|The TS-870 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 up to 15 or more dB for signal spacings from 2 to 20 kHz. Also, the blocking dynamic range will be improved for close in signals.
The roofing filter mod inserts a narrow band crystal filter after the first mixer and before the OEM roofing filter. An amplifier is included to compensate for the filter loss. 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: