AM - What's really happening?

[W3RSW]

-The Wonderous Quantum nature of electromagnetic transmission. -

The unmodulated AM carrier is contiuous wave RF, hopefully a resonably distortionless sine wave generated by whatever means at a frequency much higher than the modulation frequency.

What your not seeing on a typical waterfall or oscilloscope (unless you speed up the time base a thousand fold or more) is, even on the unmodulated carrier, a sine wave going from so much energy peaking at a "positive" value, through "zero" or no energy and thence to the same peak energy "negative."  Positve and negative are just markers to designate opposite sides of zero.  

Regarding instaneous, we can approach the infinitely small but never achieve it, so instantaneous is defined as the smallest time increment that can be measured, generated, or defined by our sadly limited outlook in the real world.  As instrumentation becomes ever more 'precise' the definition of instantaneous aproaches zero.  But as Steve says the effect is real.  .. the square wave transition from no energy to some finite energy being the summation of an infinite number of sine wave frequencies superimposed. 

We're still talking only the carrier here, or continuous wave.  Don't forget that vigorous electron energy  has "magically"* turned an antenna, say a 1/2 wave dipole into a wave generator of photons carrying energy in both the voltage and magnetic domain.  The wave front radiated, just like the class C tube or whatever generating it goes from positive energy through zero to negative energy as it is propagated outwards in space.  It goes through 'zero' or no energy millions of times a second, say 3,885,000 of cycles per second.

Now when you modulate that CW energy and wave by superimposing other much lower frequency energy, say in the audio range of 'only' 50 to 5000 cycles per second you create an 'envelope' of the previoulsly constant amplitue sine wave RF energy.  Expand the time domain scale so that you can see the individual RF sine waves being modulated by much longer wave audio.  

Your generating energy goes through zero at not only the audio frequecies but thousands of times more at RF frequencies.

Reference superposition theorm and/or algebraic addition in just about any E or EE or physics text.  

* Jury's still out how electrical energy is converted into a wave.  Once you try to capture a photon you collapse the wave front. Squirrly little creatures. Action at a distance faster than light comes into play too.

[KA1ZGC]

You are confusing looking at a particular point on a waveform with not looking at the waveform at all. Instantaneous does not mean you completely disregard what comes before or after.

...to you and I, perhaps; but some of the logic I'm being presented is asking the reader to disregard all else.

Instananeous voltage and current values are used all the time in tube parameters, amplifier design, breakdown testing, etc. That practical applications are many and varied.

I said "practical bearing", not "practical application". There's a huge difference in those two terms.

Yes, fixed numbers are used as limits. What else would we use? Please show me one situation where the disappearance of carrier at that brief instant of 100% negative modulation limits your ability to transmit RF. It doesn't.

Like you said, you cause splatter when you exceed 100% negative, where do you suppose that energy is coming from? Yes, your carrier may be "gone", but you are still transmitting energy with the tube fully cut off.

Zero voltage, zero current, yet greater-than-zero power out of your transmitter, guys! Oopsie! Perhaps focusing on the fact the tube is cut off at a specific instant and ignoring everything else around the tube in time and space is not a scientifically valid approach?

If you choose to freeze time and micro-analyze conditions in a vacuum, you can make almost any argument seem valid.

[KA1ZGC]

It goes through 'zero' or no energy millions of times a second, say 3,885,000 of cycles per second.

Actually, that would be 7,770,000 times per second. Every wave has two zero-crossings.

[W3RSW]

fast paced discussion.
-right.

assumption that 3885 was the fundamental on my oscillator?   

[W3RSW]

Let's see.
The energy of splatter on the CW zero crossing comes from too much superimposed audio. At the crossing, you've essentially created a square wave with all the consequent infinite number of frequencies.  Once the carrier is zero those spurious freq's and energy dissapates until the audio blows the carrier back up through zero causing another burst of trash.  The length of the zero carrier, cut-off region is a direct function of how much excessive energy audio you have.

[W3RSW]

thinking some more, uh oh..

The now recovering, overly 'negative' modulated CW waveform coming back up through 'zero' overshoots what would normally be a distortion free 100% modulated signal but it's now coupled with all the square wave trash picked up coming back up through zero carrier.  Superposition again. 

