The AM Forum

Say you have a transmitter that can put out 1500 watts carrier. Then you put in a modulation deck in such a way that instead of summing the modulation to the carrier for peaks, it reduces the carrier to zero. The reverse of what is normally done. opposite phase?? 

Wouldn you have a 1500 watt PEP transmission??

Why wouldn't it work??

It can be done and it can sound OK.  There are a few ways to do it.

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

When you do this you will be pumping the AGC of the reciever. Whether or not this is objectionable depends on how far you take it and the quilities of the reiciver. 
I have a paper somewhere on tests the BBC did on this on AM BC stations. I think with the 6dB of carrier reduction on 100% modulation (same thing as your suggesting) the only noticable issues were in the presence of interference . They were varying the 'carrier level' at a sylaballic rate - same as controlled carrier.
                                                                  Ian VK3KRI

I guess if the Inverted Amplitude Modulation had a good advantage that it would be in service but it isn't. 

Bob you metioned in the article that a linear in this service would be operating at very close to maximum efficiency.  This is true and it would also be operating near maximum dissipation too!

With just a quick thought, it seems that this technique can only be done with low-level modulation (followed by a linear) or series modulation.  Plate modulation with a transformer is out.

One technique that comes to mind is having to put the audio to a clamp circuit also called a DC restorer, to get one peak of the audio waveform to hold to a dc reference level.

I heard Chuck K1KW on about a year ago with his highly modified FT-1000? and linear running inverted amplitude modulation.  He is the only one I have heard on running this.

I think that Chuck achieved the inverted AM by using the transmit system ALC action and over-driving the linear and having the negative swing modulation bring the output downward from full limiting output.  (Low-level modulation and linear amp., forced ALC action.)

I agree with Tom's comments...

The only advantage I can think of, which would result from using "downward modulation"... and which could be implemented in a variety of different ways/flavors... would be that it would increase the "quieting" produced (at the receiver) by the carrier during periods between syllables, and also during times then one is transmitting, but not fully modulating (or not modulating at all).

The disadvantages I can think of are:

1. More wasted power, with the carrier moved up to full output (e.g. 1500 watts) during periods of time when it would normally be 25% of full output (e.g. 375 watts) or less. I agree with Tom that in the case of a linear amplifier the impact on electrical power consumption would be approximately 2:1. With a Class C plate modulated transmitter, the impact on electrical power consumption would be approximately 4:1. But whether using a linear amplifier or a plate modulated Class C transmitter, the electrical power consumption would be substantially higher using the downward modulation approach

2. The average plate dissipation that the output tube(s) would have to accommodate would, as in the case of the electrical power, be substantially higher for a plate modulated Class C transmitter: nominally 4 times higher. For a linear amplifier, the plate dissipation would be approximately the same. I.e., 375 watts output @ 33% efficiency => 750 watts plate dissipation; 1500 watts output at 66% efficiency => 750 watts plate dissipation.

3. Depending upon the time constant(s) with which the transmitted carrier level is adjusted (downward and upward) to accommodate the modulation, and depending upon whether the receiver's AGC circuitry is responding to the peak level or the average level of the received signal*, and depending upon the time constant(s) of the receiver's AGC circuitry, ... the AGC of the receiver might track the peak carrier level... resulting in reduced volume of the audio from the receiver compared to other signals in the same QSO; or the AGC might approximately track the changing carrier level... resulting in annoying variations in the volume of the audio coming out of the receiver.

*Most AGC circuits employ peak detectors... but in my 75A-4, I installed a precision rectifer + low pass filter circuit that tracks the carrier level of the received signal... which I use when receiving AM.

Best regards
Stu

Most of the current Icom rigs, and maybe some of the current Yaesu's, use a similar form of low-level modulation for AM. I'm sure you've seen thread titles that "my Icom is downward modulating in the AM position" or something similar.

User key's down and looks at their forward power SWR meter in their antenna tuner or some external box and sees steady carrier. As they  modulate in the AM position, the forward carrier meter pointer moves down in sync with their modulation. If you're a "seasoned amateur" (i.e. been on the air since the dawn of mankind), your perception is that you are "downward modulating". Experience from using plate modulated rigs would indicate that's not a good thing. Actual on-the-air testing has proved that this perceived malady is not discernible to the "normal" ear listening on the inside or outside speaker. Plus, if you're using this rig to drive a linear, you could tune for maximum power carrier output at steady carrier, and then when modulating the driver, not fear you're going to exceed the maximum specs for the amplifier tubes.

