[GreenKeys] using a scope for rtty

[Bill Henry]

Eugene,

The ellipses you see from your ST-6 are normal.  They are not straight 
lines and are not orthogonal loops for a good reason.  The Mark and Space 
filter bandwidths in the ST-6 are 105 Hz.  This is normal for a modem that 
will accept data rates from 45 to 110 baud - as the ST-6 does.  The 
ellipses are due to the fact that some of the received Space signal is 
"seen" through the Mark filter (and vice-versa).  Therefore a received 
Space signal at 2295 Hz produces a lot of voltage output through the Space 
filter (vertical scope channel) - and a little voltage out of the Mark 
filter (horizontal scope channel).  But, the different tuning of the two 
filters changes the relative phase of the two signals and that causes the 
"tilting".   Correct tuning is achieved when both the Mark and Space 
ellipse major axis are at maximum voltage.  That is generally with the left 
end of the horizontal ellipse (Mark) tilted up a little and with the top 
end of the vertical ellipse (Space) tilted a little to the left.  Notice 
that the two ellipses rotate as correct tuning is achieved.  With practice, 
this becomes a very useful trait as it becomes easy to predict how far off 
you are and when to quit turning the receiver knob - and in particular 
which direction to turn the knob.  In the past, some hams added extra 
filters to the Mark and Space scope output channels to get perfectly 
straight lines.  This gave a really pretty scope display but the scope 
display now shows how well your tuning matches the extra scope channel 
filters - NOT how well your receiver tuning matches the demodulation 
filters - and those are what really count.  I have played with a lot of 
different RTTY tuning indicators and hooking a good X-Y scope directly to 
the Mark and Space modem filter outputs is one of the best.

Bill Henry, K9GWT

At 12:31 AM 3/18/2007 +0000, Eugene Hertz wrote:
>Hi All,
>
>Now that my rtty station is shaping up, I decided to pull out my tektronix 
>604 XY Monitor out of storage.  And I connected it to my ST6 and had a 
>question.
>
>Seems I was expecting a vertical ellipse and a horizontal ellipse 
>corresponding to mark and space (or maybe space and mark, I forgot which 
>is which).  However the horizontal was not quite horizontal and the 
>vertical was not quite vertical!  The horizontal seemed to go up hill and 
>the vertical had a similar angle.
>
>Now, I took my precision digital function generator and programmed in 2125 
>and 2295 so I know I'm putting in the right freqs.  So why is this so? 
>Does this mean that my ST6 is not quite tuned properly? Does it mean my XY 
>scope is off?
>
>Thanks
>Eugene
>
>
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