Filter products use cavity filters as their basic building block. A cavity filter only does one of two things. One is to allow a given range of frequencies to pass through and the second is to stop a specific range of frequencies from passing through. The Sinclair Q-Circuit cavity filter has both a pass and a reject characteristic. By connecting multiple cavities together, the pass or reject frequency range can be made quite narrow, effectively passing or rejecting only one signal. Preselectors, duplexers and custom YA systems are constructed by connecting multiple cavity filters together to achieve a desired characteristic.