Understanding AM - Page 1
Forum section: electronics / rf-microwave

author: Dan Moos
author_rank: Frequent Contributor
timestamp: December 04, 2016, 06:53:30 am
content: 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?

author: Paul Rose
author_rank: Regular Contributor
timestamp: December 05, 2016, 02:50:19 am
content: I'll take a stab at this, since you haven't gotten any replies yet, but I'll be happy to defer to an expert.I might not be understanding your question correctly, maybe if you attached a diagram of what you meant by positive and negative halves being out in or out of phase.Your unmodulated carrier voltage is Vc(t) = Ac * sin( 2*pi*Fc*t )Your modulation voltage is Vm(t) = Am * sin( 2*pi*Fm*t )WhereAc is carrier amplitudeFc is carrier frequencyAm is modulation amplitudeFm is modulation frequencyand ignoring any initial phase at t=0These are both bipolarity sine waves.If you modulate the carrier with the modulation in a balanced mixer ( that outputs instantaneous voltage product  of both input volatages), you getV(t) = Am*Ac*sin(2*pi*Fc*t)*sin(2*pi*Fm*t)This is a bipolarity signal, that has components ( using trig product formula ) of0.5*Am*Ac* [ cos(2*pi*(Fc-Fm)*t) - cos(2*pi*(Fc+Fm)*t) ]The only two frequencies represented are Fc-Fm and Fc+Fm. The carrier frequency itself isn't present. This is DSBSC ( double sideband suppressed carrier).If you change the modulation voltage toVm(t) = Am * [ 1 + sin( 2*pi*Fm*t ) ]by biasing the modulation so that it touches zero, but does not got below it, and then carry out the multiplication and follow the trig product formula ( sorry I'm too lazy to type it out ), you will find components at Fc-Fm, at Fc itself, and at Fc+Fm. This is regular AM with a carrier.Even though you have biased the modulation so never go below zero, the carrier still goes below zero, and the product is still bipolarity.You shouldn't be able to get baseband out of an AM detector with DSBSC, without replacing the carrier ( traditionally this is called a BFO ), but even a little bit of carrier bleed through will be enough, and it doesn't take much imbalance in the modulator for that to happen.

author: AG6QR
author_rank: Frequent Contributor
timestamp: December 05, 2016, 04:02:48 am
content: I had to read through a few times. We usually don't use terms like "top half" and "bottom half", and the idea of the halves being in phase or out of phase is not standard terminology.Anyway, the standard graph of amplitude modulation is shown on the Wikipedia page, among other placeshttps://en.wikipedia.org/wiki/Amplitude_modulation(That animated picture also shows FM modulation, but we'll ignore it for now).Correct me if I'm wrong, but Ithinkthat, when you're talking about the "top half and bottom half being out of phase" you mean that, if you draw a curve connecting the upper peaks, and then draw another curve connecting the lower peaks, you'll see a pair of sine waves, offset from one another, so that the highest point on the upper sine wave corresponds with the lowest point on the lower sine wave.I believe that when you talk about the "upper and lower halves being in phase", you're talking about having a varying offset of the carrier wave. That is, having the carrier wave be of constant amplitude, but having it superimposed on a DC value that changes with time.If I've understood that terminology the way you intended, then the latter thing is NOT amplitude modulation. The amplitude is remaining constant -- it's the DC offset that is changing. That's not modulation, that's just superposition. It won't work for over-the-air broadcasting, because the receiver tuned to receive the carrier frequency will simply receive a constant amplitude carrier, and it won't receive the varying offset.In Amplitude Modulation, the amplitude of the wave must change. Amplitude is the difference from an upper peak to the subsequent lower peak.

author: ebclr
author_rank: Super Contributor
timestamp: December 05, 2016, 04:34:11 am
content: If you waves are in phase, your circuit is adding , but am modulation need a multiplication.

author: Dan Moos
author_rank: Frequent Contributor
timestamp: December 06, 2016, 01:29:50 am
content: I now understand my error. You guys were correct in your interpretation of my post. I wanted to do some scope shots, but you guys sorted it out before I got the chance.What I need now is some links on how to do it properly. It sounds to me like I need some sort of voltage controlled amplifier, yes?

author: Paul Rose
author_rank: Regular Contributor
timestamp: December 06, 2016, 03:48:33 am
content: You can do it with a balanced modulator that is intentionally unbalanced on the baseband (IF) portSee figure 6, and the text at the top of page 3, in the app note for the MC1496.The 1496 is a "gilbert cell" style balanced mixer IC.http://www.g1sle.com/files/downloads/MC1496&1596.pdfThe same thing can be done with a diode ring mixer by biasing the baseband input with some DC.Neither of these is the traditional way to generate AM, however. Look up "plate modulation", where the plate voltage of a vacuum tube amplifier is modulated by the baseband signal.

author: w2aew
author_rank: Super Contributor
timestamp: December 13, 2016, 01:59:54 am
content: Quote from: Paul Rose on December 06, 2016, 03:48:33 amI did a couple of videos on the Gilbert Cell. The first is the basics of the Gilbert Cell, and the second discusses AM and DSB-SC modulation with the Gilbert Cell.

author: Dan Moos
author_rank: Frequent Contributor
timestamp: December 23, 2016, 03:00:07 pm
content: Actually W2aew, I had discovered your video since I last was on this thread. I built up your example. I didn't get it working yet, but it was late and I didn't spend much time debugging. It's a rat's nest on a breadboard. You know how that goes!A video that I would love though, is help understanding the biasing network in your final example circuit. I normally don't have trouble with that sort of thing, but there is allot going on in there!As a side note, your videos are what inspired me to try my hand at RF. Keep up the fine work!