acts as a resistive element that reduces the incident signal by a certain amount of signal energy. Hence, attenuators are often designated by the decibel (dB) drop the attenuator provides.
Basic passive attenuators are both absorptive and dissipative, in that they absorb incoming signal energy and dissipate it as heat, as opposed to reflecting the signal energy or storing the energy. In this way attenuators are often used to reduce reflected signals, such as those from reflective filter topologies, and reduce standing waves that would otherwise develop between two reflective ports. These types of attenuators are also typically broadband, depending on whether the attenuator is connectorised with coaxial, waveguide, or planar transmission line ports.
There are also passive variable attenuators that enable various attenuation levels depending on a selected setting. Additionally, there are active variable attenuators that exhibit attenuation as a function of a control input. Variable attenuators can be used to control the signal level to keep the signal within a desirable range, or can even be used for modulation in amplitude modulated (AM) or quadrature amplitude modulated (QAM) communications.
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