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AM: what's really happening, or the physical reality of sidebands

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Author Topic: AM: what's really happening, or the physical reality of sidebands  (Read 7130 times)
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« on: February 01, 2011, 07:09:40 PM »

From the beginnings of radiotelephony there has been a question whether sidebands exist as physical reality or only in the mathematics of modulation theory.  In the early 20's this was a hotly debated topic, with a noted group of British engineers maintaining that sidebands existed only in the mathematics, while an equally well-remembered group of American enigneers argued that sidebands do, in fact physically exist.

Today, the issue seems settled once and for all.  We can tune our modern-day highly selective receivers through double-sideband and single-sideband voice signals, and tune in upper or lower sideband, and even adjust the selectivity to the point that we can tune in the carrier minus the sidebands.  Nearly everyone accepts the notions that sidebands do indeed exist physically.  Or do they?  Maybe it's a matter of how we observe them, and the result is modified by our measuring instruments.  Remember the Heisenberg uncertainty Principle, which states that you cannot observe something without affecting it to some degree, or more precisely, that it is impossible to know both the position (physical  location) and velocity (speed and direction) of a particle at the same time.

Imagine a cw transmitter equipped with an electronic keyer.  Also imagine that there is no shaping circuitry, so that the carrier is instantly switched between full output and zero output. Such a signal can be expected to generate extremely broad key clicks above and below the fundamental frequency because of the sharp corners of the keying waveform.  Set the keying speed up to max, and send a series of dits.  If the keyer is adjusted properly, the dits and spaces will be of equal length, identical to a full carrier AM signal 100 percent modulated by a perfect square wave.

Suppose the keyer is adjusted to send, say, 20 dits per second when the "dit" paddle is held down. The result is a 20 Hz square-wave-modulated AM signal.  Now turn the speed up. If the keyer has the capability, run it up to 100 dits per second.  If you tune in the signal using a receiver with very narrow selectivity (100 Hz or less, easily achievable using today's technology), you can actually tune in the carrier, and then as you move the dial slightly you can tune in sideband components 100, 300, 500 Hz, etc. removed from the carrier frequency. A square wave consists of a fundamental frequency plus an infinite series of odd harmonics of diminishing amplitude. Theoretically you would hear carrier components spaced every 200 Hz throughout the spectrum.  In a practical case, due to the finite noise floor, the diminishing amplitude of the sideband components and selectivity of the tuned circuits in the transmitter tank circuit and antenna itself, these sideband components eventually become inaudibly buried in the background noise as the receiver is tuned away from the carrier frequency.

Suppose we now gradually slow down the keyer.  As we change to lower keying speed, it takes more and more selectivity to discriminate between carrier and sideband components, as the modulation frequency becomes lower and the sideband components become spaced more closely together. Let's observe what happens when we slow the dit rate down to 10 dits per second. Now the fundamental modulation frequency is 10 Hz, and there are sideband components at 30 Hz, 50 Hz, 70 Hz removed from the carrier and continuing above and below the carrier frequency at intervals of 20 Hz until the signals disappear into the background noise.  In order to distinguish individual sideband components, we need selectivity on the order of 10 Hz, which is possible if we use resonant i.f. selectivity filters with extremely high "Q".  This can be accomplished using crystal filters, regenerative amplifiers or even conventional L-C tuned circuits if we carefully design the components to have high enough Q. 

As we achieve extreme selectivity with these high Q resonant circuits, we observe a sometimes annoying characteristic familiarly known as "ringing." This ringing effect is due to the "flywheel effect" of a tuned circuit, the same "flywheel effect" that allows a class-C tube type final or class-E solid state final to generate a harmonic-free sinewave rf carrier waveform.  The selective rf tank circuit stores energy which is re-released to fill in missing parts of the sinewave, thus filtering out the harmonics inherent to operation of these classes of amplifier.  CW operators are very aware of the ringing effect of very narrow filters, which can make the dits and dahs of high speed CW run together, causing the signal to be just as difficult to read with the narrow filter in line, as the same CW signal would be if one used a wider filter that admits harmful adjacent channel interference.  Kind of a damned if you do, damned if you don't scenario.
Let's now take our example of code speed and selectivity to a degree of absurdity.  We can slow down our keyer to a microscopic fraction of a Hertz, to the point where each dit is six months long, and the space between dits is also six months long.  In effect, we are transmitting an unmodulated carrier for six months, then shutting down the transmitter for six months. But still, this is only a matter of a degree of code speed; the signal waveform is still identical to the AM transmitter tone modulated with a perfect square wave, but whose frequency is one cycle per year, or 3.17 X (10 to the -8) Hz.  That means that in theory, the steady uninterrupted carrier is still being transmitted, along with a series of sideband components spaced every 6.34 X (10 to the -8) Hz.

Now, carriers spaced every 6.34 X (10 to the -8) Hz apart are inarguably VERY close together, to the point that building a filter capable of separating them would be of complexity on the order of a successful expedition to Mars, but it is still theoretically possible. Let us assume we are able to build such a filter.  We would undoubtedly have to resort to superconductivity in the tuned circuits, requiring components cooled to near absolute zero, and thoroughly shield every rf carrying conductor to prevent radiation loss, but here we are talking about something hypothetical, without the practical restraints of cost, construction time and availability of material.  Anyway, let us just assume we were able to successfully build the required selectivity filter.

The receiver would indeed be able to discriminate between sidebands and carrier of the the one cycle/year or 3.17 X (10 to the -8) Hz modulated AM signal, identical to a CW trasmitter with carrier on for six months and off for six months.  So how can we detect a steady carrier while the transmitter is shut off for six months?  The answer lies in our receiver.  In order to achieve high enough selectivity to separate carrier and sideband components at such a low modulating frequency and close spacing, the Q of the tuned circuit would have to be so high that the flywheel effect, or ringing of the filter, would maintain the the missing rf carrier during the six-month key-up period.

This takes us back to the longstanding debate over the reality of sidebands.  If we use a wideband receiver such as a crystal set with little or no front-end selectivity, we can indeed think of the AM signal precisely as a steady carrier that varies in amplitude in step with the modulating frequency.  This is physically the case if the total bandwidth of the signal is negligible compared to the selectivity of the receiver.  Once we achieve selectivity of the same order as the bandwidth of the signal, which has been the norm for practical receivers dating from as early as the 1920's up to to the present, reception of the signal behaves according to the principle of a steady carrier with distinctly observable upper and lower sidebands.  The "holes" in the carrier at 100% negative modulation are inaudible due to the flywheel effect of the tuned circuits, even though the "holes" may be observable on the envelope pattern of the oscilloscope.

An oscilloscope set up for envelope pattern, with the deflection plates coupled directly to a sample of the transmitter's output, is a wideband device much like a crystal set. It allows us to physically observe the AM signal as a carrier of varying amplitude. A spectrum analyser on the other hand, is an instrument of high selectivity, namely a selective receiver programmed to sweep back and forth across a predetermined band of spectrum while visually displaying the amplitude of the signal falling into its passband. It clearly displays distinct upper and lower sidebands with a steady carrier in between.

Furthermore, it has often been observed that the envelope pattern of a signal as displayed from the i.f. of a receiver can be quite different from that of a monitor scope at the transmitter site.  This is yet another example of how the pattern is altered (distorted) by the selective components of the receiver.

In conclusion, there is no correct yes or no answer to the age-old question whether or not sidebands are physical reality, or exist only in the mathematics of modulation theory. It all depends on how you physically observe the signal.  Sidebands physically exist only if you use an instrument selective enough to observe them. Recall the Heisenberg uncertainty Principle.

Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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