[Elecraft] K3 Reduced BW FSK

Wed Feb 13 20:12:05 EST 2013

> As anyone who was in the WPX RTTY contest this past weekend knows,
> anything that can help reduce the unnecessary interference from
> overly wide transmissions is more than welcome.

Although Andy's work showed clearly that shaped AFSK could be very
spectrally efficient, most of his testing was done with a K3 in AFSK
mode *with the AFSK filter engaged*.  Elecraft are already head and
shoulders above the other manufacturers by including and AFSK filter
and an automatic audio gain control (ALC) which makes it nearly
impossible to generate "wide" AFSK even if the user engages "all
knobs to the right".

This is not the case with AFSK on other transceivers - *particularly*
when operated with audio connected to the microphone input of a rig
designed for a dynamic mic.  Most dynamic mic rigs expect *only*
5 mV of audio drive to the mic jack and will distort significantly
when drive with more than 50 mV - something that no adjustment of
the mic gain can resolve since the mic preamp is a fixed gain stage
*before* the mic gain control.  In spite of this, many manufacturers
of low cost audio interfaces continue to sell noting but mic cables
and their documentation continues to instruct users to turn the sound
card level *all the way up*!

When one overdrives a mic stage - here's what the spectrum looks like:
http://www.subich.com/BAD_AFSK.jpg.  In this case audio was being fed
to the "mod in" pin on an Icom ACC jack but at more than 700 mV on an
input rated for 100 mV RMS it was still much too hot.  One can see the
AFSK signal, the second harmonic, the third harmonic, "suppressed"
carrier and the incompletely suppressed lower sideband.  As bad as it
is, this capture does not show the horrendous effects of hum evident
on many AFSK signals either.

While it is great that Elecraft give us the tools to make sure we are
stinking up the band but until all rigs include AFSK filtering ahead
of the modulator, I would much rather deal with the modest "key clicks"
from some of the traditional transceivers than the 3 - 5 KHz wide
mass of hum and harmonics from an improperly operated AFSK transmitter
- and I suspect anyone who has had to deal with the interference from
an improperly set-up AFSK rig will agree.

73,

    ... Joe, W4TV

On 2/13/2013 7:06 PM, Mark n2qt wrote:
> I had missed that this wasn’t discussed on  this reflector, so first a little history
>
> At the end of December, on the RTTY reflector, Andy K0SM/2 produced measurements showing that the
> use of FSK, which had been the ‘gold standard’ for generating RTTY was actually not as spectrally efficient
> as correctly adjusted, shaped AFSK generated RTTY.   This was shown in his write-up posted at
>
> http://www.frontiernet.net/~aflowers/k3rtty/k3rtty.html
>
> Note that this does NOT indicate that there was any sort of problem with the K3, just that there was a chance
> to be better.  Other manufacturers also have this same chance to improve.
>
> A little later, Ed, W0YK, also known as P49X in many contests, announced that he had successfully used
> experimental DSP firmware from Elecraft in the BARTG rtty contest.
>
> “The K3 DSP firmware engineer coded a  wave-shaping filter that narrows the transmit bandwidth similar to
> what their AFSK filter provides. The resulting spectrum image looks  like the better, i.e., narrowest, of the
> images in K0SM's  recent work.”
>
> Elecraft then allowed Ed to share this firmware with others who were interested in trying it, which is how I got it.
>
> As anyone who was in the WPX RTTY contest this past weekend knows, anything that can help reduce the
> unnecessary interference from overly wide transmissions is more than welcome.  It is great that Elecraft has
> reacted so quickly to this opportunity.  (Would we expect anything less from these guys??)
>
> Mark N2QT/WE4M
>
> From: Stephen Prior
> Sent: Wednesday, February 13, 2013 3:22 AM
> To: Mark
> Cc: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] K3 Reduced BW FSK
>
> Mark,
>
> You will have to enlighten us (well, me anyway) - just what pre-beta software are you referring to?
>
> 73 Stephen G4SJP
>
>
>
> On 13 February 2013 04:10, Mark <n2qt at verizon.net> wrote:
>
>    Just a report that I used the pre beta low bandwidth fsk software on both k3s during the
>    WPX RTTY contest.  Made over 1900 low power contacts, with no apparent problems
>    (As expected).  As close as some stations felt comfortable operating next to me, it looks
>    like it helps.
>
>    With the ever increasing popularity of rtty contesting, improvements like this helps
>    us all.
>
>
>    Mark. N2QT/WE4M
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