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Lissajous pattern interpretation...

T

Tom Wells

Finally woke up the Tektroniz 454A scope and hooked it to the AM 1620 signal. Both leads shorted at tips, hooked to output thru
.001 gave a nice sharp circle with no mod, but 45 degrees. Well, maybe 50 degrees, sligthly fuller than "half squashed".
Now one lead is 3 feet. the other eight.
Am I seeing different phase as a result of different lead lengths, as X-Y pickup differences, or what?

I have always known how to do this, never had tried with this scope, but now that I have, remembering just what circuit deficiencies impact
phase and pattern appearance is difficult.

It certainly is a great way to monitor modulation percentages, and I am happy to see my grid modulated circuit
give such full mod without distorting. I didn't think I was able to go over 100% with a Van der Bijl modulator, but I clearly can.
It "overdraws" the center while not cutting off carrier or even beginning to.
In fact, as loud as I turn up the mod, the hole in the center goes away and turns into a wide bright point, the width of
which is the +100 parts, while never cutting off carrier. The audio is smashed like a fly on the front of a truck, but not
hitting -100%. So far so good, but should I be able to adjust antenna loading or coupling to achieve a full circle?

How much are the old caps in the scope likely to contribute? Both channels would have comparable components.....

As I have not been in the industry, will the voice of experience please relate what phase angles are normally seen in good
and bad installations, and general reasons for not making a perfect circle.

I understand that an equal voltage relationship and constant phase relationship should equal a circle.

Is this the normal result of the load of the antenna?
Is a true non-inductive dummy load going to give me a circle?


This is way better than the silly scope display in windows media player or any of their "view" choices.
In looking at the textronix site, I see that Digital Oscilliscope "emulators" have a hard time or simply cannot do a real dynamic
"live" X-Y display as would be needed in broadcast work. Has anyone tried to use the Fluke Scopemeter (probably TM) on
an AM for Lissajous patterns?

So many questions on a Sunday night.
 
Extensive fiddling with lead dress, tip sensitivity selection, and fussing with A and B channel gain gives something close to 80 degrees.

I should be happy, I think. Except this proves to me that my mike impedance don't match the pre-amp nearly well enough.
Always another thing to think about. Voice looks bad on waveform or lissajous.
 
Both channels are connected to the output of the tx through a .001 cap.
Grounding to earth for the scope picks up too much AM 820, so I tied the shield to the tip right at the probes.
Instead of totally shorting the input, as would be expected if you set the channel input to ground,
this gave a good and very clean signal without hum and noise jaggies.

I expect this means my actual pickup is only in the inductance of the lead itself.
I expected since the source was same for both, and I selected X-Y on the scope, I'd get the 90 deg circle.
I wasn't looking for phase shift through the modulator by comparing x-y before and after.

The signal should be the same to both inputs, but the two scope input leads are different lengths, and I couldn't set both probe tips to the same X1, or X10 gain. Seems one needed to be higher than the other, and I had to adjust A/B gain to get the circle as
"open" as possible.
 
feed one input the RF envelope, and one rectified audio. I'd hafta dink with it, been years - but you can end up with a trapeziod which pinches to a tip at 100% negative. Great tool for setting it as close as your final limiter will allow.
 
littlejohn said:
feed one input the RF envelope, and one rectified audio. I'd hafta dink with it, been years - but you can end up with a trapeziod which pinches to a tip at 100% negative. Great tool for setting it as close as your final limiter will allow.
Click to expand...

I'm with Littlejohn. Trapezoid is the way to go for serious AM measurement.
But rather than use de-modulated audio, I would use the modulating audio (pre-transmission) as the horizontal input, with modulated RF on the vertical, in order to see the effect the transmitter has on modulation linearity.
If the sides of the trapezoid are straight, you know the linearity is good. Curvy; not so good.

