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Sure you can exceed more than 100% neg mod. 100% is exactly the amount needed to fully and properly modulate a carrier. Any more than what is required is more than 100%. When that happens tha carrier is turned off for a slight amount of time and then back on. This results in lots of modulation byproducts that appear as distortion.

 

The following are taken from www.nu9n.com/am.html and show the various waveforms of differing levels of AM modulation.

 

Remember that it is the neg peaks that cause the distortion and overmodulation not the positive peaks. The pos peaks add "loudness" to the signal. It has worked for decades in the commercial broadcast industry where low distortion is a must.I once tested a Nautel  transmitter at 100%neg and 220% pos modulation when I had modulator troubles. It looked pretty cool on the 'scope and man the sucker was LOUD but clean but I did not dare put it into the antenna system. The legal limit here is 125% pos peak mod.

 

 

 

Figure 3

Scope Calibrated for +1 and -1 Divisions

With Full Unmodulated Carrier Applied

 

Figure 4

AM at 100% Symmetrical Modulation

 

 

Figure 5

AM Undermodulated

 

 

Figure 6a

AM Overmodulated (Low-Level)

 

 

Figure 6b

AM Overmodulated (High-Level)

 

 

Figure 6c

AM Asymmetry - Correct Phase (Pos 110%, Neg 95%)

 

 

Figure 6d

 

AM Asymmetry - Inncorect Phase (Pos 95%, Neg 110%)