|The Icom IC-781 roofing filter mod consists of a 6-pole, 4 to 5 kHz wide filter followed by a high dynamic range, feedback amplifier. The amplifier provides enough gain to override the filter insertion loss.
The IMD dynamic range will be improved 7 to 14 dB for some signal spacings. The main receiver audio response will be reduced about 100 Hz in the SSB mode. Operating the noise blanker will not result in as much degradation of the dynamic range.
The mod inserts a narrow band crystal filter in between the pin attenuator following the first mixer and the grounded gate first IF amplifier. Reducing the bandwidth at this point in the radio helps to keep strong off-frequency signals out of the second mixer where they can cause intermodulation. Transmission is not changed as it does not pass through the roofing filter.
Please see our document titled "A Few Words About Roofing Filters" for more details.
Full installation instructions are available here: