The AM Forum

Check out the article by Bob, K1VUB in the Tech section of The AM Window Web Site.

http://www.amwindow.org/tech/htm/k1vubmodulator.htm

"Sometimes sine of 90 degrees equals to 2"

Guys, more than 100% modulation does not exist. It is up to 100% modulation by distorted signal.

Hello
OHHHH YESSSS there is more than 100%

100% modulation is the maximum NEGATIVE PEAK for AM, beyond that you cause splatter on the bands and can damage the mod tranny (no load) in a plate modulated Tx.

150% POSITIVE is your loudness!!!

But according to some 150% distorts most detectors on the receive end

I'm happy to see peaks of 125%  caaaamown
Fred

Sure.

If say the audio amp clips on 100W it can give 150 W of power, but human ears will distort the signal no matter how quiet it is on the distance! :lol:

My voice needs 150 to 200 % positive peak to sound like me on the radio.
I just reduce the carrier to be able to hit it cleanly.
compression is adding distortion

Hi Wavebourn and I hope others can clarify a little,

You are on the correct track of thinking for an audio amplifier.

And in a sense it's the same thinking for a Linear RF amplifier. You cannot exceed positive peaks or the P.E.P. rating of that linear amplifier.
 
I always had a little trouble understanding the neg and pos too. If you look at a properly excited RF carrier on a scope, the negative peak is in the center of the display, and the positive peaks are hopefully rising equally at the top and bottom of the modulated carrier. The UNmodulated carrier looks like a thick green display usually 2 increments above and below the the center of the screen.
 
There's a big difference from the audio amplifier world and the RF world. The Audio world is trying to achieve undistorted RMS audio to your speakers. Gone are the days of the peak audio ratings of the 70's.

As was mentioned earlier you can reduce the RF power and turn up the modulation. But there is always limitations due to the power supply, etc.

There was a thing called Ultra Super Modulation and there are tech articles explaining how that was accomplished.

If you read carefully the posted article from WB3HUZ, Steve. The writer explains how this is done in more technical terms.
This should be a nice thread.
I hope I explained what I could here.

Fred

Fred;

single ended amp distorts, giving odd and even harmonics. Also, due to distortions it gives a "rectified" part of distorted signal that modulates the carrier also. If distortions works the way average power is higher when carrier is modulated people say that modulation is more than 100% positive, if it is less people say it is negative. But it is not true if to follow strictly.
Try to experiment. Take a modulation choke with the center tap connected to b+, and feed your transmitter tube from one tap connected to modulation tube's plate, then switch it to another tab. Compare results. You will hear and see on the oscilloscope nearly the same signal after detector whether you modulated positive or negative.

Now use push-pull modulating amp. Get 100% modulation. Pre - distort input audio signal and see results. You will see the same if your modulating amp is capable to follow very low frequencies. It does not matted mathematically where you get needed for "positive modulation" distortions, before the amp or in the output stage. However, it is technically simplier to achieve it due to non-linearity of the output pipe.

But if you really want more than 100% modulation by clean sound, here it is:

(http://wavebourn.net/images/200percent.gif)

Such an approach gives desired "rectified" part of distortions, but do not produce extra harmonics of audio signal, except when the amp is overloaded on fronts of a signal. But simple diode detectors in receivers for sure will give extra distortions. Anyway, AM can't be more than 100%, it may be just a trick with usage of distortions in the output stage for more power on peaks of audio signal.

http://www.qsl.net/wa5bxo/asyam/aam3.html

Wavebourn,
The link above this posting tells the whole story.
thanks
DAVE

FRED

My opinion is, AM _must_ be Hi-fi forver. Otherwise SSB is more effective in terms of power per bandwidth.