Kind Regards,
David
 
Tom Wells said:
The signal should be the same to both inputs, but the two scope input leads are different lengths, and I couldn't set both probe tips to the same X1, or X10 gain. Seems one needed to be higher than the other, and I had to adjust A/B gain to get the circle as
"open" as possible.
Click to expand...

If the two channels of a scope set for "X-Y" are identical (and they're useless if they're not!), and the gains are set the same, then an identical signal on both inputs should result in a perfectly straight line at exactly 45 degrees, NOT a circle!! The effect of a few feet of lead length difference should not be visible.

Kind Regards,
David
 
David Reaves said:
Tom Wells said:
The signal should be the same to both inputs, but the two scope input leads are different lengths, and I couldn't set both probe tips to the same X1, or X10 gain. Seems one needed to be higher than the other, and I had to adjust A/B gain to get the circle as
"open" as possible.
Click to expand...

If the two channels of a scope set for "X-Y" are identical (and they're useless if they're not!), and the gains are set the same, then an identical signal on both inputs should result in a perfectly straight line at exactly 45 degrees, NOT a circle!! The effect of a few feet of lead length difference should not be visible.

Kind Regards,
David
Click to expand...

My very-first connection resulted in a darn-near-if not-45 dgree angle line, so I guess I shouldn't fuss.

I should refer to some texts ( ARRL handbook Terman, etc) but I'm stuck in color-analysis software school at X-Rite in Grand Rapids, Michigan .
When I return to home I will sample ( rectified ?) audio on ch A, RF on ch B.
Non-linearity, I presume, is seen as a "fat" or "skninny" trapezoid shoulders.
Certainly one of my Valpo tech texts spells this out explicitly.

I tell you for sure it never worked very well on my 1956 Du Mont scope. Gotta fix this one. too pretty to ditch.


Anyone need a gigantic Tektronics roll-about scope? I have one on-cart, with one "extra" plug-in amp ( not sure which mebbe hi-speed))
and another of same size without cart. Both with a Major-mfr pedigree. Bell Labs , I think.
AT & T , Western Electric or some such.. I'm out of room, and someone needs 'em.

Reasonable offers will be very seriously considered, and need be such (fair enough ) as to permit me to share any sales price with the guy who got laid off from AT & T. ( Even though I've just had them sitting around for years I'd be sharing with him.)
 
Connecting to audio vs RF gave me a fine trapazoid with a bit of fatness.
I was surprised to see modulation over 100% appear as an extended peak above beyond the perfect trapezoid cutoff "top".

A bit more fiddling with gain shows that the "line" at the top and bottom of the trapezoid IS the previously seen circle, squished flat.

If I really fiddle I can get get a partial "tilt" on the trapezoid, and see a live 3-D modulation conic section display.

Nobody needs a scope the size of a big cooler?
 
Tom Wells said:
Connecting to audio vs RF gave me a fine trapazoid with a bit of fatness.
I was surprised to see modulation over 100% appear as an extended peak above beyond the perfect trapezoid cutoff "top".

A bit more fiddling with gain shows that the "line" at the top and bottom of the trapezoid IS the previously seen circle, squished flat.

If I really fiddle I can get get a partial "tilt" on the trapezoid, and see a live 3-D modulation conic section display.

Nobody needs a scope the size of a big cooler?
Click to expand...

Any deviations from perfectly straight sides of the trapezoid (such as curviness, 'fatness', etc.) indicate imperfections in the modulation (or the measuring technique). Distortion, phase shift, etc.

The 'point' of the trapezoid indicates 100% negative modulation. Overmodulation in the negative direction will make the point extend out in a straight line.

The wider end of the trapezoid indicates positive modulation. By setting the number of scope divisions from the center, (dead carrier, no audio) to the 'point' of 100% negative modulation, you can take that measurement the opposite direction to show where 100% positive modulation occurs. Since there is no limit to the positive side, you can see how positive modulation can extend well beyond 100%. If all is working well, that is. ;)


Glad you're having fun!
Kind Regards,
David
 
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