[RTTY] Digital Oscilloscope for tuning RTTY?

[Kok Chen]

On Jul 29, 2011, at 10:37 AM, Mike Harris wrote:

> What advantage is gained from the XY scope that isn't available from a 
> waterfall display from the point of view of signal tuning?

You can see many RTTY characteristics with just a single tuning tool, and as a tuning tool (with an analog scope), it is very, very fast.

You can easily notice selective fading (one ellipse shrinking relative to the other, sometimes in the blink of an eye).  You can also see multipath (when one ellipse is large while the second ellipse does not completely vanish at the same moment).  Most waterfalls will not catch fast selective fades, and waterfalls are practically useless to determine if there is multipath.  You can use that information to quickly pick some post-processing steps in your modem that optimizes for selective fading or for multipath.

IMHO, waterfalls are better, of course if you use mouse clicks to tune.  Cross ellipses are better if you use the VFO knob to tune.

However, for fast *fine* tuning, a cross banana is still better than the waterfall.  cocoaModem lets you tune with a mouse click in a waterfall, but it also displays a crossed ellipse for you to fine tune with the mouse scroll wheel (or the Griffin PowerMate).  

For signals that have poor SNR, being able to get within 10 Hz is often helpful.  cocoaModem's waterfall has a resolution of 2.7 Hz per pixel on the screen (your mileage may vary with the modem that you use).  It is much easier to get better fine tuning with the crossed bananas than by staring at a noisy waterfall.

Do not base how good or bad a cross ellipse display is by what you see from a software modem.  Their implementation quality varies.  Get a good analog scope that is fed with some analog filters.  ... then, attempt to emulate that response with software.   I will even wager money that some software modem developers (the ones W6WRT calls lazy :-) have never used an analog crossed ellipse display in real life (and I am a cheap skate when money is concerned; no linear amplifiers, no commercial beam antennas).

73
Chen, W7AY