Where did the distortion products go on the down swing through zero in an overmodulated signal?  The were radiated as multiple frequency components and gradually dissapated like any other damped wave, just a lot dirtier.

There's nothing wrong with 'positive' overmodulaton as long as the energy is not allowed to negatively cutoff the carrier.

[KA1ZGC]

Right, but I'm talking about something a bit more fundamental.

From the moment you strike 100% negative, you begin to emit square-wave energy (odd-order harmonics, 3f, 5f, 7f, and so on).

At that point, the tube is cut off, and no longer contributing energy.

But that square-wave energy has to come from somewhere. Energy is neither created nor destroyed. Something other than the final has to supply that energy.

That's my point: looking only at the final at a frozen moment of time is just changing the test to match the answers.

[The Slab Bacon]

The carrier is cut off EVERY TIME the grid swings negative, however that is at a much faster rate than the modulating audio. That is why you have the tank circuit in the final plate line for the "flywheel effect". When your scope has the sweep speed right to display the audio waveform, the sweep speed is way, way to slow to show the individual sine waves from the RF component.

Going back to the flywheel effect, that is why tou have to design the tank circuit differently for a class C amp vs a class AB1 amp etc.

Thom,
        for someone who knows so much, how come you put out such a piss-weak signal??

                                                            The Slab Bacon

[KA1ZGC]

The carrier is cut off EVERY TIME the grid swings negative, however that is at a much faster rate than the modulating audio. That is why you have the tank circuit in the final plate line for the "flywheel effect". When your scope has the sweep speed right to display the audio waveform, the sweep speed is way, way to slow to show the individual sine waves from the RF component.

You mean the carrier is cut off every time the grid swings positive.

Thom,
        for someone who knows so much, how come you put out such a piss-weak signal??

Grow up, Frank. The last time you pulled that one on me, you didn't know I was actually at Tim's place, and strapped you several times during that QSO. That was two years ago, and we haven't worked since then. Go find someone else to crap on.

[Steve - WB3HUZ]

No energy is emited when the carrier is off. The energy comes from the abrupt turn off and turn on or the sharp rise and fall times. The sharper the rise and fall time, the wider spectral content.

Right, but I'm talking about something a bit more fundamental.

From the moment you strike 100% negative, you begin to emit square-wave energy (odd-order harmonics, 3f, 5f, 7f, and so on).

At that point, the tube is cut off, and no longer contributing energy.

But that square-wave energy has to come from somewhere. Energy is neither created nor destroyed. Something other than the final has to supply that energy.

That's my point: looking only at the final at a frozen moment of time is just changing the test to match the answers.

[The Slab Bacon]

The carrier is cut off EVERY TIME the grid swings negative, however that is at a much faster rate than the modulating audio. That is why you have the tank circuit in the final plate line for the "flywheel effect". When your scope has the sweep speed right to display the audio waveform, the sweep speed is way, way to slow to show the individual sine waves from the RF component.

You mean the carrier is cut off every time the grid swings positive.

Thom,
        for someone who knows so much, how come you put out such a piss-weak signal??

Grow up, Frank. The last time you pulled that one on me, you didn't know I was actually at Tim's place, and strapped you several times during that QSO. That was two years ago, and we haven't worked since then. Go find someone else to crap on.

Thom,
         How does the tube cut off when the grid swings positive?? when the grid swings positive the tube is turned full on. When the grid swings negative the electron flow diminishes until you hit the point of full cut off. Therefore increasing the amount of negative biass on the grid diminishes the electron flow, therfore reducing the zero signal plate current.

<snip>
Grow up, Frank. The last time you pulled that one on me, you didn't know I was actually at Tim's place, and strapped you several times during that QSO. That was two years ago, and we haven't worked since then. Go find someone else to crap on.
<snip>

You made the first poke by criticising my spelling. I was merely tossing one back
Would you expect less of me??
It kinda looks like you can dish it out but you cant take it.

And when you are home you are at times rather tough copy down here. I actually do at times have trouble hearing you.

                                                                  the Slab Bacon

[KA1ZGC]

        How does the tube cut off when the grid swings positive?? when the grid swings positive the tube is turned full on.

Yeah, I got my wires crossed there.

When the grid hits zero, the tube is fully on. When it swings positive, it becomes a second anode and draws power from the tube. So you might say that when you swing positive, you're no longer fully "on", as fewer electrons strike the plate as would with the grid at zero.

