Lissajou patterns

Guest
Hi!

I am having problems with displaying Lissajou patterns (for educational
purpose) with a Tektronix 2246 and I need some help.
I have one and only one function generator set to display sinusoidal
signal. I put a T BNC connector on the output plug of the generator on
plug to 50 ohms BNC to it and I have connected the 2 ends to the scope
on CH1 and Ch2.
I have inverted the CH2 to have the exact same signal and frequency
inverted. I have changed the mode to XY and I am getting 2 diagonals
lines (like this <).
From there I cannot get any Lissajou figures. I have tried to change to
different mode, different type of current and different everything but
Click to expand...
nothing works.
I would really appreciate a little help from confirmed technician which
I am obviously not...
Thank you.
 
cubik@yahoo.com wrote:
Hi!

I am having problems with displaying Lissajou patterns (for educational
purpose) with a Tektronix 2246 and I need some help.
I have one and only one function generator set to display sinusoidal
signal. I put a T BNC connector on the output plug of the generator on
plug to 50 ohms BNC to it and I have connected the 2 ends to the scope
on CH1 and Ch2.
I have inverted the CH2 to have the exact same signal and frequency
inverted. I have changed the mode to XY and I am getting 2 diagonals
lines (like this <).
From there I cannot get any Lissajou figures. I have tried to change to
different mode, different type of current and different everything but
nothing works.
I would really appreciate a little help from confirmed technician which
I am obviously not...
Thank you.
Click to expand...
Apparently your output is exactly right. The whole point of Lissajous
figures (cue "The Outer Limits" music -- "Do not adjust your screen.
We control the horizontal. We control the vertical") is comparing
frequencies and phases. If they're both identical, you aren't going to
get any more than what you have.

You might want to try getting a second function generator, or cobbling
up a PLL circuit or something else to get two different frequencies to
compare.

Actually, Lissajous figures are still vitally important in electronics.
If I have to read a manual or data sheet or do something important on
their problem that doesn't require wrench twiddling, and they want some
wrench twiddling on their problem RIGHT NOW, I have been known to set
up a Lissajous figure on my bench scope with one frequency drifting
slightly to cause slow but constant motion on the display. I then put
the scope on my bench next to the protoboard, with a large sign stating
"CAUTION -- TEST IN PROGRESS!" That has a tendency to keep
non-technical people out of my hair so I can think. For some reason,
they stare at the display for twenty seconds or so, then walk away with
either a blank or frustrated glance. I think there's something about,
say, a slowly drifting 4:3 or 3:2 display that causes great inner
confusion in non-technical persons. ;-)

Good luck with the homework. Try googling with a different spelling --
there are a lot of results for "Lissajous figure setup" (no quotes).

Chris
 
cubik@yahoo.com writes:
Hi!

I am having problems with displaying Lissajou patterns (for educational
purpose) with a Tektronix 2246 and I need some help.
I have one and only one function generator set to display sinusoidal
signal. I put a T BNC connector on the output plug of the generator on
plug to 50 ohms BNC to it and I have connected the 2 ends to the scope
on CH1 and Ch2.
I have inverted the CH2 to have the exact same signal and frequency
inverted. I have changed the mode to XY and I am getting 2 diagonals
lines (like this <).
From there I cannot get any Lissajou figures. I have tried to change to
different mode, different type of current and different everything but
nothing works.
I would really appreciate a little help from confirmed technician which
I am obviously not...
Thank you.
Click to expand...
Well I used to design these things for Tek but that was LONG ago.

With only one signal generator you are getting exactly what I would
expect.

Forget scope for a moment. Just think algebra for a moment, you
are going to plot a point (x,y) and the value for x is exactly the
same as the value for y. So you get a point somewhere on a diagonal
line y=x. You change to a different x, but the new y will be the
same as the new x, so you get another point, again on that diagonal
line.

How about this instead. Find a little "wall wart" plug in power
supply that outputs AC, maybe 6 volts or so. Hook one of your
inputs to that. Probably better use your scope probe to attach to
that, instead of connecting it directly, don't want to toast your
input channel.

Then set up your signal generator to be say 120 hz. Hook that to
your other input. See if that gives you something other than a
line. Then gently rock the generator frequency up and down a bit.
 
cubik@yahoo.com wrote:
Hi!

I am having problems with displaying Lissajou patterns (for educational
purpose) with a Tektronix 2246 and I need some help.
I have one and only one function generator set to display sinusoidal
signal. I put a T BNC connector on the output plug of the generator on
plug to 50 ohms BNC to it and I have connected the 2 ends to the scope
on CH1 and Ch2.
I have inverted the CH2 to have the exact same signal and frequency
inverted. I have changed the mode to XY and I am getting 2 diagonals
lines (like this <).
From there I cannot get any Lissajou figures. I have tried to change to
different mode, different type of current and different everything but
nothing works.
I would really appreciate a little help from confirmed technician which
I am obviously not...
Thank you.
Click to expand...

