[Elecraft] K3 - A different RTTY question

[Ed Muns]

NF4L asked:
> I've found I copy much better without using dual passband. I 
> have the 400Hz 8-pole filter. I'm using AFSK-A. I use MMTTY.
> With DPB on, even the noise sounds a bit like a RTTY signal, 
> and the MMTTY  shows 2 little peaks on the FFT display.
> Do I have a configuration problem, or is that normal?

I've used the DTF (Dual Tone Filter) exclusively since Lyle first coded in
in early field test.  It works great, just like the TPF (Twin Peak Filter)
works well in the Icom radios.  The K3 implementation is different than
Icoms and for a few months before I got my second K3 I used a 756Pro3 with
the K3 in SO2R.  I could not prove any superiority between the two although
I prefer the K3 implementation much more.  The K3 simply puts two 50 Hz
8th-order, i.e., steep, DSP filters on the two tones.  The Icom approach is
to peak the IF around the two tones.  Accordingly, you need to back the Icom
RF Gain way off to a base level of about S9+15dB for it to work well.

As to your question on using DTF or not, i.e., regular DSP filter, I don't
have data to prove one is better than the other.  I just haven't used the
regular DSP filter enough (actually not at all), but anecdotally I feel I
copy weak signals extremely well and can slice through big pile-ups readily.
My belief is that the DTF is superior, but perhaps not by much and probably
only in certain instances.

You do need to tune carefully with the DTF because it is effectively putting
a sharp 220 Hz filter in your IF.  A wider filter allows the decoder to see
an attenuated signal and decode it when you are off-frequency a bit.  Being
a long-time CW contester, it is second nature for me to use RIT, or a
receive-only VFO, constantly as needed while operating.  However, by using
such a narrow RTTY filter (the DTF), it means that I don't work
off-frequency callers until the on-frequency callers have been worked out
first.  Hopefully, this encourages stations to zero-beat my TX signal
carefully.  ;>)  

Ed - W0YK
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Ed Muns
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