[Elecraft] K3 RTTY spectrum

[Kjeld Holm]

"The AFSK TX Filter cleans up any hum on your sound card from bad
connections, grounding issues, or distortion from over drive.  However, with
the K3 one can get the same narrow transmit spectrum by using FSK without
the issues of AFSK." Written by Joe Subich, W4TV

Not as I read the article - but I may be wrong. 
" Effect of the K3 AFSK Transmit Filter
Rather than hacking the K3 to transmit
through a narrow roofing filter, an equal or
better result can be had by simply enabling
the AFSK transmit filter in the configuration
menu. This places a 400 Hz filter before
transmit audio arrives at the RF modulator
(this only applies to AFSK-A mode,
not DATA-A mode). This filter is centered
around the tones configured under the
radio's PITCH menu. The effect is similar
to what MMTTY and other programs do in
their software, but this is done in the radio's
DSP firmware.
The radio's manual says this filter can
serve to filter any noise that might be on
the audio input, and it certainly will do that,
but it has the additional benefit of filtering
keying sideband energy. Figure 9 shows
the result of MMTTY's unfiltered phasecontinuous
AFSK audio into the K3 with
the K3's AFSK filter enabled. Comparing
Figure 9 with Figure 3 demonstrates the
effect of the K3's AFSK filter, as both have
exactly the same audio input to the K3.
Attempts to overdrive the radio resulted
in no significant change in signal bandwidth.
Even when transmitting wideband
noise into the K3, the AFSK filter limits the
bandwidth. While it is possible to transmit
trash that is difficult or impossible to copy,
the K3's AFSK filter makes it unlikely that
it would generate much interference on
adjacent channels."

"An important point: Even given the K3's
transmit IMD, the occupied spectrum using
shaped FSK is much narrower than
that when using the internally generated
FSK synthesizer. FSK keying sidebands
are not unique to the K3; every radio that
uses phase-coherent for its internal FSK
generator will generate a wider spectrum
at essentially any power level. The only
differences will be in cases where the IF
filter cuts off the sidebands.
As an experiment, I decided to see
just how wide I could make my signal by
overdriving the K3's line input. I cranked
the PC's headphone output to 100 percent
and drove the ALC as hard as I could by
setting the line input gain to maximum (see
Figure 7). Some strange spurs show up in
the spectrum, but even those are below the
keying sidebands of the FSK transmitter
(see Figure 2). Even trying to transmit absolute
trash I wasn't able to make the AFSK
Figure 4 - AFSK with MMTTY using a 512 tap TX BPF with a
passband of from 2000 to 2400 Hz, 0 dBm
Figure 5 - AFSK with MMTTY with 512 tap TX BPF, 100 W
signal as wide as that of the internal FSK
generator. The rise in the noise floor over
the 2.8 kHz bandwidth occurs, because the
K3 amplifies noise from somewhere; at this
gain setting it is present whether or not anything
is plugged into the K3's line in jack.
This behavior is almost certainly specific
to the K3, because the K3 scales audio in
DSP before it ever enters the RF stages.
Other radios are not going to protect you
from yourself nearly as well, so you might
expect harmonic distortion and other bad
things with no limiting prior to the point of
RF modulation."
Oz1ccm Kel