Help understanding AM - Page 1
Forum section: electronics / beginners

author: Dan Moos
author_rank: Frequent Contributor
timestamp: December 05, 2016, 02:35:16 am
content: Ok, hopefully this isn't a breach of forum etiquette. I posted this in the RF section, but got crickets. What follows is a direct cut and paste of my original post.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: helius
author_rank: Super Contributor
timestamp: December 05, 2016, 02:51:10 am
content: "180 degrees out of phase" is equivalent to inverted polarity for primarily sinusoidal signals. You can't have any phase difference between "positive and negative halves" of a signal: that's mixing up the time axis with the f(t) axis.AM is not "modulating the DC operating point". It is modulating the amplitude, which is most often symbolized by a direct multiplication of the carrier with the signal. By superimposing your signal with a capacitor, you've essentially done adding instead of multiplying. The results appear similar for SMALL modulation indexes. When you make the modulation index LARGE they are very different and I doubt you will cleanly recover the signal anymore.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: December 05, 2016, 02:55:51 am
content: What you did is not amplitude modulation. Amplitude modulation mathematically involves a product of the modulating signal with the carrier. Something like (1+a*sin(modulation signal))*sin(carrier signal). a is the fraction modulation. The multiplication is why the upper and lower halves are in phase. What you have done is a summation of the two signals.If you looked at your signal in the frequency domain you would see one spike at the modulation frequency and another at the carrier frequency. An AM modulation signal would show a spike at the carrier frequency, with two other spikes just above and below the carrier, spaced off by the modulation frequency.Your receiver may have produced a response to this signal for a number of reasons. The most likely is that a non-linearity in the input stages actually produced AM modulation which then worked in later stages.

author: Dan Moos
author_rank: Frequent Contributor
timestamp: December 05, 2016, 03:15:55 am
content: Ok, I sort of get it now. I'm basically taking a carrier of constant P to P amplitude, and modulating the center point, rather than actually modulating the amplitude.Two questions. First, why isn't this a valid method? If you draw a horizontal line at the average mid point of my modulated signal, the two halves individually look like a demodulated, but yet to be filtered signal. I realize it's not the correct way, but the end result seems right. To my eyes, a detector should get the same result as real AM.Second, correct me if I'm wrong, but instead of modulating the operating point, I want some sort of voltage controlled amplifier, right?

author: hamster_nz
author_rank: Super Contributor
timestamp: December 05, 2016, 04:06:26 am
content: I'm doing some thinking and it all comes down to this:cos(x)/2+cos(-x)/2 = cos(x)andsin(x)/2+sin(-x)/2 = 0Because your modulation signal lacks the quadrature information it is acting as half the sine of positive frequency summed with half the sine of negative frequency, which gives a constant quadrature value of 0.When you scale (multiply) your carrier by the modulation signal you end up with the final three signals in the output.- a proportion of the carrier speed up by f_modulation- an equal proportion of the carrier slowed down by f_modulation- the reminder being the rest of the carrierThe balance between the three depends on the modulation depth, at 100% modulation there will be no carrier left in the final signal, at 0% modulation it will be all carrier

author: CatalinaWOW
author_rank: Super Contributor
timestamp: December 05, 2016, 04:19:21 am
content: Quote from: Dan Moos on December 05, 2016, 03:15:55 am

author: Dan Moos
author_rank: Frequent Contributor
timestamp: December 05, 2016, 04:50:30 am
content: Thanks for the help! The net seems to be devoid of well explained fundamental rf stuff. It 's particularly disappointing that the Art of Electronics says so little on the matter. Strange gap in an otherwise great text if you ask me.My goal is to build a decent receiver that doesn't depend on modern digital stuff. Towards that goal, I figured I needed a weak transmitter to test it with. I am close to purchasing a better signal generator (siglent dsg2042x) that will do AM, but even then, I want to roll my own for the learning experience.What would be a good method to amplitude modulate a signal that doesn't require more than jelly bean parts?

