[Elecraft] K3 - AFSK anomaly? No replies, try again

[Bill Breeden]

Mike,

Glad to help and glad it made sense!

When you tune across an RTTY signal using your 400 Hz filter with the 
dual passband filter turned off, first one shift, either Mark or Space 
depending on the direction you are tuning,  will enter the passband of 
the filter and then both shifts.  Assuming you have the Shift control on 
the front of the K3 centered, when the Mark and Space energy is centered 
on either side of the center of the filter (and the decoder), the RTTY 
will decode properly.  Besides seeing the decoded text, the CWT 
indicator in the meter area of the K3 display will show you when you 
have the Mark and Space energy centered for proper decode.   As you 
continue to tune same direction, you will lose the first shift you 
acquired as it passes out of the other side of the filter passband, and 
then finally you will lose the other shift as you continue to tune away.

73,

Bill - NA5DX



On 2/19/2016 10:06 AM, Mike Murray wrote:
> Bill,
>
> Excellent analysis and I suspect you are correct in exposing my 
> current, very limited knowledge of RTTY signals. Now that I think 
> about the problem, it makes sense that the radio is doing exactly what 
> it is supposed to when I hear the inverted tones 170 Hz down from the 
> original signal.  Rich, VE3KI, suggested turning off the dual passband 
> filter to see if that has any affect, so I'll try that in the near future.
> Thanks for the response and ongoing RTTY lessons!
>
> Mike - W0AG
>
> On Fri, Feb 19, 2016 at 8:22 AM, Bill Breeden <breedenwb at cableone.net 
> <mailto:breedenwb at cableone.net>> wrote:
>
>     Mike,
>
>     What I see in your video is that you have the RTTY signal properly
>     tuned at 14,091.883.  "Properly tuned" means that you have the
>     Mark energy and the Space properly aligned in the dual passband
>     filter and the data conveyed by the mark and space energy is being
>     decoded and displayed.  In a normal "ham radio" RTTY signal the RF
>     energy for the Mark and Space is 170 Hz apart, with the Mark
>     frequency 170 Hz higher, in RF terms, than the Space frequency. 
>     When demodulated by a receiver operating in Lower Sideband (LSB)
>     mode, this results in two audio tones, with the audio tone
>     representing the Space energy 170 Hz higher than the audio tone
>     representing the Mark energy.  In the video, when your radio is
>     tuned to 14,091.883, the tone representing the Mark energy is
>     passing through the "Mark" side of the dual passband filter and
>     the tone representing the Space energy is passing through the
>     "Space" side of the dual passband filter, and the RTTY data is
>     decoded properly.  When you tune down to 14,091.720 in the video,
>     a 163 Hz difference, you have the tone representing the Space
>     energy passing through the "Mark" side of the dual passband filter
>     and the tone representing the Space energy is outside of either
>     portion of the dual passband filter.  There is no anomaly revealed
>     in your video, that's just the way that RTTY, the receiver, and
>     the dual passband filter works when you tune away from a properly
>     tuned RTTY signal.
>
>     The reason the data on the Space tone sounds like an inverted
>     version of the data on the Mark tone is because that is exactly
>     what it is.  In an RTTY signal, the same data is carried by the
>     both the Mark and Space energy and the state of one is the
>     inverted state of the other.
>
>     73,
>
>     Bill - NA5DX
>
>
>     Message: 23
>     Date: Thu, 18 Feb 2016 09:56:06 -0700
>     From: Mike Murray<w0agmike at gmail.com <mailto:w0agmike at gmail.com>>
>     To: "Wes (N7WS)"<wes at triconet.org <mailto:wes at triconet.org>>
>     Cc:elecraft at mailman.qth.net <mailto:Cc%3Aelecraft at mailman.qth.net>
>     Subject: Re: [Elecraft] K3 - AFSK anamoly? No replies, try again
>     Message-ID:
>     <CAP7zeEJBvJCVEY-=uu21OQTN=EavupUY_WF-LwZ5_QoamEHTOw at mail.gmail.com <mailto:EavupUY_WF-LwZ5_QoamEHTOw at mail.gmail.com>>
>     Content-Type: text/plain; charset=UTF-8
>
>     After doing some more testing in last weekends RTTY contest, it
>     appears
>     that I made an erroneous statement as to what I was hearing and where.
>     What I found is that as I tune down in frequency and find a signal
>     that
>     will decode, if I then tune down another 170 Hz I'll hear the inverted
>     signal.  Still using dual passband DSP filter at 400 Hz and 400
>     Hz, 8 pole
>     roofing filter as before.  I have a brief video on uTube showing
>     the effect
>     at:
>
>     *http://tinyurl.com/hxykq9c <http://tinyurl.com/hxykq9c>*
>
>     Anyone have additional thoughts as to what's causing this or where
>     I should
>     look next?
>
>     Mike - W0AG
>
>