Tom - depending on the degree of modulation, dissipation actually may not be much higher.  For a 1.5 KW output linear, dissipation is about 1,000 watts at 1500 carrier output.  At the carrier level for 100% positive modulation (375 W carrier output), dissipation is about 875 watts, and at the carrier level for 150% positive modulation (240 W carrier output), dissipation is about 760 watts.  Dissipation in a linear amp reaches a broad peak at 50% efficiency, so if it is set up for 1500W peak output, dissipation will peak at about 1041 watts dissipation with about 1041 W carrier out.

Hi Bacon,

My reference to increased dissipation refers to comparing the normal AM signal with modulation versus the "inverted modulation" signal with modulation, both passing out a linear amplifer with 1500 Watts PEP output.

Wouldn't the average dissipation in the linear with the normal AM signal be about 560 Watts, compared to the 1041 Watts for the inverted modulation signal?

Tom
Bacon
et. al.

It would seem to me that, as a practical matter... we would want to make the comparison for the case where the operator is silent (between syllables or thinking about what to say next). Even for those of us (including me) who are old buzzards... most of the time, the audio level will be close to zero... and that is what will govern such things as average power consumption (using electricity/heating the room) and plate dissipation.

The exception might be people who spend their on-air time transmitting a 100% modulated sine wave tone.

Best regards
Stu

Dissipation with a typical linear amp tuned for 1500W would be 540 watts at a carrier output level of about 97.5 watts.  Efficiency would be about 60 * ((1500/97.5) ^ 0.5) = 15.3%, so input power would have to be 97.5/.153 = 637.5 watts, and dissipation would be 637.5 - 97.5 = 540 watts.  This would permit positive modulation of ((1500/97.5) ^ 0.5) - 1 = 2.92 or 292%.  Alternatively, positive carrier control could be used, and positive modulation could be more like 200%, 150%, 100%, etc, or even 67% or 50%, etc, if the voice polarity was inverted.

Operation at 50% efficiency at 1041 watts output would allow 1500/1041) ^ 0.5) - 1 = 0.20 or 20% positive modulation, allowing a 5:1 asymmetry in the speech audio, or allowing whatever negative dynamic adjustment would be needed for the required modulation percentage.  Carrier input power would be about 2082 watts.  This might be best, so that the leading edge of an initial sound would not be clipped in the positive direction before the carrier control could act, although a slight delay in audio versus carrier control would allow these dynamic distortions to be avoided.

Figure we are using a class B linear amplifier tuned for 1500W PEP.  Figure six conditions: an unmodulated carrier of 1500W output, a carrier level of 666.667 watts for 50% positive modulation (for 2:1 asymmetry, negative pointing), a carrier of 540 watts for 66.667% positive modulation (for 1.5:1 asymmetry, negative pointing), a carrier of 375 watts for 100% positive modulation, a carrier of 240 watts for 150% positive modulation, and a carrier level of 167.667 watts for 200% positive modulation.

full pep output
carrier output power: 1500 watts
carrier efficiency: 60%
input power: 2500 watts
dissipation at carrier level: 1000 watts

50% positive modulation, 1500W pep output
carrier output power: 666.667 watts
carrier efficiency: 40%
input power: 1666.667 watts
dissipation at carrier level: 1000 watts

66.667% positive modulation, 1500W pep output
carrier output power: 540 watts
carrier efficiency: 36%
input power: 1500 watts
dissipation at carrier level: 960 watts

100% positive and negative modulation, 1500W pep
carrier output power: 375 watts
carrier efficiency: 30%
carrier input power: 1250 watts
carrier dissipation: 875 watts

150% positive modulation 1500 pep output
carrier output power: 240 watts
carrier efficiency: 24%
input power: 1000 watts
dissipation at carrier level: 760 watts

200% positive modulation, 1500W pep output
carrier output power: 166.667 watts
carrier efficiency: 20%
input power: 833.333 watts
dissipation at carrier level: 666.667 watts

Yes, dissipation is generally higher at the higher carrier levels, but not that much.  Is the extra few db of receiver quieting between syllables worth it?  Maybe.  Or, we could live with existing limitations, or we could switch to audio-derived AGC and use lower carrier levels.

