[Elecraft] K3 Filters for digital work

[Jim Brown]

On 11/29/2013 6:25 AM, Joe Subich, W4TV wrote:
> > My K3 copies RTTY fine through the 200 Hz filter.
>
> This is much like the old ads "I worked 150 countries with my
> Gotham vertical" ...  When one tries to pass a FSK signal through
> a "too narrow" filter it doesn't just stop working.  However, the
> tight skirts of the filter cause the energy in the mark and space
> channels to smear (pulse stretching) into the next bit period and
> cause errors in decoding. 

Yes.  Those of us who worked extensively in pro audio in the last 30 or 
so years are well aware of the significance of the phase response of 
filters and systems. We learned, for example, that varying amplitude 
response has an associated variation in the phase response. Thus, I was 
quite wary of using the "double humped" response provided by the K3

Quoting from W7AY's website,  "With a software demodulator that includes 
a matched filter or a raised cosine filter that matches the baud rate of 
the RTTY signal, the receiver's filter should be only as narrow as 
needed to keep interference from clipping the sound card. "

David Wicks, G3YYD, author of the new and highly regarded RTTY decoder 
called 2Tone, advises use of a filter no narrower than for RTTY 
decoding. I was not at all surprised to read it in his documentation.

73, Jim K9YC