OK. I have taken the liberty of copying out of the 1996 ARRL Handbook. It is an amateur radio book put out by the American Radio Relay League. Chapter 15 covers: Mixers, Modulators and Demodulators.
Incidentally, I will keep the discussion leaning towards AM.
What is a Mixer?
Mixer is a traditional radio term for a circuit that shifts on signals frequency up or down by combing it with another signal. The word mixer is also used to refer to a device used to blend multiple audio inputs together for recording, broadcast or sound reinforcement. These two mixer types differ in one very important way: A radio mixer makes new frequencies out of the frequencies put into it and an audio mixer does not.
Mixing Versus Adding
Radio mixers might be more accurately called “frequency mixers” to distinguish them from devices such as “microphone mixers,” which are really just signal combiners, summers or adders. In their most basic ideal forms, both devices have two inputs and one output. The combiner simply adds the instantaneous voltages of the two signals together to produce the output at each point in time. The mixer, on the other hand, MULTIPLIES the instantaneous voltages of the two signals together to produce its output signal from instant to instant. Comparing the output spectra of the combiner’s output contains only the frequencies of the two inputs, and nothing else, while the mixer’s output contains new frequencies. Because it combines one energy with another, the process is sometimes called heterodyning, from the Greek words for OTHER and Power.
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I skipped the section about the multiplication. It contains formulas and also talks about how real world mixers are not perfect and are optimized to keep the distortion products down to a low level. Distortion products make for a ‘dirtier’ output as the book puts it.
There is a section called “Putting multiplication to work” that I will cover the last paragraph. The ‘multiplication’ refers to the multiplying that is discussed in one of the above paragraphs. The last paragraph and following text reads as follows:
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It turns out that the mechanism underlying multiplication, mixing, modulation, and demodulation is a pretty straightforward theing: Any circuit structure that NONLINEARLY DISTORTS ac waveforms acts as a multiplier to some degree.
Nonlinear Distortion?
The phrase nonlinear distortion sounds redundant but isn’t. Distortion, an externally imposed change in a waveform, can be linear; that is, it can occur independently of signal amplitude. Consider a radio receiver front end filter that passes only signals between 6 and 8 Mhz. It does this by LINEARLY DISTORTING the single complex waveform corresponding to the wide RF spectrum present at the radio’s antenna terminals, reducing the amplitudes of frequency components below 6 MHz and above 8 MHz relative to those between 6 and 8 MHz. (Consider multiple signals on a wire as one complex waveform is just as valid, and sometimes handier, than considering them as separate signlas. In this case, it’s a bit easier to think of distortion as something that happens to a vaveform rather than something that happens to separate signals relative to each other. It would be just as valid – and certainly more in keeping with the consensus view – to say merely that the filter attenuates signals at frequencies below 6 MHz and above 8 MHz.) The filter’s output waveform certainly differs from its input waveform; the waveform has been distorted. But because this distortion occurs independently of signal level or polarity, the distortion is linear. No new frequency components are created; only the amplitude relationships among the wave’s existing frequency components are altered. This is amplitude or frequency distortion, and all filters do it or they wouldn’t be filters.
PHASE or DELAY DISTORTION, also linear, causes a complex signal’s various component frequencies to be delayed by different amounts of time, depending on their frequency but independently of their amplitude. No new frequency components occur, and amplitude relationships among existing frequency components are not altered. Phase distortion occurs to some degree in all filters.
In a NONLINEARLY distorting circuit, however, the output’s root-mean-square (RMS) voltage or current values don’t track the input’s corresponding RMS voltage or current values in linear proportion. The distortion therefore varies with the signal’s amplitude (and in some nonlinear structures, polarity); the stronger the signal the greater the distortion.
Nonlinear distortion may take the form of HARMONIC DISTORTION , in which integer multiples of input frequencies occur, or intermodulation distortion (IMD), in which different components multiply to make new ones.
Any departure from absolute linearity results in some form of nonlinear distortion, and this distortion can work for us or against us. Any so-called linear amplifier distorts nonlinearly to some degree; any device or circuit that distorts non linearly can work as a mixer, modulator, demodulator or frequency multiplier. An amplifier optimized for linear operation will nonetheless mix, but inefficiently; an amplifier biased for nonlinear amplification may be practically linear over a given tiny portion of its input-signal range. The trick is to use careful design and component selection to maximize nonlinear distortion when we want it, and minimize it when we don’t. Once we’ve decided to maximize nonlinear distortion, th etrick is to minimize the distortion products we don’t want, and maximize the products we desire.
Why we call it amplitude modulation
We call the modulation process described in equation 8 amplitude modulation because the complex waveform consisting of the sum of the sidebands and carrier varies with the information signal’s magnitude. Concepts long used to illustrate AM’s mechanism may mislead us into thinking that the carrier varies in strength with modulation, but careful study of equation 9 shows that this doesn’t happen.
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Ok, I will quit copying here. There is a WHOLE LOT MORE to the chapter but I don’t feel like typing it all in. Just go buy the book. I also left the equations out but you can trust me that I typed it word for word. If you question the validity, once again, buy the book. You can contact the organization who puts out the book at:
American Radio Relay League
225 Main St
Newington, CT 06111-1494
Go ahead and tell them I plagiarized their book, they will love it since it is a possibility they will sell a book and recruit one more radio amateur.
4Newton: I recall your post saying: The intent of AM is to increase and decrease the peak voltage of the RF in a linear manner proportional to the applied audio. Hmmmm. As you can see this is not true. It is the apparent result of amplitude modulation, but the strength of the carrier does NOT vary.
I've been dying to know. 