If we use reverse carrier control, the receiver AGC time constant needs to be rapid for best performance.  But most of us use fairly fast AGC with envelope detectors, because we want the receiver to recover quickly after a strong signal.  And the audio processing and TX audio delay versus carrier control at the transmitter can compensate for the receive AGC tme constant.  In fact, the attack time of a transmit speech processor applies the right kind of compensation.

Positive carrier control emphasizes the noise between syllables, and it doesn't sound good.  Reverse carrier control sounds pretty good, and it reduces noise between syllables on an average-envelope derived AGC such as we have in classic AM receivers.  The the peak-envelope derived AGC of typical solid-state receivers stinks, because it compresses the receive audio, which messes up the received audio dynamics.

The issue is the extra power consumed during times of no transmit audio, and to a lesser extent the power dissipated in the transmitter during periods of no speech.  But this is very much like the argument about using a carrier at all.  We could keep our quality, and save a lot of power, by using DSB reduced carrier, and synchronous detection, and well-designed AGC.  And, we could get get useful noise reduction by using dbx or Dolby compansion with that (I think something like Dolby B would be best, so as not to blast us in the ears when interference overwhelms our signals).  But we prefer full-carrier AM.

It's just a thought.

750 watts carrier (output), 100% modulated (in the positive and negative direction - sine wave) comes out to 1500 watts total, peak power on any spectrum analyzer.  Computations as follows:

A 750 watt (carrier) (RMS) 100% modulated (sine wave) signal breaks down like this ->  1/2 the (RMS) power is in the sidebands, or 375 watts of total sideband power (RMS).  The PEAK *power* in the sidebands is 749.7735 watts (750 watts), plus the carrier power of 750 watts comes out to 1500 watts.  This comes about because the peak *voltage* is 1.414 times the RMS voltage, which, when squared comes out to be 2 Power = (Voltage squared / Resistance), so the peak audio (sideband) *power* works out to be twice the RMS power.

You can think about it like this:  You have 750 watts of carrier.  That doesn't change.  You have a modulator that must supply 375 watts of RMS *power* into a known load (the RF ampflier).  Let's say you're using an RF amplifier that is 100% efficient, and has an impedance of 500 ohms (any numbers will work - I just chose these).  The RMS audio voltage required is 433 volts.  The peak audio voltage is 612 volts.  The peak audio *power* is (612 **2) / 500 = 749 watts.

For "real" RF amplifiers, with less than 100% efficiency, the output of all parts of the RF signal, sidebands and carrier, must be multiplied by the conversion efficiency of the RF amplifier, but everything still works out.  If the RF amplifier were, say, 75% efficient, you would then put 1kw of DC power into this RF amplifier to get 750 watts out.  The modulator power needed for 100% modulation would go from 375 watts to 500 watts (RMS).  The PEAK *power* from the modulator would go from 750 watts to 1kw.  The carrier RF output would be 750 watts (1000 * .75), and the PEAK sideband output power would be 750 watts (both sidebands added together) - 1KW of PEAK sideband *power* multiplied by the .75 efficiency factor.

No power was created or destroyed in this demonstration.

There is no such thing as RMS power. Its average power and that's what you measure with most spectrum analyzers.

HUZ Steve is right, Watts RMS is not a valid term. 

The peak power of a 750 Watt carrier with 100 % sine wave AM modulation is 6000 Watts.

Power terms can be confusing and the values are not intuitive - average power, peak power, peak envelope power. 

Since the power values of various rf modulated signals are not intuitive, therein lies some of the reason for the continuing "arguments" over what the "fair" power value is for AM versus SSB.   

You can’t break down a modulated signal to its individual sidebands and then add the peak powers of the individual frequency components.  This is adding of time independent values.  The total peak power of the AM signal is time dependent on each of the modulation components.

The peak power of the 750 Watt carrier is 1500 Watts.  The power in each sideband is 187.5 Watts and the peak power of each sideband is 375 Watts, but I don’t see that you can pull the 2 sidebands out of the AM signal and just add them together (you can, but...).  This addition is 1500 pk +  375 pk + 375 pk = 2250 Pk., which is incorrect.  The composite signal’s peak power is 6000 Watts.

