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Here is a scope display of an AM carrier overmodulated by a continuous tone. If, as you claim, under these conditions the final collector voltage is reduced to zero, and the carrier is cutoff, then explain to me HOW THE UNIT CONTINUES TO TRANSMIT, AND HOW THAT TRANSMISSION CAN BE DETECTED BY A STANDARD AM RECEIVER IF NO CARRIER IS PRESENT. Common sense and logic tells me that the carrier is there.
The misunderstanding comes from the way a scope displays a modulated AM signal. There are three components to an AM signal: the carrier, the upper sideband, and the lower sideband. However, a scope cannot display these components seperately in a single trace. What the scope does is to ADD THE THREE SIGNALS TOGETHER AND DISPLAY THE VECTOR SUM. What happens is that as modultion % increases, the negative modulation peaks are subtracted making it APPEAR that the carrier is varying in amplitude. When modulation exceeds 100%, it APPEARS as if the carrier is cutoff, but this is not the case. If it was cutoff, transmission would cease.
If you look the conditions in the scope trace shown above on a spectrum analyzer, it will show that the carrier is present when overmodulated, and does not change in amplitude.
BTW, I posted the AMwindow link because I thought Steve did a good job of explaining how the circuit works, and that members might like to look at the additional info on the site.
Also thanks for the reply on the D1 question. That eliminated a lot of the confusion. 73s.
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