Re: K3 in CQ WW RTTY


ve3ki
 

Part of the difficulty with the 200 Hz filter in RTTY might perhaps
be the inability to set the filter Fc accurately enough with the
current firmware to match some of the selectable RTTY tone pairs.

The worst case is the 1445 Hz pitch setting. The centre frequency is
1530 Hz, but the current granularity of Fc is 50 Hz and Fc NORM is
actually set to 1500Hz. At a filter bandwidth of 400 Hz, this 30 Hz
error is not important, but if you were trying to use a 200 Hz filter
an error of 30 Hz would probably destroy the ability to copy.
Alternatively, you could retune to centre the received signal in the
filter and establish copy, but at the price of losing the CWT
indicator and also of transmitting 30 Hz off in FSK and perhaps being
outside the filters at the other end of the QSO.

Pitch settings of 1275 and 2125 Hz are better, because the error in
Fc is only 10 Hz. The 915 pitch setting has zero Fc error, but for
those using AFSK that might not be an optimal choice (second and
third harmonics both within the Tx filter bandpass).

I am not sure whether a granularity of 10 Hz for the front panel
shift control is reasonable or not, but even if it isn't, setting the
NORM Fc to the exact value corresponding to the selected pitch should
be possible (e.g. NORM Fc in PSK D is 1010 Hz), and I would hope that
a 10 Hz granularity for the IS; software command would also be
feasible.

While on the topic of software control, I note that the new MG;
command gets and sets the mic gain setting; I'd like to see the same
capability extended to the line input setting (I use Line 4 for PSK31
and Line 8 for AFSK RTTY - I'd like to be able to have my software
set these automatically when changing digital modes).

73,
Rich VE3KI


--- In Elecraft_K3@..., wayne burdick <n6kr@...> wrote:

Ian White GM3SEK wrote:

The 200Hz 5-pole filter for the K3, on the other hand, does
deliver
pretty much the bandwidth it says on the label - and that is
definitely
too tight for 170Hz shift. Even if tuned very precisely, it
causes a
noticeable deterioration in copy.
Hi Ian,

We designed this filter specifically for narrow-shift RTTY, PSK31,
and
CW use in high-QRM situations. The bandwidth was selected in
consultation with some of our field testers who specialize in RTTY
contesting. I believe they've had very good results with it.

That said, you could easily widen a 200-Hz 5-pole filter as an
experiment. I just looked at the model. You only need to change two
capacitors -- C4 and C5. I'd suggest using 820 pF or 750 pF, which
result in a -6 dB bandwidth of about 270 Hz or 300 Hz,
respectively.
This should also decrease loss through the filter by about 1 dB
(set
your FLx GN menu entry accordingly). Return loss and passband
flatness
are essentially unchanged.

73,
Wayne
N6KR


---

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