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Re: FT-847 and 38.4k baud modifications

-----Ursprüngliche Nachricht-----
Von: Bob Carlson <n0aot@san.rr.com>
An: amsat-bb <amsat-bb@AMSAT.Org>
Datum: Dienstag, 4. April 2000 10:05
Betreff: [amsat-bb] FT-847 and 38.4k baud modifications

Bob Carlson wrote:
 >I'm mulling over ways to get the FT-847 going on 38.4K baud. The SYMEK IFD
 >board is under active consideration but I had another idea and would
 >your comments...
 >The FT-847 has separate 2nd IF filters for FM-N and FM-W modes. The FM-W
 >mode selects the normal 15khz filter in the second IF, but the narrower
 >filter for FM-N is pretty much useless (to me).
 >I was thinking that perhaps the 8khz filter could be swapped out with a
 >suitable 50khz filter for use with 38.4K? Everything downstream would
 >the same, so radio and modem connections would remain intact. Also, it
 >be a trivial matter to have the tuner software select FM-N mode via the CAT
 >for 38.4K operation.
 >In other words, 38.4k baud operation would be identical to 9600, and 1200
 >baud operation except for selecting FM-N instead of FM-W. As an experiment,
 >I was thinking of bypassing the FM-N filter with a capacitor just to see
 >it would work until I can locate a proper 50khz wide filter...
 >The only hitch I can see is the whether or not the first IF filter is wide
 >enough for all this to work. Can anybody see why this wouldn't work?

That's the point where problems will occur! The first IF will be
too narrow for the 38kBaud. It should be 70-120 kHz wide.
That is the reason, why the IFD is connected directly after the
1st mixer and before the first filter. Trying anything after the
first filter will be OK for 19200 but not for higher speed data.

The FM filters for audio are designed for flat top and
steeply rise from stop band to passband. This results in a
group-delay which is not constant over frequency. For data
applications, group-delay optimised filters should be preferred.

 >Where could a compatible 50khz IF filter be found?

The 1st IF of FT847 is 45.705 MHz (a non standard IF). I think there
will be no manufacturer for 80 kHz wide filters for this specific
frequency. It could be possible to use 45.000 MHz 1st IF instead.
Then, the displayed frequency has to be set to a value 705 kHz above or
below the desired RX frequency and the complete IF chain has to be
totally readjusted.

73! Ulf, DK9SJ

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