A couple of weeks ago I managed to acquire a cheap ICOM FL-100 500Hz CW/RTTY narrow crystal filter for my ICOM IC-746 amateur radio transceiver. I thought this would be interesting to try with fldigi and digital modes such as PSK. The IC-746 manual implies that any combination of the optional crystals can be used in any radio operating mode. Unfortunately this is not the case: the IC-746 will not allow a CW (or RTTY) labelled crystal to be activated when in SSB mode. This is a problem as I work digital modes using software like fldigi with the radio operating in SSB. Fortunately there is a solution, which will be explained below.
Before picking up the ICOM FL-100 narrow filter, my radio already had an FL-96 (2.8KHz SSB) and FL-223 (1.9KHz SSB) fitted. The FL-272 is the standard factory fitted 2.4KHz SSB filter.
My 500Hz FL-100 filter is installed in the #2 filter socket. The radio was configured to use an FL-100 in this position via the menus:
However when in an SSB mode, the 500Hz filter is not listed as an option in the filter configuration screen:
Switching to CW mode, the 500Hz filter becomes available in the filter menu:
The solution is to tell the radio that there is an SSB crystal filter in the position where the CW/RTTY filter is installed. Below I change the FL-100 to an FL-103 2.8KHz SSB filter in the settings menu. Simply pick an SSB filter model which is not actually installed in the radio (to avoid confusion).
Returning to the filter options screen while in SSB mode the fake FL-103 can now be selected. Even though the menu screens will refer to this filter as a 2.8KHz filter, the radio will be filtering it through the 500Hz narrow filter:
It’s now possible to activate this narrow filter while the radio is hooked up to the fldigi software and working in an SSB mode. This should come in handy for pulling out weak signals. Keep in mind depending on how your own digimode software works, it may be necessary to adjust the PBT (passband) controls on the front panel of the radio to get the desired effect.
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