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I conducted an experiment where I constructed a Full AM signal and changed the message signals amplitude to change the modulation index of the Full AM signal. I then increased the amplitude of the modulator chip until a DSB-SC signal was made. Although they look really similar, I was wondering what the difference between them is.

It is clear from the pictures that the amplitude of the DSB-SC signal is lower than the full AM. I also understand that the DSB-SC signal does not carry a carrier signal.

Can somebody tell me what the difference of the two pictures I have uploaded are please?

Image 1 Image 2

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  • \$\begingroup\$ Inspect one cycle of modulation and closely look at the way the carrier changes as you approach and exceed 100% modulation. Have you done the math yet? \$\endgroup\$ – Andy aka Feb 5 '18 at 14:44
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Look at the two closely. There is a significant difference, even if just looking at the amplitude envelope.

Look at just the top envelope of the AM signal. It is the same sine as the carried signal. However, note that the tops of the other signal are effectively the absolute value of the sine. This results in two obvious differences you really should be able to spot. The dips are "sharp", and the frequency is double.

What you can't see on the scope at this magnification is that the lower carrier has its phase flipped 180° every hump. One way to think of this is over-driven AM. When the negative peaks of the carried wave go lower than what causes the carrier to have 0 amplitude, it causes a sortof "negative amplitude" which is the carrier with inverted phase.

To get some intuition, start with nearly 100% modulated AM, then crank up the amplitude of the carried signal and see what happens at the negative peaks. As the carrier gets multiplied by a negative number, it's phase gets flipped. As the magnitude of that negative number increases, the amplitude of the carrier increases, but still with flipped phase. This is exactly what you should expect to happen when a sine (the carrier) is multiplied by a negative number.

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With simple (less than 100%) modulation (as used in broadcast music channels) you get this: -

enter image description here

If you go over 100% modulation, the troughs in the modulated waveform above start to become small peaks with a reversal of phase of the carrier: -

enter image description here

In this scheme, because the carrier reverses phase each half cycle of the modulator there is, in effect, no carrier frequency amplitude and hence it is called "suppressed carrier".

Here is a picture of three DSB waveforms with different levels of modulation: -

enter image description here

Hopefully you should be able to see in the bottom picture the anomaly in the carrier as modulation rises past 100%. This prevents it from being used for transmission to radios that have a simple diode envelope detector.

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In AM the carrier has no polarity reversals beyond those which belong to normal sinusoidal form. In DSB the carrier sine is inverted when the low frequency signal is negative.

If your low freq signal happens to be zero, the AM signal has typically amplitude=50% of the maximum, the DSB signal is zero.

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