[Elecraft] RTTY Filter

[wa9fvp]

The next two photos shows the phase relationships as the tone transitions
from mark to space.  The butterfly filter maintains a perfect out of phase
relationship using the AFSK, single filter, data mode.  With the dual
filter, the phase relationship is sometimes in phase and then transistions
to an out of phase condition. The phase non-coherence can cause a distortion
in the data symbol and increase the bit error rate.

The bottom line is.  If your using an external RTTY box like a PK-232, KAM
or even RTTY decode software,  use the AFSK (single filter) and not the FSK
(dual filter).

http://elecraft.365791.n2.nabble.com/file/n6757028/rtty_afsk.bmp 
http://elecraft.365791.n2.nabble.com/file/n6757028/rtty_fsk.bmp 

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Jack WA9FVP
Willco Electronics
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