I agree with STAYVERTICAL. The 500 hz mechanical filter is the best place to start (either the Yaesu (Collins) or INRAD). If you go with the 300hz filter you will likely find that it is a bit too narrow to have engaged while tuning around. You can always add the 300 hz filter as the second filter later on if you feel that you really need it.
Likely part of your dissatisfaction with just the audio DSP filtering in the FT897D is that even though
the DSP audio filtering may filter out the audio of of interfering stations, those stations are still within the passband of your 2.8 KHZ IF filter and thus they will have an impact on the AGC, causing it to pump. No doubt you have
experienced this. Usually you will hear a bit of a thump and the station you are trying to copy drops out because
the rig is responding to the overload caused by the stronger interfering signal, thus drastically reducing the
Personally I find that the FT897D with the 500hz Collins filter + the DBF feature of the
built-in DSP audio filter works pretty well for CW operation. If you find that the DSP filtering is too narrow for
your liking I believe that the bandwidth can be varied by one of the internal menu settings so you might want to
experiment with this once you have a narrower IF filter installed.
Best of luck
Michael VE3WMB / VA2NB
I have an FT897D and fitted the 500hz Yaesu filter (which is actually a Collins mechanical filter).
I chose the 500hz filter over the 300hz because 500hz is a commonly used digital modes bandwidth and it is not far from 300hz in any case.
Very easy to install (just take off the top cover and plug in the filter onto pins already fitted - no soldering required).
The FT897D has two places for optional filters, so if you really want, you could fit both a 300hz and 500hz filter if desired.
I have the second slot populated with a 2.3khz Yaesu mechanical filter, but would not say it is really worth fitting.
A second filter of 300hz would be my choice if I had to do it over.
I use the DSP setting "DBF" which gives a CW peaking filter and personally find it very useful and easy on the ears, but that may just be my preference.
Using the 500hz filter and the DBF DSP setting makes for a pretty good CW environment.