author: jh15
author_rank: Frequent Contributor
timestamp: December 05, 2016, 05:10:47 am
content: Decent receiver? germanium diode or galena cat whisker, coils on oatmeal box, black high impedance headphones.

author: Dan Moos
author_rank: Frequent Contributor
timestamp: December 05, 2016, 05:40:50 am
content: Ha! No, I am going for a a super het type.

author: AG6QR
author_rank: Frequent Contributor
timestamp: December 05, 2016, 06:18:18 am
content: Quote from: Dan Moos on December 05, 2016, 04:50:30 amSimplest: Take one of those standard four-pin crystal oscillators in a can, and instead of feeding it steady DC power, feed it a varying DC signal.The result will be low-power, crude, and full of harmonics (use a low-pass filter to clean them up if that's important to you, and please don't try to transmit the signal at any significant power), but it can be detected nearby with a simple AM radio tuned to the frequency of the crystal.As an alternative, use an ordinary signal generator, and modulate its output by combining it with the modulating signal using an Analog Devices AD633 analog multiplier IC. The data sheet has an example circuit to do linear amplitude modulation. That's probably more expensive, and it requires that you have some sort of RF oscillator already, but you aren't restricted to using a fixed frequency crystal controlled carrier.

author: DimitriP
author_rank: Super Contributor
timestamp: December 05, 2016, 08:43:55 am
content: QuoteTime for another couple of books:ARRL Radio Handbook and Experimental methods in RF designAs far as designing with discrete components go take a look here:http://www.n5ese.com/2n2222.htm

author: C
author_rank: Super Contributor
timestamp: December 05, 2016, 02:18:27 pm
content: One old method to create AM is to use an audio transformers secondary as power source to RF amplifier.Connect audio transformer secondary as follows,One connection to DC Level.Second connection to RF amp's DC source.You now have a DC level that changes +-audio level.100% modulation is when the audio input level is such that the lowest DC voltage peak just turns off RF output.One trick I have seen is using a second secondary winding of audio transformer as a power source to boost to the DC source. The result is a carrier of x watts with no modulation with transmitter power increasing to a higher wattage output at 100%.

author: radiogeek381
author_rank: Regular Contributor
timestamp: December 06, 2016, 01:42:53 pm
content: The reason your radio detected an AM signal is that you actually produceda very slightly modulated AM signal, in addition to adding the low frequencyoffset.If I understand your circuit correctly, you have two resistors in series betweenthe emitter and ground. You are driving the node between the two resistorswith your AF generator. We'll call this node E1, and the emitter E. (Baseand collector will be B and C).If the AF generator impedance is low WRT the emitter resistors, then E1 willwiggle up and down. But the voltage at B will, largely, be affected only bythe RF generator, as its impedance is surely lower than the effective impedancelooking into the base terminal. That means that the voltage between B andE1 will be modulated by the AF generator. This voltage determines the base-emittercurrent and, because it is a BJT, the collector-emitter current.Another way of saying this is, if you raise the voltage at E1 very slowly (relativeto the signal at the base) then you move the operating point of the transistorand thus reduce the gain.If you were to take the output of your "modulating amplifier" and pass it through a.01uF cap that is then shunted on the output by a 1K resistor, you'll remove mostof the low frequency "DC" offset from the output and you might even see a(small?) amount of modulation on a carefully synchronized oscilloscope. This willlook like the textbook AM envelope.

author: TimFox
author_rank: Super Contributor
timestamp: December 06, 2016, 09:16:25 pm
content: It is important to note that the audio signal that is multiplied by the carrier sinusoidal waveform is of the form M = 1 + A(t) where A(t) > -1 for all time. Therefore, M > 0 for all time. The simplest modulation would be an audio tone A(t) = 1 x cos (2 pi f t), where f is an audio frequency. When A(t) = 0 (between words), the RF output is an unmodulated carrier, which is otherwise a waste of power but is needed to demodulate the AM signal in a simple detector.