Fred,

my conclusion from the article is that some certain transmitters were poor because developers saved costs on a quality offering "more than 100% modulation". If you mean that there are couple of limits, both for positive and for negative parts of signals, it is always true, it is called clipping. But you don't modulate your carrier by some kind of positive signals to certain percents and by negative signals to certain percents, you are modulating it by a the single your own natural voice  that has both polarities and is symmetrical after at least one R-C network or transformer. No magic here. You may sacrifice quality of your audio   to be heard farther. The question is, what part of quality are you willing to sacrifice.
You may loose the 1'St harmonic of your voice. The result will be tiny, but loud scream, very symmetrical on oscilloscope. You may add extra harmonics from limiter, but get louder voice.
Or, you may loose dynamics using compressor, but power usage will be better. The last is preferrable, however... But anyway it is a distortion, it is boring if you make a long speach...  ::)

Conclusion: AM means Hi-fi forever!  ;D

I think that diode circuit is going to do something interesting going toward the negative peak.

It increases B+ proportional to amplitude of audio signal. However, positive peak can't be endless...

the diode will go poof  and maybe the transformer going to the negative peak

Also output transistors will go poof if you underestimate peak power consumption that is really huge. In this trick modulator acts also as an audio controlled extra power supply.

Or you can build a PDM modulated rig and never worry about trannies..(snicker) ;)

B

But then you gotta worry about cleaning up MOSFET shards.....

<running and ducking>

my 1983 6DQ5 modulators in the Viking 2 PDM rig are good as new and the FETs in my 1995 160 meter rig hanging in.

And did you try to pour a water in it?  ???

I tried this scope test to find my voice asymmetry but I just couldn't see it.   It's not that my voice is symmetric, it was just changing faster than my old eyes could latch.   How do you pros set up to do this?  And what do you say -- "The rain in Spain falls mainly on the plain" or "How now brown cow" or maybe "The quick brown fox jumped over the lazy dog"?  Wait a minute.  That last one was for typing, wasn't it.  Seriously, do you just say "ah" and watch the scope?

My intact MOSFETs transmitter even has a cute little jumper for reversing the voice polarity (thanks, Steve) but I've never set it up.

Jon

PS.  I liked the idea of the "reverse controlled carrier" in the web link.  I've turned my T-4XC upside down.  Watch out ionosphere.  Here I come

Jon,
It sounds like you're almost there. The standard is to have a dead carrier from your TX picked up by a small wire or coil near the source of the RF and adjust the vert height 1 graticule above and below the CENTER of the display. With the tx in a dummy make a long yaaaeellloooooo into the TX microphone and adjust the sweep speed of the trace so that it is not drifitng around on the scope. You should get it close to locking in where it's not dancing around too much. If you can input a steady tone from a sig gen it would show better on your scope. Be careful for the audio freq and the amount of input to the Tx.

Now, as you talk or you will see a fair representation of your voice. The positive peaks are the little spikes going upward on the screen. If you see bright green spaces in the CENTER of the display, those are NEG peaks and you are overmodulating. Look closely at the display pics from Dave's link.
My voice characteristics look like a Christmas tree on its side, with the top of the "tree" pointing to the right of the scope screen. If my mic polarity is incorrect the top of the tree is to the left of the scope screen and there are smaller positive peaks and more bright traces in the center(NEG peaks).
I hope you can see how valuable it is to monitor your tx with a scope. A broadcast mod monitor cannot show you what a scope can.

Fred

Jon, I saw FETs with crane shout flied toward the south just under ionosphere, upside-down! :D

 Nice job! ;)

Use the words "five fiayve fiayyyyyve". Rinse, repeat.

The difference in audio polarity can be staggering with certain voices. Even a P.E.P. Wattmeter shows about 30% more power with my voice in the proper polarity.

Tolly: I've seen a few talented people fight this concept. Sorry, but it's true. The total energy on both sides of zero is the same but the slope is not a mirror image. Voice isn't a sinewave. Some are close but many look nothing like a sinewave.
Some transmitters will not show much difference when switching "phase". Plate modulated boxes using a common supply and high numeric modulation transformer ratios are the worst culprets. Those transmitters are producing distortion. Distortion is anything that changes the waveform from original.

Dave, you have to turn the output pipe you plate modulate upside - down, inverting voice phase will not help. Period.

