Abstract: This invention is a modular active filter that can be inserted between the antenna and a filter of a pre-existing unit, so that it can still function normally, without noise from cellular phones. This unit is modular. A heterodyning unit is used with a prefilter/amplifier unit and a post-filter/amplifier unit. After down converting the signal, while heterodyning, the signal is put through at least one crystal filter to filter out the noise introduced by cellular phones. To make sure that the unit works well with a wide variety of antenna-receiver pairs, the resonant frequency of the filters of the pre and post amplifier/filter units is set to the center frequency that the antenna and receiver are designed to handle. In this way, only one type of unit needs to be manufactured for each frequency band. An example of such an operating frequency might be 900 MHz. This invention has two types of embodiments. In one type embodiment the local oscillator produces only one fixed frequency.