<snip>
Grow up, Frank. The last time you pulled that one on me, you didn't know I was actually at Tim's place, and strapped you several times during that QSO. That was two years ago, and we haven't worked since then. Go find someone else to crap on.
<snip>

You made the first poke by criticising my spelling. I was merely tossing one back

It's still a pretty tired line, and it's a claim you make even when I out-strap you. Therefore, I can't rely on signal reports from you. You will always tell me I'm piss-weak.

Yeah, I picked on your spelling, in retaliation for claiming that people don't deserve to have ham licenses because they hold a different view than you do.

Would you expect less of me??

I know what to expect out of you, Frank. No suprises there.

It kinda looks like you can dish it out but you cant take it.

I took it just fine, and dished it right back.

And when you are home you are at times rather tough copy down here. I actually do at times have trouble hearing you.

I wouldn't know, we haven't spoken in at least two years. The last time I remember working you from home was on my Ranger. That would explain it.

[KA1ZGC]

No energy is emited when the carrier is off. The energy comes from the abrupt turn off and turn on or the sharp rise and fall times. The sharper the rise and fall time, the wider spectral content.

I'm confused... you say no energy is emitted when the carrier is off, then explain where that energy comes from. Is there energy, or isn't there?

[The Slab Bacon]

Thom,
        It kinda sounds like you need a little refresher course in vacuum tube theory.
When the grid swings positive, it actually acts as an accelerator increasing the electron flow through the tube. Just like the screen grid in a tetrode or pentode.
The current that the grid draws when it is swung positive does not come from the tube itself, but from the preceeding stage. That is why a tube running class B or class C require more driving power than a tube running class A or AB1. The power to swing the grid positive comes from the preceeding stage. Also tubes running A or AB1 are never cut completely off, and never biassed to the point of cutoff. Anyone who has ever messed with big triodes and grid leak bias only (old school) has adjusted the driver circuit and grid tank with the HV to the finals turned off, to be sure he has enough drive to insure he will be able to drive it into class C operation.

<snip>
I'm confused... you say no energy is emitted when the carrier is off, then explain where that energy comes from. Is there energy, or isn't there?
<snip>

There is no output produced at the point of cutoff. It cant be. You still see energy on an analog outpoot meter because the pointer cant respond fast enough.

                                                                 The Slab Bacon

[KA1ZGC]

There is no output produced at the point of cutoff. It cant be. You still see energy on an analog outpoot meter because the pointer cant respond fast enough.

I'm not talking about that. I'm talking about splatter. That happens when the tube is cut off. There has to be a source for that energy. The shape of a waveform is not an energy source, the actual energy radiated as splatter has to come from somewhere.

[The Slab Bacon]

There is no output produced at the point of cutoff. It cant be. You still see energy on an analog outpoot meter because the pointer cant respond fast enough.

I'm not talking about that. I'm talking about splatter. That happens when the tube is cut off. There has to be a source for that energy. The shape of a waveform is not an energy source, the actual energy radiated as splatter has to come from somewhere.

Sometimes its just better to accept the fact that something is rather than boggling ones brain trying trying to figure out why it exists. I have never really tried to create and examine it, even though I have a spectrum analizer. One day when I nave nothing better to do i will have to create and observe it. However I would rather spend the little free time I have these days doing something more interesting.

My best guess at it would be that the splatter is caused by the stopping and restarting of the flywheel effect of the tank circuit, and / or the voltage transients caused by the shutting off of the voltage at the plate feed caused by negative the modulation peaks being reapplied to modulated final amp. After all these voltage transients when the circuit "unloads" is also notorious for wiping out the insulation in mod transformers.

Sometimes it is just better to accept the fact that something exists than to kill ones self trying to figger out why it exists when you know that it does exist.

                                                                             the Slab Bacon

[KA1ZGC]

Sometimes it is just better to accept the fact that something exists than to kill ones self trying to figger out why it exists when you know that it does exist.

Strange, that was the exact sentiment that caused me to jump in. We've come full-circle! All the debate of whether sidebands really exist, whether math is real, all spawned from a pretty simple question. I was going to just ask "If W8NB emits a sideband signal and nobody's there to hear it, is he still a pain in the ass?" and let it go, but I thought Chas' original question deserved an answer.

