> I'm considering purchasing a K3. Which filter do I want for RTTY (both
> DX & contesting)? 200 Hz 5-pole or 250 Hz 8-pole? Or is there some
> other configuration I should consider?
Between those two, the so-called 250 Hz 8-pole filter. Some things to
consider:
1. The primary IF filtering the K3 is done via the continuously adjustable
DSP. The crystal filters in the K3 are designed to be wider than the DSP
filter bandwidth to protect the DSP from very strong signals nearby that
would otherwise keep the DSP from doing its job. Don't think of the K3
crystal filters like IF filters in older radios. They are more like
"roofing filters" which used to be on the order of 15 kHz and in modern
radios are going down into CW bandwidth. Still, their purpose is to protect
the DSP, not be the radio's only filter.
2. The 200 Hz 5-pole filter is about 224 Hz wide. The 250 Hz 8-pole filter
is nominally 370 Hz wide. The 400 Hz 8-pole filter is 435 Hz wide. The 500
Hz 5-pole is about 530 Hz. Any of the three widest filters are great for
RTTY. They are essentially the same for the purpose they are intended.
3. Generally, 300 Hz is a good bandwidth for completely passing a 170 Hz
shift RTTY signal, assuming you are centered in it. Operationally, I've
found that a 200 Hz bandwidth works great in huge pile-ups because it passes
the fewest signals that are still good enough for the decoders. Yes, the
outer skirts of the two frequencies are rolled off, but the trade-off is
less signals presented to the decoders, assuming they are not all perfectly
zero-beat. Turns out to be a net win in heavy pile-up conditions.
4. The DSP filter is used to vary between 300 and 200 Hz as conditions
warrant. If you use the Dual-Tone Filter, also a DSP constructed filter, it
effectively narrows the bandwidth to about 200 Hz anyway, regardless of
where you set the main DSP filter.
I operated the entire Round-Up with a 370 Hz crystal filter (marketed as
"250 Hz 8-pole"), 300 Hz DSP and the DTF. So my effective bandwidth in both
K3s was about 200 Hz and it was protected by a 370 Hz roofer. If the
pileups had been smaller (no thanks!) or if I had the opportunity to tune
around more (running both radios is good when you can do it), then I would
have possibly turned off the DTF and used a 300 Hz bandwidth. But, even the
limited tuning around I did do, was not hampered by the 200 Hz filtering.
Ed - P49X (W0YK)
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