All of this discussion is completely moot if no one gets on the air with the Inverted Amplitude Modulation.  You’d think that Don K4KYV would be one of the first on with inverted modulation and would be doing it for many years now, because this would allow him to run 1500 Watts carrier output legally, essentially twice what was allowed BEFORE Johnny Johnston’s rule change!

You can make it as complicated as you want.....

But right or wrong, when the little black vans pull up with their scope and dummy load,

They'll use a very simple equation....


Po=V(rms)/R


How they determine how you USUALLY operate your station...I have no clue.....

Dust on the knobs?

Well if the men in black use that equation, every ham will come out o.k.

But if the one of the men (or women) in black is a degreed engineer, I expect he (or she) will use:

P = E²/R.

Having dealt with the FCC before, I can tell you this:

Before they EVER come knocking on your door, they already know your effective Pout, off the front side of the antenna.  It doesn't take an idiot to see that it takes more than a 5 element beam to equate to a megawatt of ERP.  The FCC, contrary to what we all WANT to believe, doesn't hire idiots.

OTOH, 11 meter enforcement has picked WAY up, I've heard.  Three stations busted running high power this last month...  Very wide signals (running Johnson tube TXs, widebanded, etc).

The last station was allowed to keep his final PA.  They took his "driver" and his two export style radios. 

Yes, they have been to my house, no they didn't get any equipment.  Yes, I got into an argument about just the same kind of thing here:

1.  Legal output is 4 watts carrier at 100 percent modulation.

How can you have a 100 percent modulated carrier when your limited to 12 watts PEP?  That was a question that NONE of the people sitting in my radio room could answer...  And it really ticked off the local FCC Field Inspector, with a 15 yr old kid asking him a question he couldn't answer.


They came into my house with a termaline load.  I'm expecting since it was CB related, they didn't expect to find any ladder line or other such stuff.  The dusty old tram tube DSB transmitter did a whopping 4+ watts output.  He told me to retune the output network for 4 watts, as delivered into a dummy load.  I wasn't going to ask what happened when I hooked up a reactive antenna and the Pinput shot up.

Since I moved to the amateur radio category, no more neighbor problems, etc.  No more FCC at my door, either.

Shane

From Part 95 on Personal radio services -

95.410 (CB Rule 10) How much power may I use?
(a) Your CB station transmitter power output must not exceed the following values under any conditions:
AM (A3) – 4 watts (carrier power) SSB – 12 watts (peak envelope power)

(b) If you need more information about the power rule, see the technical rules in subpart E of part 95.


In subpart E:
95.639 Maximum transmitter power. ( I didn’t see any further ramifications for voice phone.)

In 95.667, there is a 10 W dissipation limit on the final amplifier components, using the manufacturer’s component data.
- - - - - - - - - - - - - - - - - - - - -

The rule seems quite explicit to me.  They should have been able to answer the question.  Maybe the field people are idiots.

The 12 Watt PEP rule applies to SSB mode, not AM.  In AM with a 4 watts carrier out, the PEP is 16 watts, which is fine.

The problem arises with AMC / ALC (and in the earlier rigs, the generation method of SSB / DSB)  in the CB radio rigs.  When adjusted (the cheaper radios, or the old OLD radios) for 12 watts PEP, they won't DO 16 watts PEP linearly.

When adjusted (drive / mic / AMC / ALC levels) for 16 watts PEP, they don't DO 12 watts PEP.

Compound that with radios that didn't have amc / alc (before the rules effected them), and you have real problems.

I had a DSB, limiterless radio at the time.  Talk about having to "ride the mic gain" to keep legal... hahaha.  It was designed and marketed around the time of 5 watts DC input.  It did that, on output.

Later in life, to get around the ambiguity, manufacturers tried all sorts of things, including different output networks for SSB and AM. 

Of course, I come from the school that to do it PEP, an amplifier must also do it in carrier power.  That blows away ICAS, though. 

And another question, then.  If a 4 watt carrier equates to 16 watts PEP, then how do you arrive at 6Kw for a 750 watt carrier?  Wouldn't it be more like 32 watts PEP?  UGGGH more confusion.

Shane

Well, it could be that all of my college professors, when I was getting my EE were wrong :-)  But, we did in fact use RMS and peak (and average) for various calculations.

