MMTTY AFC, O-scope (Re: [RTTY] Re: Noise with MMTTY

[Kok Chen]

On Mar 24, 2004, at 8:06 AM, Peter Laws wrote:

> Speaking of the virtual oscilloscope ... When someone is transmitting 
> with
> a shift of other than 170 Hz, I've been tuning so that the vertical 
> bar is
> vertical (which means that the other bar is not quite horizontal).  Is
> that right, or should I tune so the horizontal bar is actually 
> horizontal.
>
That usually means the phase relationship of the mark and space filters 
are not "right."  You need the filters to behave so that the mark 
filter has a 90 degree relative phase shift at the space frequency, and 
vice versa.

What I did on my digital display is not to play with the filter phases, 
but to apply a rotational transform to one of the output channels 
before drawing to the screen.  Adjusted it so that a 170 Hz shift 
signal is fully orthogonal, and everybody who transmits at 170 Hz comes 
into my shack correctly orthogonal.  You can do that with "real" scopes 
too, just two resistors if you are lucky, or an extra op amp to reverse 
the voltage polarity of one signal if you are not so lucky, but at most 
one op amp and 3 or 4 resistors.  Easier than tinkering with filter 
phases.

Funny thing is that there are a couple of big guns (well, RTTY 
regulars) who are off 170 Hz.  Perhaps an off adjusted Omni in FSK 
mode?  VY1JA had a really off tuned FSK signal (like 100 Hz or 
somewhere there) some years ago but no one told him about it until I 
mentioned it.  J has since calibrated the FSK capacitor on his Omni.

73
Chen, W7AY