Wavebourne - plot cos(f0)+((cos(2f0))/2)

The waveform is asymmetrical.  The human voice is like this, lots of second-overtone and even-overtone energy.  I think it must be from the way vocal cords vibrate with air.  When they move together, they pinch off the flow fast; when they move apart, they widen the opening only a little bit.  So the excitation of the vocal cords is asymmetrical, and the pressure waves that result are asymmetrical.  I think that must be why a voice is asymmetrical.

I played with electrical phase shifting at frequencies around 100 Hz, and it strongly affected voice asymmetry, even though frequency response was not changed, and nothing non-linear was in the audio path.  See the "Bass Phase Aligner" in "Asymmetrical Peak Limiting" in AM Press/Exchange issue 79.
http://www.amfone.net/AMPX/issue_number_79.htm

Bacon - I can plot the entire Fourie transformation for a rectangular signal of the big porosity. Then, I can multiply it on a function of the single R-C differentiating network to get rid of asymmetry.

Asymmetrical vaveform means DC presence. It is gone after the 1'st coupling capacitor or transformer. Due to non - linearity of output tubes and of a tube you modulate it is back again because of distortions. Also, dynamic resistance of a power supply gives us some. It is simple like 2x2.

No, this is just a time plot of two cosine waves added together, one twice the frequency of the other, and the higher frequency one about 1/2 the amplitude of the lower frequency one.  When you do this, there is no DC offset.  The waveform has a relatively wide, flat negative cycle at a medium amplitude, and a relatively narrow spike of higher amplitude on the positive side.  Here there is no distortion, just two sine waves added as they would be in an audio mixer, but you get an asymmetrical output waveform.  But if you do this with sine waves instead of cosine waves, you get a strange but symmetrical output waveform.  With harmonic combination, the relative phase is important.

Or think of a square wave.  Its peak value is equal to its RMS value.  But a sine wave's peak value is 1.414 times its RMS value.  Take the positive peak of a sine wave of peak amplitude of +1.414V, and then switch to the negative peak of a square wave of -1V peak amplitude.  You have the same energy in the positive and negative peak, which means no DC offset, but you have a 1.414:1 waveform asymmetry.  But in this case you have nasty distortion.

The key is equal watt-seconds per half-cycle.  If they are the same, you have no DC, but the peaks do not have to be the same.  When you have even-harmonic addition in the correct amplitudes and relative phase, you get significant asymmetry, and the voice has a lot of that.

What? Positive half-sine of 1.414 and negative half-sine of 1 and no dc? Do not make laugh my slippers!, please. :D


You do not transmit just a lalf of a cycle, you transmit the whole Hhhyaaallloooooowwwwwfffff! :)

Try to invert your voice signal and ask some DX-er to compare S-readings.

No - positive half of sine, negative half of square, same energy in each, but different peak amplitudes.

Anyway, Bob's circuit soft-clips peaks on one side, which is set up to be the negative side, and some distortion does result.  We all find that we need to do something like that to prevent overmodulation by occasional peaks.  This is the only way a symmetrical wave like a sine wave can be made asymmetrical, but most voice waveforms are asymmetrical.  When the asymmetry of the voice is aligned with the negative peak soft-clipping of a circuit like Bob shows us, distortion is light, and modulation is louder.

When a waveform is soft-clipped on one side like this, there is some rectification, and a DC component is introduced.  Bob's circuit is DC coupled after the asymmetrical soft-clipper, so this will result in some positive carrier shift.  This DC offset is sometimes removed by AC coupling after the soft-clipping.  Peak control is reduced when this is done, but extra aymmetry is still produced, because one peak is dented.

Natural voice aymmetry is reduced if low frequency response rolls off and attenuates the lower voice frequencies.  Also, some voices are not very asymmetrical.  But with low frequency response down to 50 to 80 Hz or so, most voices are significantly asymmetrical.

Bacon;
to align asymmetry of voice you do not need pre-distortions, you need just one coupling capacitor or one transformer. You do not need even a capacitor; the entire mic is acting as a high-pass filter. Sometimes when it's low end frequency is too low so called pop-screen helps.

For better sound quality predistortion should have different characteristics than just rectification to increase positive peaks. If you want to fool FCC and transmit more energy it is fine. But you may transmit no carrier neither one of sidebands, and it will be absolutely legal. But please do not call it AM then.