Icom IC-740

AF Filter 100 Hz wide @ 700 Hz

The narrower filter for CW on 455 kHz is 250 Hz wide in my IC-740. Yet, sometimes one needs even narrower passband to pick the only one station from a pile-up. The AF filter can fit this task.

There is no button on the front panel that could be used for this extra filter. As it makes very little use to switch on the NOTCH filter on CW mode, the logic was slighly changed. On CW mode, the NOTCH button sets the AF filter, on all the other modes it sets the notch filter as usual.

The final pass curve of the filter:

The curve was taken from the Spectrogram application running on PC. The sound was fed to the sound card. This means that the curve is not much accurate, probably.
In spite of this it shows how sharp and steep the filter is, at least.

The filter consists of three sections. Each of them is made by two ferrite pot cores diameter 14mm.

The sections are separated by a stage of an OA. The OA only makes for the attenuation inserted by the section.

Placement of the filter in IC-740. It is located close to the REG UNIT. Four holes had to be drilled to the chassis.

Schematics diagram

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