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Reverse Carrier Control "seems" to be an excellent way to insure the "1500 Watt PEP Rule " isn't broken.   

"IF" you have the tubes capable of running 1500 watts carrier output.....
A pair of 833s or 4-400s and you don't mind warming up the hamshack in the winter,  what would be the downside to "Reverse Carrier Control" ?

I have the iron to plate modulate, but it seems that plate modulating a pair of 833s with another pair of 833s would be no more efficient if run at 1500 watts pep, than a pair running RCC.

What say you experts ?  ( ready to get back on AM after retirement )

Don W4DNR   Huntsville, Alabama





 

I do not think that the RF world is that critical that we have to insure that the P.E.P. NEVER jumps up past 1500 watts.
I do not think someone is monitoring anything much these days that close.
The new focus we should have is how to clean up our transmitters from distortion products and splatter.
And a lot of that is purposeful or accidental adjustment of the equipment.
I'm not taking shots at your post. It would be interesting to see more responses

Fred

No Problem Fred,

Hope you are doing well.

I just thought that a pair of tubes running at 1.5KW carrier and being modulated downward would have to be fairly clean.

Don W4DNR

Actually, a form of "reverse carrier control" (Bacon's tag for it), has been used in a number of Icom and Yaesu transceivers over the last bunch of years. There are several threads here over the years and in a number of other forums where users of modern transceivers complained of "downward modulation" as viewed on their external wattmeters or SWR meters. i.e. I key the transceiver, see carrier, and when I talk, the meter swings downward. However, if you put a scope on the RF output and watch the waveform, it's doing exactly what Bacon describes in his blurb. What it effectively means is that you can drive a linear amplifier all the way to maximum legal output with only carrier. Under normal modulator gain settings, you really never exceed the legal limit when modulating. If you set the modulator gain control too high, at the zero base, the positive and negative going waves can flatten out with the result of distortion in your audio.

Many amateurs try to defeat this "feature" of "reverse carrier control" by diddling with the ALC control or applying some variable negative voltage to the ALC jack to cause the transmitted signal to show positive peaks. The problem with that is that you have to carefully monitor your output signal so as not to overdrive the linear on audio peaks. One additional nice feature with "reverse carrier control" is that with a scope on your output transmitted signal, you can clearly see how well your modulating the carrier.

Mark has very succinctly and accurately described this action in the ts440 thread I started.

RF based alc on a am signal is fubar.

--Shane
KD6VXI

I never heard a good sounding AM signal with the carrier pumping around like it does when you get into the alc on a rice box.
I think that is different from downward only modulation.
The AGC action of the alc makes the carrier jump around, not just be modulated in the downward direction.

Every ham radio implementation of reverse carrier control I have heard sounded horribly distorted.

Some modern AM broadcast stations use something like this (others use regular carrier control -- go figure) and, done with proper digital modulation algorithms, it can sound almost as good as standard AM. When I tune such stations in on a very high fidelity AM receiver, though, there is a subtle subsonic component that is slightly annoying.

All the best,

Kevin, WB4AIO.

Many of the "older" rigs like the TS-440, and maybe some of more recent economy models, used a balanced modulator. Many transceivers (Icom and Yaesu specifically - haven't followed what Kenwood does) within the last 10 or 15 years use a DSP-type modulator. Given that the rig hadn't been diddled with, you can generally achieve a near perfect looking AM signal using a DSP modulator terchnology. At least that's what I've seen or heard on a number of Icom rigs, and several Yaesu's over the years. When modulating these rigs on AM, there is no movement (increase) of carrier beyond the limits you see with just carrier and no modulation. There is also no pumping action.

I don't have a TS-440, so that is one reason that RCC looks good to me.
I could use my DX-40 to generate the RF, and I wouldn't mind playing around with a cathode or grid modulator .... Solid State or Tube...

Don   W4DNR

I like the idea of reverse carrier control when an amplifier is to be used. The Icom here a IC735 does that.. Set the carrier, and watch the rig on the scope and when you speak it just almost disappears to be replaced with a well modulated wave form. 

I don't know what the receiving station hears on that because I don't use that rig for on-air work but as a pig for amplifier testing.

I also like the idea of a big carrier and mostly negative going modulation, if that is how the voice goes.

Some TX tubes have a spec for this 'modulation essentially negative'. I like the idea.. but have not tried it. I'd like to know how it works if it's done.  The aforementioned article is more of a suggestion and not showing a mechanism for doing it.

I'd sure also like to learn more about these things.

It's interesting, to be sure. And useful for the tests you were running.

The AM broadcasters who are doing this use digital modulation and they just write software making the DC component in the pulse train rise to any selected level when audio modulation is low.

But think about it.

Compared to standard AM operation, all it does is make the carrier 6 dB stronger whenever you're not talking. Just a bit of additional quieting, in the range of "slightly noticeable." And it will certainly stress all but the most scroteful of linear amplifiers.

