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
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