RF Attenuators are widely used throughout the RF/microwave systems to reduce the signal strength in a signal chain for a variety of reasons. In some cases, it is merely advantageous to lower the signal strength at the output of a RF attenuator without otherwise distorting the signal characteristics. As RF attenuators are typically passive reciprocal devices, an RF attenuator is an ideal in-line solution to lowering the signal strength. Some RF attenuators are based on PIN diode or transistor technologies, which may result in non-reciprocal behavior. In the case of coaxial transmission lines, RF fixed coaxial attenuators are well suited for this purpose, as they provide an accurate and reliable fixed attenuation value in a compact and in-line form factor. In some cases, these coaxial attenuators can even double as adapters if different coaxial connector types are used on either ends. An example of a basic fixed coaxial attenuator are N-type 3dB Fixed Attenuators.
Outside of simply providing a set attenuation value for a passing signal, fixed attenuators are also used to improve the impedance match between two ports that are mismatched (i.e. at different impedance levels). This is a consideration as improperly matched ports result in reflection that is proportional to how disparate the impedances at the ports are. The reflection coefficient is the difference of the two port impedances divided by the sum of the two port impedances. If the impedances at two ports are matched, there is no reflection. However, even a slight mismatch can result in a voltage standing wave ratio (VSWR) other than 1:1. When this is the case the reflected wave from one port can also be partially reflected from the other mismatched port. The result of this is the development of a standing wave. Standing waves can be particularly problematic in cases where a high standing wave can pass as DC through the RF ports or induce undesirable voltage levels at the inputs or outputs of active devices. Such voltages could result in damage or disruption of amplifier, mixer, oscillator, or other active devices, and could even cause arcing, shorting, or over temperature conditions for even passive RF components.
RF fixed attenuators can be used to thwart the development of a standing wave by providing a small fixed attenuation of the reflected signals such that the reflected voltages decay more rapidly, and thus do not form a standing wave. This is a common use for low-value fixed coaxial attenuators, such as the N-type 3dB Fixed Attenuators mentioned earlier. As the reflected signals pass through the fixed attenuator several times, the signal strength of the reflected signals steadily decay with the signal energy being safely dissipated as heat in the fixed attenuator.