I can just see the carrier on my Clipperton going up to 1kW out every time I pause -- better not pause too long!

I've thought about it before and I'm not too sure it would be as much stress on the amp as one would think. RF powerwise, yes since the components are now being hit hard with a lot more power than before, but as far as tube stress, I don't think it would be any harder than class C plate modulated service, and here's why. When running conventional AM, we have to tune the amp for peak power then reduce the carrier down to be able to have headroom for positive peaks. Since what we are actually doing is modulating the efficiency of the amplifier, the efficiency also drops at carrier. This generates a ton of heat. If we run AM that has 0% positive modulation, then the carrier level can be set to whatever the amp can safely put out and not be too stressful on it. During modulation, the power would drop due to peaks. As long as you didn't get into an old buzzard transmission, I don't think it would be any harder than running full power CW at a moderately slow rate.

Would this amp run in class C or AB?

In normal plate modulation, it can run deep into class C and the plate voltage is modulated.
In downward modulation, you could only do it that way if you reduce the plate voltage only.
If you make a waveform and then want to amplify it, you need a class AB amp to do it cleanly.

Most sideband amps are low duty cycle, tubes and power supply are under rated because of the low duty cycle.
Amplifying downward modulation would be somewhat like running fsk or other 100% duty cycle modes. Even AM is not that bad as the carrier is only 1/2 peak power, not full power.
It just seems like a bad idea, it is hard on amps and has no advantage.

RE:  "Reverse Carrier Control" as postulated in this thread...

Other things equal, does a carrier power that is amplitude-modulated so that total power during positive-going modulation peaks does not exceed the peak voltage/power of that unmodulated carrier, and negative-going peaks approach -99% modulation produce the same S/N at the receiver as when that original carrier power is modulated +/- 99%?

Compare the power in the sidebands, and total ERP for those two configurations.

You would need to clarify:
1500 watts downward modulated VS 375 watts carrier modulated 100% to 1500 watts peak?

Same signal to noise ratio with modulation, more carrier power without, it would seem like an under modulated carrier, it would have a really strong sounding carrier, then with modulation not sound very loud.

I would think it would be better to go the other way, normal carrier and LOUD modulation.

I was thinking that with an amp specially built with components rated to handle the power level and biased lightly into class C (a class C AM linear, yes it can be done, it has been discussed on here several times) it might work out. I'm gonna have to try some experiments and see what the real difference in volume is between conventional AM, AM that is 50% positive modulation and 100% negative, and AM that is 100% negative modulation only. Then as a second test, start reducing carrier to go over 100% positive peaks ans listen for the difference there too.

Sometimes the simple way to look at it is also good. The 'area under the curve' after the RF had been detected in the listener's receiver would seem to have something to do with it. This is just my opinion about doing the comparison without a bunch of math an my opinion that if modulation is done fully for each case then it does not matter very much at the other end to the listener.

Take the drawings in the article and cut out the waves and paste them either way..

Since the listener's speaker is a balanced thing moving back and forth from -x to +x with a given polarity-heavy speech wave, that finite movement's polarity should not affect the sound loudness because the speaker will spend more time at +x than -X, or vice versa, because the real result is the same difference in air pressure.

The only advantage I see from this graphical method is that a heavy carrier at zero modulation would make for a quieter 'channel' between syllables or words.

The reverse carrier control does not subtract from the modulation level because the carrier is adjusted with the modulation level instantly and the listener's AGC should move with that. Not all can..
The lack of a control on the other hand would leave a huge carrier and a varying modulation level during speech. It would seem well to use audio processing to keep the average modulation as high as possible, just as with the 1/4PEP carrier, or with a very low carrier. I mean that the carrier=x% of the PEP is a myth if the audio is controled right, but carrier=1/4PEP is a very easy thing to do.

To me with the graphical view, it looks like the loudness of the modulation using either reverse modulation or reverse carrier control is about the same as the regular AM method so-called carrier=1/4 peak, as long as the audio level is such that the modulation is kept highest.

The waveforms in the article show different ways of modulating and all show a case where full modulation just short of clipping was used. Taking these, and inverting some of them to get the shapes, it shows me that what matters is that the envelope, not matter what its amplitude at a given time, be kept full of audio, and other things are secondary.

At the other end where a speaker is being used it does not matter which way the spikes point. At the transmitting end it might matter because of the power levels needed to do it one way or another.

The only advantage I see from a high carrier is a quiet space between syllables.  A disadvantage that comes to mind is how the poor old antenna matcher or balun or other victim along the line is going to feel with a 1500W carrier as the default level. This is all opinion of course I can be wrong about it.

And isn't that what it's all about ?   IF  you can afford the extra stress on your equipment to produce a
better experience for the listener during high static or QRM , and still stay below FCC limits, isn't that what we are all trying to achieve ?

