AM - What's really happening?

[WD8BIL]

Clearly, the carrier that is injected after the modulation process is continuous and goes right on out the linear PA section of the rig.

Carrier injected after the modulation process...... hmmmmmm
If you add the carrier after the modulation "process"... what was "modulated" ?

[Steve - WB3HUZ]

Welcome back Buddly! :-P

[WD8BIL]

Thanks Steve !!
Great trip.
Film at 11 !!

[Philip, AB9IL]

Steve,

Thanks for the phasor diagram.  I had been imagining three wheels rotating: LSB, CAR, and USB.  Each one was rotating uniformly faster than the other one, and understanding their phases was driving me nuts!

My wife saw me pointing up, down, and left or right and must've figured this radio theory stuff had gone way too far!  But referencing to the carrier and seeing the other vectors rotating in opposite directions is so much better for digestion...

Now to imagine this combined with Don's 6 month square waves and I'll finally have a grip on the vector model of AM.  To say that modulation is a mixing process has now taken on a more complex meaning.



Y'all have fun with the aurora tonight; I'm pulling the switch early...

[Rob K2CU]

Referring to the TR-7, I stand corrected on the details. The carrier is added back in a summing  amplifier after the USB filter, not at the input of the PA.  I was not trying to get into the details of the TR-7, but mearly point out that the carrier can be added back to an SSB signal to create AM.

A simple demonstarion od this principle occurs in radios where the BFO is added to the IF signal prior to detection in a diode AM type detector.  Some others have "talked over" certain braodcasters by using SSB and the other stations carrier.  

Still not convcinced?  Tune in an SSB station with an AM receiver and then add a low level CW  (carrier) signal from a signal generator at the AM receiver's antenna terminal.  You will be able to receive the monkey chatter as if it were an AM signal.

As to the carrier being constant in an AM modulated signal. ponder this: You tune in an AM station with someone in an old buzard monologue.  Someone is tuing up 1 KHz away. You hear the 1 KHz beat note in your speaker. IF the carrier of the station with the talking were not constant, you would not hear a clean continuos beat note.

[WD8BIL]

Get an audio spectrum analyzer and look at a 10 cycle carrier and tell me what you see !

[Steve - WB3HUZ]

As to the carrier being constant in an AM modulated signal. ponder this: You tune in an AM station with someone in an old buzard monologue.  Someone is tuing up 1 KHz away. You hear the 1 KHz beat note in your speaker. IF the carrier of the station with the talking were not constant, you would not hear a clean continuos beat note.

You forgot to consider the time constants of the detector. Unless the carrier was cutoff longer than these, the carrier and any related heterodynes will appear constant.

[w3jn]

The beat note of the two carriers, because they are mixing (and therefore intermodulating), is the result of the mixing product "carrier" turning on and off at an audio rate.   Your example thus proves that the carrier does turn off instantaneously.

Or so it seems to me.

73 John

[w3jn]

The beat note of the two carriers, because they are mixing (and therefore intermodulating), is the result of the mixing product "carrier" turning on and off at an audio rate.   Your example thus proves that the carrier does turn off instantaneously.

Or so it seems to me.

73 John

[WA1GFZ]

I think we can all agree that the plate voltage is zero at 0 % modulation
and 2X plate voltage at 100 % modulation.

[WD8BIL]

think we can all agree that the plate voltage is zero at 0 % modulation
and 2X plate voltage at 100 % modulation.

Yes !
And what is the power output of a final with 0 plate voltage ?

[w3jn]

And what is the power output of a final with 0 plate voltage ?

Uhhh..... 1.21 gigawatts?

[WA1GFZ]

1.5 if you are running class e

[W8ER]

GFZ said:

I think we can all agree that the plate voltage is zero at 0 % modulation
and 2X plate voltage at 100 % modulation.

Uh ..  did you mean at 100% or are you in standby?

--Larry

[W2VW]

Yes !
And what is the power output of a final with 0 plate voltage ?

The same as when the conduction of the Class C final is off.

[WD8BIL]

Isnt that classE, Larry ?

'Course... let's not mention circulating currents in the output tank !

[WA1GFZ]

Eddie Haskell was a cop in  L.A. when I lived there in the '80s
I saw him on TV a couple times.

[WA1GFZ]

Does the crank shaft in a motor stop between power strokes?