The point of lissajous figures is that the frequencies of the x and y
components are related by some ratio. Thus, two signal generators are
required.

You can easily build a sine wave generator, using an opamp and a few
other components. Google for 'wein bridge'. There are also signal
generator chips that can be had for a few bucks that will generate very
nice slow sine waves, which is what you want. The waveform needs to have
a stable frequency, or it's hard to match.

I saw an exhibit at the SF exploratorium, in which thin vertical rods
were fixed at the bottom, and thinned along their axis in particular
directions so they would oscillate at different frequencies in different
directions when 'plucked'. That isn't a very good description, but the
result was a lissajous figure traced out by the end of the rod when
plucked. The amount of thinning along the rod determined what figure
would emerge, figure 8, three loops, etc. It was very cool.

--
Regards,
Bob Monsen

If a little knowledge is dangerous, where is the man who has
so much as to be out of danger?
Thomas Henry Huxley, 1877
 
On 20 Jul 2005 09:14:48 -0700, cubik@yahoo.com wrote:

I would really appreciate a little help from confirmed technician which
I am obviously not...
Click to expand...
It helps if the input frequency on ch1 and ch2 is not the same or
near. For example, put one channel on 60hz and another on 65hz and
you get a rotating circle. If the second frequency about double (but
not exactly) than the first, you'd get shifting figure 8.

It would help if you have 2 different frequency generator (both on
sine wave) and you vary the setting of one.
--
When you hear the toilet flush, and hear the words "uh oh", it's already
too late. - by anonymous Mother in Austin, TX
To reply, replace digi.mon with phreaker.net
 
cubik@yahoo.com wrote:
Hi!

I am having problems with displaying Lissajou patterns (for educational
purpose) with a Tektronix 2246 and I need some help.
I have one and only one function generator set to display sinusoidal
signal. I put a T BNC connector on the output plug of the generator on
plug to 50 ohms BNC to it and I have connected the 2 ends to the scope
on CH1 and Ch2.
I have inverted the CH2 to have the exact same signal and frequency
inverted. I have changed the mode to XY and I am getting 2 diagonals
lines (like this <).
From there I cannot get any Lissajou figures. I have tried to change to
different mode, different type of current and different everything but
nothing works.
I would really appreciate a little help from confirmed technician which
I am obviously not...
Thank you.
Click to expand...
Not only do you want two signal generators, but you should get two
Function generators, which are able to generate other types of
waveforms apart from sinusoidal. You can get some real funky lissajou
displays with waveforms like triangle waves, ramps, square etc

Dave :)
 
On Wed, 20 Jul 2005 15:42:05 -0700, Bob Monsen wrote:

Google for 'wein bridge'
Click to expand...
"Wien bridge" would be better.

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
(Stephen Leacock)
 
Fred Abse wrote:
On Wed, 20 Jul 2005 15:42:05 -0700, Bob Monsen wrote:


Google for 'wein bridge'


"Wien bridge" would be better.
Click to expand...
yes, it would. thanks.

--
Regards,
Bob Monsen

If a little knowledge is dangerous, where is the man who has
so much as to be out of danger?
Thomas Henry Huxley, 1877
 
As others have pointed out, with just one function generator all you will
get is a diagonal line. What you see with a Lissajous setup is the
graphical relationship between two signals. If both signals are the same,
then what you see is what you get.

If you can get your hands on another function generator, then you're all
set. Or set up a circuit to turn your sine wave into a cosine wave
(shifting it by 90 degrees) and you'll get a nice circle. But it'll be
easier just to get another generator.

Plus, you can experiment with wave shapes -- triangle, square, etc.

If you really want to ham it up you can get a third generator and play with
the insensity of the beam -- the Z axis. You can turn the beam on and off
and get some really interesting patterns. This is what laser shows did
before they got computers to control everything.
 
"Bob Monsen" <rcsurname@comcast.net> wrote in message
news:LfGdnVRxR74oZULfRVn-vQ@comcast.com...
Fred Abse wrote:
On Wed, 20 Jul 2005 15:42:05 -0700, Bob Monsen wrote:


Google for 'wein bridge'


"Wien bridge" would be better.


yes, it would. thanks.

--
Regards,
Bob Monsen

If a little knowledge is dangerous, where is the man who has
so much as to be out of danger?
Thomas Henry Huxley, 1877
Click to expand...
wein bridge = 634 hits
wien bridge = 5500 hits

surprisingly many people confused about how its spelt.

Colin =^.^=
 
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