[RTTY] Digital Oscilloscope for tuning RTTY?

[Kok Chen]

On Jul 29, 2011, at 7:22 AM, Kim A. Hinceman wrote:

> Rigol has an attractively priced digital oscilloscope (DS1052E) available.  Can digital scopes be used to view "crossed bananas" for tuning?  If so, any caveats?

Check their specs to see if the horizontal amplifier is switchable from the internal sweep to an external input.  Most of the DSO allow that.  My Tektronix TDS 2024B, also a cheapish DSO, does it by allowing one of the four "vertical" channels to replace the sweep generator of the horizontal channel.

That said, you need to go to higher sampling rates or you will get the sampled data appearance of crossed bananas.  

The advantage of the old analog scopes is that cross bananas and Lissajous patterns show up as continuous curves.  Software modems do not plot cross bananas as prettily as analog scopes until you use sampling rates north of 48000 samples per second. 

On the other hand, the advantage of a DSO or software crossed bananas (unless you use a CRT with your computer) is that you will not get phosphor burns (I am lucky that my ST-8000 does not have phosphor burns).  At one time in the early 1990s, I has used a Tektronix XYZ display (nice 5" rectangular CRT that had cost an arm and a leg when new, but I'd picked it up at a surplus store) as my crossed ellipse display and I had fed the rectified amplitude of the RTTY signal into the Z channel to dim the display when the both mark and space channels are low.

IMHO, anyone who has not seen a good crossed ellipse display on an analog display don't know that they are missing.

If you are going to use a scope with a software modem, check first to make sure that your modem can feed the filtered Mark and Space output back into a separate sound card. Most of them can't, and you need to check if the source code is available for you to modify them to send filtered Mark and Space out to your scope. 

If you are using a fixed tone pair in your RTTY operations, I would recommend feeding your scope from a pair of analog filters that are tuned to the Mark and Space tones and tap that from your receiver.  (88 mH coils are good for that, I'll bet WS7I still has some he is willing to part with HI HI).

You need to make sure that the Mark filter produces a 90 degree phase shift when tuned to the Space frequency, and likewise, the Space filter produces a 90 degree phase shift at the Mark frequency.  Without those properties, your crossed bananas will not cross each other at right angles when you are perfectly tuned to a 170 Hz RTTY station.

In the computer graphics world, you can do it easily without resorting to adjusting filter phase shifts.  In cocoaModem, I had used a transformation matrix to "rotate" the points from the mark and space filters (http://en.wikipedia.org/wiki/Transformation_matrix).  But if you are using an external scope, and if the Mark and Space filters are not designed for crossed ellipse display, you will need some pots between the sound card outputs and the scope inputs to adjust the slopes of the two bananas.  You will need to readjust when you change shift (even a 170 Hz to 200 Hz shift difference is huge -- that is how we easily tell a 200 Hz shift on a crossed ellipse display that is tuned for 170 Hz shift).

Another warning is that Mark and Space filters that are designed for demodulation are often too narrow for a pleasing crossed banana display. (cocoaModem uses different filters, using an FIR for the demodulation Matched Filter, and using a pair of simple IIR for crossed bananas).

When the filters are too sharp, the ellipses become skinny, and it is harder to find out which direction you need to nudge the VFO know to bring the signal in tune when the off tuned signal is for example 120 Hz away.  You need to try one direction and if wrong, reverse the VFO tuning direction; thus wasting valuable time.  After all, a crossed banana's main purpose is to tune rapidly.  You want to design all the parameters "just right" for quick tuning.  Nothing other than AFC, and I mean nothing, is as fast a tuning tool as a well crafted crossed ellipse display.

73
Chen, W7AY