The whole discussion about measuring power on AM is very interesting, and has been going on for many years.  An AM signal is, in fact, a composite signal comprised of 3 individual, distinct components.

If I operate a double sideband transmitter with a total output of 375 watts - 1/2 of this power in each sideband, and operate another transmitter putting out a 750 watt carrier, and assuming everything is in phase and properly combined, or is transmitted into two different antenna systems, where can you prove to me that there is 3000 watts PEP?

I guess if we really want to get technical about it, my spectrum analyzer displays voltage :-), and it will measure peak, quasi-peak, average and RMS depending on how I set it.  Nice for this sort of thing!

Regards,

Steve

I've got it covered: I'm going to mount my final amp right at the antenna feedpoint.

When the black van shows up, I'll just point to the top of an eighty-foot tree ....

Bill

well, yes, I skipped that didn't I....?
oh well,
no ones prefect......

Steve
et. al.

First, as a tongue-in-cheek comment... these analyses remind me of the analyses that "technical experts" put forward in legal proceedings... in which each side is trying to redefine whatever it is that needs to be proven... so that they can prove whatever it is they wish to prove.

An AM signal at the carrier frequency of 3.885 MHz has the form:

v(t) = A sin (3,885,000 x 2 x pi x t) x [ 1 + m(t)]

where m(t) is the modulating signal, and will be assumed to take on a peak positive value of 1... i.e., 100 positive peak modulation.

A is the amplitude of the unmodulated carrier

pi = 3.14159.....

t = time (seconds)

Furthermore, we will assume that m(t) has a bandwidth that is much, much less than 3.885 MHz.

If we observe v(t) on an oscilloscope, then it looks like a sine wave whose amplitude is slowly varying, and where the peak amplitude (for many, many cycles of the 3.885 MHz carrier) is 2A.

During those times when the amplitude of v(t) is close to 2A, the power associated with v(t) is 4 times (2 x 2) larger than when the amplitude of v(t) is "at a carrier" (i.e. =A)

So.... if we define peak envelope power as proportional to the maximum of the square of the amplitude of the slowly-varying (modulated) carrier... then we conclude that a 100% modulated carrier has a peak envelope power that is 4x the power of the unmodulated carrier.

Getting back to my comment about arguments that "technical experts" put forward in court rooms during litigation...

If you define peak envelope power in a totally different way... for example, the sum of the powers in the carrier and each of the two sidebands of an AM signal that is produced when a carrier is being AM modulated by a sine wave audio signal, then you get a different answer for what the peak envelope power is.

If you define peak envelope power in yet another way... e.g., the way that Steve is defining it... then you get yet another answer for what the peak envelope power of a 100% AM modulated (by a sine wave audio signal) carrier is.

The real question is: what objective (measurable/reproducible/unambiguous) definition does the FCC have in mind when they refer to the peak envelope power of a modulated carrier.

I believe that (for better or worse from the perspectives of those who wish it was something else), they meant the square of the amplitude of the waveform  v(t) = A sin (2 x pi x carrier frequency x t) x [1 + m(t)], where m(t) is a modulating signal whose bandwidth is much, much less than the carrier frequency, at a point in time where m(t) assumes its peak positive value.

Best regards
Stu

Indeed it is possible to have some sort of isolated dual-feed antenna and put 750 W PEP DSB into one port, and 750W carrier into another port, and the peak measurable transmitter power, measured as the FCC prescribes, will be 1500 watts RMS, and yet a 3000W pep dBd erp output will be produced (minus any minor antenna losses).  And it's perfectly legal - because if that's illegal, then hams with beams had better drop their TX output!  The only problem might be neighborhood EM limits - just as with a beam.

Somewhere, the FCC defined peak power as the maximum RMS RF waveform value at the peak of modulation.  Because modulation is very slow compared to the RF waveform, and the RF is relatively narrowband, the RF waveform is essentially a sine wave (except for Ultra-Wideband, which we don't use below UHF), and there are plenty of cycles of RF to measure during a modulation peak whose level does not measurably change during the measurement.  When we have a sine wave like that, it is a simple matter to determine the power, whether we report it as instantaneous peak, RMS, etc.  I think that the FCC has that one covered pretty well.

