Four Filters in FT-897D
The below modification enables you to use as much as 4 filters besides the standard CERAMIC filter, which is soldered directly on the main PCB.
The TCVR is configured in the way that the CERAMIC filter is always used for TX. This is not a must but one can be sure that the stopbands are not compromised by the modification.
For RX, for SSB modes the FIL1 bank is selected and for CW modes FIL2 bank is selected. The small switch on the top cover then toggles two filters on each bank. Therefore, if the mode is set to LSB or USB, the switch toggles 2.7 kHz and 2.0 kHz widths, if the mode is set to CW, the switch toggles 500 Hz and 300 Hz widths.
The extra board carries four filters:
Inrad #712-F (300 Hz)
Inrad #705-F (500 Hz)
Inrad #720-F (2.0 kHz)
muRata CFJ455K13 (2.7 kHz)
They are switched by two Teledyne relays (DPDT).
The bottom view of the board. To keep the other connections short one of them is realized by a tiny shielded cable.
The rezistor T-networks in front of collins filters were kept in the same way as on the original filter boards from Inrad.
Also the filter width recognition logic is kept towards the tcvr. This way, the tcvr "thinks" that slot FIL1 is occupied by ssb-width filter, while FIL2 is occupied by cw-width filter.
The board is directly inserted on the pins intended for the two optional filters.
The filters are toggled by the small switch on the top.
This switch replaced the clumpsy switch for batteries (I use internal PSU anyway).
The rear view of the switch. It just sends 8V to both relays, or not.
Before I designed this special
filter board I had to figure out the standard Inrad filter
boards. The non-signal pins are used to identify the kind of the
filter. This is, in fact, the information for the tcvr for the
filter selection menu.
The logic is as follows:
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