I'm not racking my brain trying to figure it out, I'm just saying that this stuff is never as simple as we all make it sound. There's always a "gotcha" somewhere.

...but yeah, I think we agree that we've gone down this rathole far enough. Passed that point last night some time, actually.

Now just get a real antenna so you can hear me, 'kay? 

[K1ZJH]

There is no output produced at the point of cutoff. It cant be. You still see energy on an analog outpoot meter because the pointer cant respond fast enough.

I'm not talking about that. I'm talking about splatter. That happens when the tube is cut off. There has to be a source for that energy. The shape of a waveform is not an energy source, the actual energy radiated as splatter has to come from somewhere.

Same as "key clicks."  True, the shape of the waveform is not a source of energy, but the rapid rise and fall times imply a broad spectral BW.

Pete

[k4kyv]

I have heard it argued that overmodulation on negative peaks does not cause splatter because the sharp corner of the waveform as the carrier cuts off in the negative direction occurs only when the rf output from the transmitter has already very closely approached zero.

[W3RSW]

Boy you sure know how to get us excited! (pun intended)

It could be posited that the sharper the square wave the greater the generation of spurious hash and trash right up through infinity.  Hard to prove otherwise...  I mean a perfect square wave doesn't exist. .. else the whole world would be zapped instantaneously. 

What I'm saying is even a small signal can generate spikes far beyond the much smaller, p.w. CW component right before it hits the zero line....  the sharper the dirtier, with 'spikes' approaching infinity.

[DMOD]

I know this is old hat for most of us, but a review of a basic triode Class C amplifier might help.

We will assume only sine wave RF exitation to the grid for 1.9 Mhz and 100 volts Peak or 70.7 V rms, a resonant circuit at the plate tuned to 1.9 mHz with   no modulation , and a 50 ohm load. We will also assume the resonant circuit will filter out any harmonics outside the passband.

The design specs are:
Cutoff bias for the tube is speced at -30 Volts
Plate supply voltage at tube is a well regulated and constant 1000 Volts. Plate current of 100 mA.  Max plate voltage speced at 2,500 Volts.
Grid bias voltage of -90 volts. So the tube is biased three times beyond cutoff.
Grid exitation sine wave voltage has a full cycle of 526 nanoseconds = 360 degrees.

Somewhere on the positive going (first half-cycle) of the grid voltage (total of 263 nS), the grid voltage must rise above -30 volts in order to start turn on. In this case, we drive the grid to a positive +10 volts in order to draw a high peak plate current in a relatively short period of time. So max plate current at 100% is 100 mA without modulation.

At approx. 75 nS into the positive first half-cycle of exitation, grid current and plate current are now flowing and are about 30% Max.

At approx. 132 nS, the RF exitation voltage has peaked to 100 volts and has driven the grid to a full 10 volts positive, so the maximum grid and plate current occur here at 100%.

At approx. 142 nS, the grid and plate current fall to 30% once again.

At about 208 nS, the grid and plate currents have fallen to less than 10%. At 263 nS, all grid and plate currents are zero. The negative half cycle of the RF exitation voltage starts to ramp negatively, but we neglect it since we can't use it.

We have then a plate current pulse of approx. 67 nS duration. This 67 nS (rounded, not rectangular) pulse occurs every 526 nS and shock excites the resonant circuit.

If the Q of the tank circuit is adequate, enough energy will be pulsed into the resonant circuit, such that the resonant (tank) circuit completes a full cycle. If the plate current pulse does not come soon enough (periodically), then the sine wave in the resonant circuit will decay exponentially over time.

If 263 nS represents 180 degrees of the sine wave, then each nS represents about 0.68 degrees. So for this example,  the plate current flows for only 46 degrees of the input sine wave.

An oscilliscope is an amplitude verses time device. On an oscilliscope with suitable probes, one could see the RF input exitation voltage as a complete 200 Volt Peak-to-Peak sine wave, and a plate current pulse of say 100 mA with a duration of 67 nS.

If the overall tube/tank circuit efficiency is 67%, the average power dissipated in the load is 67 Watts. Then with an oscilliscope across the 50 ohm resistive load, one would see a continous sine wave of approx. 164 volts Peak-to-Peak across the load.

