[AMRadio] The Physical Reality of Sidebands in the AM signal


Oliver Steiner steinerviolinist at gmail.com
Thu Sep 29 14:36:42 EDT 2016


Don,

Thank you for this very informative and beautifully written contribution.

Ollie
W2QXR

On 9/29/16, Donald Chester <k4kyv at charter.net> wrote:
> 	
> 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 engineers
> 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 notion that sidebands do indeed
> exist physically...  or do they?
>
> Maybe it's a matter of how we observe the signal, and our result is
> modified
> by our measuring techniques. Those who have studied quantum mechanics will
> recall the Heisenberg uncertainty Principle, which states that it is
> impossible to know both the position (physical location) and velocity
> (speed
> and direction) of a particle at the same time, along with the related
> "Observer Effect", which states that you cannot observe a system without
> changing something in the system. In the following thought experiment, we
> take this to an analogy with an amplitude modulated radio signal.
>
> 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 we can hear sideband
> components at 10 Hz, 30 Hz, 50 Hz, 70 Hz removed from the carrier,
> continuing above and below the carrier frequency at intervals of 20 Hz
> until
> we reach a point  where 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 receiving 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 with a wider filter, even one that admits harmful adjacent channel
> interference.  Kind of a damned if you do, damned if you don't scenario.
>
> Now, let's continue with our thought experiment, taking our example of code
> speed and selectivity to 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 the 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 likely be of complexity on the order of a successful
> expedition to Mars, but 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 one cycle/year or 3.17 X (10 to the -8) Hz modulated AM
> signal, identical to a CW transmitter 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 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 always 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 the late 1900's up 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 those same "holes" may
> be
> observable on the envelope pattern of an 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 at each instant. 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 a scope connected to the i.f. output of a distant
> receiver can be quite different from what is  seen on 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. Putting it another way, their
> existence depends on whether we observe the signal in the time domain or
> the
> frequency domain. Remember the Heisenberg Uncertainty Principle and the
> associated Observer Effect?
>
> Don k4kyv
>
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