Don W4DNR

Using Icom's version of reverse carrier control in a 756 PRO II driving a SB-200, working AM stations on the west coast or over in Europe on 75 meters, I have less problems with stations on the other end hearing me. A healthy carrier cutting through the QRM static, seems to provide better capture for the receiving station.

Of course, if you have to think before you say every word, (The...........rig...........here..........is..........running...........100...........watts) you might put some extra stress on your RF components.

Here is one way of developing inverted carrier operation using a direct-coupled audio amplifier to modulator, then on to the screen Grid of the modulated RF amplifier.

The higher the audio amplitude, the lower the carrier level and vice versa.


Phil - AC0OB

Then using transistors - am I to understand that basically you would set to the rated transistor voltage, and make sure that the heat sink is large enough?

in other words - my amplifier board with 25V transistors - I can operate with 24V - instead of 6V?  I do not have to worry about that 4-1 voltage ratio?

Cliff

That is the way screen modulation works, the plate voltage you put on the final tube is the maximum voltage the parts will see.

Much different from where the final tube is run though, in screen modulation as in AB amplifiers resting carrier is about 1/3 plate dissipation and 1/4 peak power output.
Running it at full power and modulating it downward would be brutal on the tube and lower power output as the plate dissipation would limit things.
I have three 4-400's I screen modulate with low voltage on the plates (3000 volts) and at 300 to 400 watts carrier and about 1500 watts pep, and the plates are glowing dull red.
I have no problem adjusting things so the deck flat tops at 1500 watts pep and runs higher carrier
outputs but never tried how far I could go or what it sounds like.
I always liked doing 350 watts carrier and 1500 watts pep.
To get more carrier you just unload it a bit.
I will try some tests doing that with someone listening at the far end and see what it sounds like.
I doubt it will sound good... 

That's why you would pick a tube type that can run "full bore" with no modulation present.

Plate voltage on a class C RF tube may run as high as DC + the RF peak voltage.

Phil - AC0OB

I was thinking 833s or 4-400s but I do have a 4CX7500 on the shelf in the garage .

Build the 7500.  I've got a 4cx5k, socket,  filament xformer and 3 amp ccs Dahl plate xformer.   Within the next couple months,  vac variables for tune and load.


I'm scream modulating it,  a la continental 50k xmitter basis,  although I stead of a series mod,  I'm going pwm (already have a pwm capable of 1.5 kv,  hence the reason).

Build yours,  and let me know what problems lie ahead lol...

--Shane
KD6VXI

You're screen modulating the 4cx5k with a PWM signal? That ought to be interesting.

Phil- AC0OB

It's going to be my first choice.   I had a series mod built,  from a schematic posted here,  but have since dismantled it.

If the pwm drive is no Bueno,  I'll build up the series mod again.

I asked QIX about this idea a while back and I believe it got lost in the shuffle.   As such,  only way to find out  if it's going to work is build it....   :-)

--Shane
KD6VXI

You only need about a 30W modulator that varies the voltage between 250V and 500V at 50 mA max.

A PWM modulator seems like overkill and overly complex for SM.

Phil - AC0OB

It is.   But here's the rub.

I have the pw built,  using air wound coils and then 600 volt rated #14 (iirc).   The mosfets are 1.5 kv .   I've since purchased Teflon  16 gauge.   I had built a series mod and cannabalized it fto make a hv pwm.

I modulated a triode with it,  fine business.   Learning experience.

That's what this xmitter is...   Learning experience.

If this doesn't work,  I have a 3000 new in box I'm sure I could use as the switch.

It's a case of run whatcha brung.   The modulator and processor is built.


--Shane
KD6VXI

My opinion - hope that is OK.  This whole concept seems like an awful waste of power and certainly will stress most amateur grade components

Merry Christmas, Al

I agree.

I think the reason it has obtained a toehold at a few AM broadcast stations is that they are in bad shape economically and are desperate to reduce their power bills. Thus the 50-kW station using such a system is 50-kW only during pauses in speech or other programming. During full modulation, the carrier drops down to 12.5 kilowatts or so. That really saves on the power bill.

Of course, they'd save even more on the power bill if they just used standard AM and dropped down to 12.5-kW steady carrier. And it would sound better too, and be much easier to implement.

But... the FCC says that you must stay within 15 per cent. (if I recall correctly) of your assigned carrier power. If you don't there are fines and penalties. And they might recalculate your protection from other stations if you apply for a power cutback.

So the "reverse controlled carrier" operation lets them save power and still pretend they are running 50 kilowatts.

73,

Kevin.

Note that a "standard AM" station with a 12.5 kW carrier modulated +/-100% has a smaller useful coverage area than one using a 50 kW carrier modulated +/-100% (other things equal).