|The FT-920 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 mod is inserted after the first mixer and before the OEM roofing
filter. 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.
The IMD dynamic range will be improved up to 20 dB for signal spacings from 2 to 20 kHz. Also, the blocking dynamic range will be improved for close in signals.
Please see our document titled "A Few Words About Roofing Filters" for more details.
Full installation instructions are available here: