The 4KFM Audio Filter

Few things are more irritating to me than listening to an analog repeater with low deviation and low audio bandwidth; it’s just grating on the ears, all midrange and no fidelity. Low deviation also makes you constantly adjust your radio’s volume for different users. This article describes a repeater audio accessory that allows you to increase the bandwidth of an analog FM repeater to 4,000 Hz, while adding an extra layer of protection against over-deviation. It also eliminates any residual CTCSS (PL®) tones.

The dual filter features a 5-pole low-pass filter set to 4,300 Hz, strongly attenuating audio above 4,300 Hz. The slope of the filter is such that it provides an additional 5 dB of rejection at 5,000 Hz, the FCC allowable limit for narrowband FM deviation. The 4,300-Hz frequency will give repeater operators room to adjust the deviation somewhat higher, as