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In my lab experiment (http://www.ele.uri.edu/courses/ele436/labs/expA2.pdf), My amplitude modulated wave is not displaying the vertical lines-the high frequency waves of carrier (as shown in accompanied figure).

top- message signal; bottom = modulated wave(here overmodulated)top- message signal;bottom = modulated wave(here overmodulated)

top- message signal;bottom = modulated wave(here m<1) top- message signal;bottom = modulated wave(here m<1) What could be the possible reason for it. Has flickering display of oscilloscope caused the problem?. How it can be corrected?

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    \$\begingroup\$ "the vertical line(as shown in accompanied figure)". What figure/vertical line are you referring to? \$\endgroup\$ – jippie Oct 17 '13 at 18:35
  • \$\begingroup\$ Looks like the scope is in dot mode, displaying a dot for each sample, rather than the connecting lines in between them. Can you verify that? \$\endgroup\$ – jippie Oct 17 '13 at 18:37
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    \$\begingroup\$ What "vertical line" are you talking about?? It looks like your carrier wave is too low in frequency. \$\endgroup\$ – Scott Seidman Oct 17 '13 at 18:37
  • \$\begingroup\$ Sorry for confusion, By 'vertical line' I meant the almost vertical lines of high frequency sinusodial or simply the curve. \$\endgroup\$ – mohit Oct 18 '13 at 11:46
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Your digital o-scope is aliasing the high frequency content of the modulated waveform. This is a common "feature" in digital oscillosopes.

You have the time base set to show the modulating signal (orange trace) and no problem here so far but the o-scope won't be sampling at its full bandwidth because it doesn't need to. If you turned the timebase up to run more quickly you'll get to a point when you start to see the carrier frequency in all its glory.

Here is a decent document that shows what happens and here's a little video you can watch.

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  • \$\begingroup\$ Couldn't it be dot mode as explained by @jippie in above post? If only dots are displayed instead of connecting lines, the display might be the same. \$\endgroup\$ – mohit Oct 18 '13 at 11:48
  • \$\begingroup\$ Is there a 'timebase' option on oscilloscope or it automatically adjusts?? \$\endgroup\$ – mohit Oct 18 '13 at 11:54
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I got myself a Hantek 200 MHz digital scope a couple of years ago and was very disappointed when I saw the mess it made of an amplitude-modulated signal (carrier freq around 25 MHz). It was just a mess. I've worked all my life with analogue scopes and they have no problem with AM signals of any frequency within their b/w. The Hantek had only a fairly small display memory and I think this the problem. A digital scope with a much bigger memory may be the answer, but there may still be a limit on how high the carrier freq could be before aliasing sets in.

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