On the spectrum analyzer with a probe across the output, one would only see a single spike centered at 1.9 Mhz with no sidebands, since a spectrum analyzer displays dB amplitude verses frequency. With modulation, the trigonometric product of the modulation wave and the carrier wave will show a spike at the carrier frequency with sidebands.

BTW, here is an animated AM modulation site. The modulation frequency is varied and the modulation appears to be a constant 95%:

http://www.williamson-labs.com/480_am.htm

[KA2QFX]

At the risk of being redundant in this long thread…  The power of the RF stage when the scope shows zero is indeed zero. BUT, the energy of the carrier is still a critical component of that zero value.  Were it not for that value of carrier the vector sum of the sidebands alone would yield a very different waveform; namely DSB suppressed carrier. 
Just because the particular instantaneous values of the three waveforms (for a single pure modulating sinusoid) combine at some instant to equal zero DOES NOT infer that their individual values are all zero as well.  Hence the carrier IS actually still there even though the NET output is zero. Just as when crossing zero (twice each cycle), the pure (unmodulated) carrier’s instantaneous value of zero is no less an important component of the waveform as it’s peak value.
I'm sure that clears up everything LOL

P.S. I think Bacon's first reply said it all as well.

[kc6mcw]

The carrier is cut off EVERY TIME the grid swings negative, however that is at a much faster rate than the modulating audio. That is why you have the tank circuit in the final plate line for the "flywheel effect". When your scope has the sweep speed right to display the audio waveform, the sweep speed is way, way to slow to show the individual sine waves from the RF component.

This is how I am understanding this as well. So its not like the carrier is coming and going. It might be a different story if the modulating frequency was the same as the carrier frequency. The signal would then be all screwed up when you said "hello".

[Gito]

Hi

It's true what DMOD wrote about Creating a carrier
but The Created Carrier amplitude(voltage) is depended of the plate voltage Final RF tube,So when the plate voltage is 600 volt you get a higher  amplitude(RF voltage) than 300 v on the plate.
Plate modulation is the way to vary the plate voltage of the Final RF amplifier.
You can manually  " modulate" a RF amply.
With a variable power  supply   ,first we set the plate voltage at 300 volt than look at the carrier amplitude/RFvoltage ,than slowly rise the variable power supply to 600 volt,than slowly make the power supply down to 0 /zero volt.
Than compare the  Amplitude of the Carrier  with 3 different setting Power supply ( 0 volt,300 volt,600 volt)

So it's not how the Carrier is created ,but the Amplitude of the created Carrier with the varying  plate voltage of the Final
 in actual modulation ,the varying Plate voltage of the RF Final is caused by the proses of modulating it.


 A little Question :

If a 10 Hz sine wave  has a 20 RMS voltage we measured with A DC voltmeter, I think the needle of the volt meter is trying to go up and down in phase with the  sine wave,
. its like looking a carrier in a osciloscope ,(WAve -envelope pattern)

If the same sine wave is measured with a AC voltmeter ,we can see a steady voltage on the meter( the needle stay still) is it because it measured the average voltage? why  the voltage does not go up an down like the sine wave?

Because it' design to see the average  amplitude of the wave ! ,rectified filtered with a capacitor than the result is a rectified voltage.that is always there when the sine wave is still on/running.

So the voltage is always present,but actually the voltage going up and down.

Like looking at a spectrum analyzer,we always see a carrier,why? because it is design to measured the average amplitude voltage of the carrier.
actually the Carrier it self is a sine wave which goes up and down.

Regards

Gito.

[K1DEU]

   When W1ECO and I were kicking around the benefits for 20 Db. of negative feedback in modulation design. One advantage was; When there was an instantaneous higher Z (no reasonable load) from the RF stages the modulator tubes would produce extreme distortion/harmonics which would result in off channel shot noise (not sidebands). But with now more than 20 Db. of negative feedback resulting from less secondary load the modulator gain is momentarily reduced, minimizing perhaps eliminating off channel splatter.

    Therefore I always support a stable 20 Db. mid band gain negative feedback loop. http://hamelectronics.com/k1deu/pages/ham/transmitters/am/pages/negative_feedback_design.htm

    And If we must really process, ego slam the audio the WA1QIX Load device should alsso be added, even to Class E design. http://www.classeradio.com/3-diode.jpg

Regards John