[Elecraft] K3s AFSK frequencies

[Ed Muns]

RTTY audio tones are personal choice only affecting the audio at your
station.  I can use 2125/2295 and you can use 1275/1445 when we communicate
with each other.  This is analogous to me using a CW pitch of 800 Hz and you
using 400 Hz in the same QSO.  The local audio tones we each choose have no
impact on the other station.

Two caveats:

1.  With AFSK, but not FSK, transmission, using low tones, e.g., 915/1085,
allows their second harmonic to fall within the radio's TX IF passband,
e.g., 2800 Hz, creating QRM to the extent of the second harmonic.

2.  Most radios show the SSB suppressed carrier frequency in AFSK mode
rather than the Mark frequency which is the convention for denoting RTTY.
The suppressed carrier frequency will differ from the RF Mark frequency by
the amount of the local audio Mark frequency.  This usually leads to
inconsistent frequencies being communicated, for example, in a spotting
network.  The K3/K3s, however, show the RTTY Mark frequency on the dial in
both AFSK or FSK transmit modes.

With most of today's software decoders (MMTTY, 2Tone, GRITTY, Fldigi, MixW,
etc.), an IF bandwidth of at least 500 Hz is recommended.  The decoding
algorithms make use of the sidebands outside the 170 Hz shifted frequencies
to decrease error rate.  Narrower IF filters may be appropriate in very rare
situations where near-by strong signals can be reduced from overloading the
IF.  But, this comes at a cost of higher RTTY error rate.  I seldom, if
ever, go below 500 Hz even in the worst of QRM situations.

This also means that slightly lower error rate is achievable with the K3/K3s
Dual-Tone Filter DISabled, contrary to conventional intuition.

For RTTY, AGC should be Off, another conflict with conventional thinking.

Ed W0YK
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Mike N4CF wrote:

Hello to the group. I just put my new K3s/100 into service, and have a 
question about the Mark frequency to use with AFSK. Before the K3s, I 
used a SignaLink with my KX3 and MMTTY. With that setup, I set MMTTY to 
use 915 Hz for the Mark frequency, and everything worked fine.

Fiddling with the K3s and its internal USB sound card (I love that!), I 
can select various higher Mark frequencies using the PITCH button, and 
set MMTTY accordingly, and the K3 still decodes the RTTY.

So my question is, what is the "standard" or common Mark frequency to 
use for AFSK on the K3? Or is it a matter of personal preference -- 
which sounds better to my ears when tuning RTTY signals?

Also, what bandwidth is commonly used for 170 Hz-shift RTTY? I stocked 
this K3s with four 8-pole filters, including 400 Hz and 250 Hz. The 250 
Hz filter works with RTTY, but the MMTTY X-Y scope pattern looks like I 
might be pushing things with that narrow a filter. the 400 Hz filter 
looks better.

But David (I think) at Elecraft told me when I made my list of filters 
that I'd be using 2.1 kHz or 2.8 kHz for RTTY. That doesn't make a lot 
of sense to me if QRM is present. OTOH, MMTTY is decoding specific 
tones, so maybe filter width doesn't make much difference.