Roofing Filter Design

A close up of the roofing filter section of the frontend board.

Roofing filter design.

For more demanding uses like listening to weak cw or ssb signals a roofing filter is required that gives a smaller passband appropriate to the reception mode in use. Basically a roofing filter is simply the first if filter in a radio. Kenwood ts 950 series roofing filters. Roofing filter basics where.

The amplifier provides enough gain to override the filter insertion loss. 250 hz 500 hz or 1 8 khz for ssb would be acceptable values. First filter as close to the antenna as possible 1 st if typical why. Using a more narrow roofing filter can eliminate this unwanted signal and eliminate the agc pumping.

These allow a much better filtering curve than a roofing filter which often uses a high first if above 40 mhz. Think of the roofing filter as a roof on a house the wide filter allows the dsp and hardware agc to respond to all the rain falling on the roof. Protects following circuits from nonlinear operation due to strong out of band signals what. Roofing filters are usually crystal or ceramic filter types.

To the left is the 2x 4 pole ssb filter to the right is the 2x 3 pole cw filter. The term roofing stems from the fact that it protects the rest of the radio following it from out of the passband signals. To reduce the amount of rain falling on the roof a more narrow roofing filter is required. Note the shield to improve the stop band beyond 100db and the small trimmers needed to align the cw filter to center exactly on 9 000000mhz.

There are two basic designs for modern radios. Sized to pass the widest mode of operation improve dr3 begins with. Mcf or discrete crystal construction width.