Cavity Filters: A Short Guide

Cavity filters are the main building blocks of duplexers, multi-couplers and pre-selectors. However, the uses of cavity filters are not limited to only these applications. Solo or cascaded cavities may be utilized for a wide array of interference fighting activities, like cleaning the performance of existing filters that have inadequate isolation or off-channel interference rejections.

At crowded antenna sites, cavity filter manufacturers designed these filters for eliminating noisy transmitters or for the prevention of transmitter IM fusion. Receiver-end selectivity can be significantly increased with the utilization of extra filtering, thus removing a lot of desensitization, IM and overload issues.

When utilized in association with a spectrum analyzer or service monitor, cavity filters can permit a detailed analysis of lower-level transmitter noise. This noise is the main core source of interference at multi-transmitter sites. These filters can also operate alone as pieces of experimental tools for analyzing a lot of receiver IM problems. It can also help identify the best filter type to use for a permanent fix.

Noteworthy Details about Cavity Filters

Cavity filters are known for two things: a high Q factor level and a low insertion loss.

The Quality Factor (Q Factor)

In physics and in engineering, the quality factor (Q factor for short) is the dimensionless parameter that describes how under-damped a resonator is, and it labels the resonator’s bandwidth relative to its core frequency. A higher Q factor pinpoints a lower energy loss rate relative to the resonator’s stored energy; oscillations die down slower.

The Insertion Loss

Insertion loss refers to the attenuation, or loss in signal power, resulting from the component’s insertion, like connectors or splices, in a circuit. This is then measured as a signal power comparison at the point when the incident energy hits the component and the signal power leaves the component. Generally, insertion loss is measured in decibels (dB), though it may also be expressed as either a coefficient or a fraction.

In a fiber optic transmission system (FOTS), insertion loss is a measure of loss all over the circuit because of all of the factors. These factors include absorption, bending loss from macro-bends and micro-bends, diffusion, dispersion and leaky modes.

Other Facts about Cavity Filters

The usual individual cavity size ranges from 50mm (2″) square profile, 101mm (4″) OD, 168mm (6.625″) and 254mm (10″) diameters. Normally, manufacturers would prefer cavities in their inch sizes for the most part, and each classifications of cavity filters can be configured to provide pre-identified responses.

The pre-identified responses are configured using various types of coupling loop designs, they are:

Band Pass: The band pass configuration passes through one narrow frequency band at the cavity’s resonance frequency and by symmetrically constricts frequencies with an increased attenuation above and below the pass frequency.
Band Pass Reject: The band pass reject configuration done by cavity filter manufacturers has a loop that passes through a relatively narrow frequency band and rejects (notches) a wide band of frequencies either higher or lower of the frequency pass.