[Rob K2CU]

FC, doesn't zero percent mean no modulation at all and that the plate voltage would be DC plate supply voltage? Isn't 100% defined as being when the plate voltage, under modulation has a crest value of 2 x Vp and a modulation valley value of zero.  That is, of course, unless you live in "The Valley".

OH yeah, forgot your advice about not trying. Apparently, you did too.

I guess your analogy is to the typical Class C RF power amplifier where the conduction angle is notably less than 180 degrees.  So, even though the plate voltage is swinging from 2 x Vp to zero, the RF amplifier tube is only conducting for less that a half RF carrier cycle anyway.  So then I guess it might be more correct to say that the carrier is chopping the audio voltage on the plate of the tube and not that the audio is chopping the RF.  Now, which is the chicken and which is the egg?

In a class E transmitter, the FET switch devicea iare turned on and off by the carrier drive and the drain supply voltage is a audio varying  voltage with a positive DC offset.  Again, the continuous, yet varying, supply voltage is being chopped by the carrier.

Doesn't experience with the production of, and need to filter the odd harminics from a class C output stage agree with this? Then is the typical resonant PI matching network an actual bandpass filter or jsut a ringing circuit. Ah yes, the old flywheel effect, as it was explained when we barely understood algebra, let alone, Fourier analysis.

[WA1GFZ]

Yes Rob,
100% negative peak and I like my alge braless myself.
It all just so magic, a modulation transformer and all that.
doin it without a modulation transformer now there may
be a law against that kind of stuff.

[WB6VHE]

Greetings, Gentlemen!

Maybe I can help shed some light on this dilemma.  I think the confusion arises when
considering how the modulated waveform looks in the time domain versus how it looks
in the frequency domain.  When you look at an a.m. signal on a 'scope (time domain)
you see the carrier with the modulating signal swinging the amplitude up to twice the
amplitude of the carrier alone and down to zero at the modulating frequency.  This means
that in the time domain (where the signal is observed as a function of time), the COMPOSITE signal (carrier plus modulation) goes to zero every cycle of the modulating signal.   So, yes, the signal really does go to zero (this means that the
carrier and the sidebands go to zero).  Now, looking at the same sig in the frequency
domain (on your spectrum analyzer) means taking the Fourier transform of the sig, that is, you have integrated over time, so time no longer enters into the picture!  You see the carrier
and two sidebands (keeping it simple with just a sinusoidal modulating signal).  Now you
cannot ask it the carrier or anything else goes to zero at some point in time because time

is no longer a parameter: the time domain picture and the frequency domain picture are
COMPLIMENTARY, with each one giving some information about the sig that the other
cannot give.  That is, if you want to know what the sig does in time, look at it in the time
domain; if you want info about the spectrum, look at it in the frequency domain;  the frequency domain picture is independent of time, that is, to get the freq spectrum you
have to take a "weighted average" of the time-dependent sig over all time (the Fourier
transform), so it is not meaningful to ask what happens at a certain time when looking
at the sig in the frequency domain!  Because the spectrum analyzer does the transform
over and over again at some rate determined by the analyzer you are using, the
spectrum appears to change in time with a complex modulating waveform, but what
you are actually seeing are "snapshots' of the frequency distribution with different modulating waveforms.  So the answer is, YES, the signal really goes to zero with
100% modulation, and YES, the sidebands are real.

Hope this helps some and doesn't increase the confusion!

[wavebourn]

Here is an illustration.
What do you see on the picture?

[wavebourn]

It's a man riding a bycicle and wearing a sambrero hat.

[kc6mcw]

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 maths, 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.  But 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 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, and 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 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 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% 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. 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.

Very well explained! I have always wondered about the "carrier holes" too. Its all about the FLYWHEEL affect! It puts the carrier back in the holes. I know this is an old thread, but it was there so I read it.

[Opcom]

You all are making my brain hurt! The picture is a red ball with a drain hole in it hanging along a cable. Side bands exist on paper, and they exist in reality, as proof I offer that I have tuned through them with a narrow filter on the receiver.

Is it wrong to simplify this and say that the upper sideband might be produced by the positive swing of the audio cycle and the lower sideband might be produced by the negative swing, except that the RF frequency is so high that this relationship is reversed every half cycle of the RF wave?