Ok, this is a pretty noob question.
I am a long time electronics hobbyist, but just dipping my feet into radio. Something has always confused me about AM. All the texts I see show the positive and negative halves of the modulated signal as being 180 degrees out of phase with each other. This always seemed strange, since to my mind, it would seem that basically modulating the DC operating point of a sine wave would result in having the top and bottom halves be in phase.
Ok, so I built (and Spice simmed) a circuit to investigate. I made a simple, low gain BJT common emitter stage, with the carrier injected at the base. I used two resistors from emitter to ground, and injected my modulating signal through a coupling cap to the junction between the two resistors. Thus, instead of the emitter being grounded, it was modulated by my signal. AM, right?
I got the result my thinking expected, instead of the text book signal. My new modulated signal's top and bottom halves were in phase. What's more, it seemes to me that that should still be a valid way to do AM. Since the detector dumps half of it anyway, phase between the two halves should be irrelevant, right?
And so I experimented. I tuned a transistor radio to my carrier frequency (about 600kHz), and was able to hear a clean tone. Carrier is from an opamp based relaxation oscillator. Modulation from my sig gen at 1k.
So what am I missing? Is the textbook version wrong? I doubt it, as its what you see in virtually every text I read. Or am I doing AM wrong? Also seems odd, since, by experimentation, i found my way to get a proper result.
Any thoughts?