It is true that the instantaneous peak level of a sine wave is 2X the RMS level, so a 1500W RMS sine wave would have an instantaneous peak power of 3000W.  But that does not mean that the sine wave power is 3000W RMS; clearly, 1500 does not equal 3000.  The FCC spec refers to the RMS RF level at the modulation peak.

It's a shame, because in reality the peak permissible AM PEP power was unlimited before June of 1990.  The FCC stooped to impossible splatter limits to bust W3PHL way back when; 100W PEP would have failed their infinite suppression requirement too.  They might have limited us to 125% positive like broadcasters, or maybe 150% or 200% positive, but what about SSB or DSB suppressed carrier, they must have a million percent positive.  So the FCC smirkingly went with PEP, costing us (typically) 3dB or more.

A 3dB loss isn't much when a station has good audio and a good antenna.  Even a 6dB loss isn't deadly with the rock-crushing signals even a 500W transmitter can produce with a good antenna system.  What's a shame is that it was done at all, and that it was presented deceptively by the FCC - 'level playing field' jive, and speciously citing K1MAN's supreme court challenge as coming from all AMers - but jive from the FCC was nothing new, as PHL Fred knew.

Well, yes :-)  That was/is the whole point !! 8)

Ultimately, and I've been saying for many years that we need a footnote in the FCC rules regarding an alternate and sensible power measurement for standard double-sideband, full carrier, AM.  In no other places/industries where AM is used (that I know of) is the power measured in PEP.  Every other industry in which I've been involved measured AM power in terms of the CARRIER output (or power input as an alternative [indirect method]), and the precentage of modulation is also specified.

That's what we need here.

Regards,

Steve

Steve

I agree that the definition of what is allowed should be based on something that is easy to measure and unambiguous, and driven by some rational, underlying imperative.

As we discussed once on the air, perhaps a year or more ago... when it comes to regulatory agency rules or other legal matters, people will always try to "game" the system by finding a way to argue for an interpretation of the meaning of the rule that favors their desired outcome. Therefore, rules need to be a simple and easy-to-interpret as possible... even if they are not totally "fair".

In any event... one of the principal goals of the FCC at this point in time is to find ways to use the spectrum more efficiently.

Therefore, any attempt to cause (e.g., convince or force) the FCC to revisit the FCC's current rules on AM would carry along with it a great danger that the FCC will place tight bandwidth restraints on any AM signal above a modest power level. Thus, we might end up with a new set of rules for AM that we like a lot less than the current rules.

To illustrate the danger... the (hypothetical) new rules might say something like "any AM signal whose carrier level is above 5 watts must conform to a spectral mask that requires spectral components more than 4.5 kHz from the carrier frequency to be greater than 50 dB down from the carrier level (defined as the total power measured within any 100Hz bandwidth centered more than 4.55 kHz from the carrier frequency)".

A rule like the (hypothetical) rule, above, would be very tough to meet with a typical plate modulated transmitter... and would also make some Class E transmitter operators unhappy about having to put a brick wall filter on their audio.

Best regards
Stu

From Part 97 Definitions:

"(6) PEP (peak envelope power). The average power supplied to the
antenna transmission line by a transmitter during one RF cycle at the
crest of the modulation envelope taken under normal operating
conditions."

Now, on with the discussion  :D

Correct, Shane (and Congrats!  you're keeping up!   ;D )

There's no doubt that the current -legal limit- output power is 1.5kW "output".  That's Power Output or P_out, as measured at the antenna.

Don Chester says "running your AM rig without a Scope, is like driving your car at night without headlights". 

So, we've got our scope, we set the carrier envelope to two graticules, we modulate to 100% with a sine wave, and the audio peaks 'grow' to consume 4 graticules, at the same time the negative peak is at the baseline.  This ratio of 4:2 is the same as 2:1, and is considered 100% modulated.

Now, when we speak with our human asymmetrical voice and let's say our Symmetry Ratio (SR) = 4, and we have enough audio power in the modulator to faithfully reproduce our voice, then our SR of 2 (2:1) changes to an SR of 4 (or 4:1 ratio on the scope).

Now, let's put some numbers in that.  Let's say you're running 100w of carrier output.  If you're modulating it to 100% with a sine wave, the PEP output is 400wPEP.  This is with an SR of 2.  An SR of 3, increases your PEP output by twice as much (or 3db) or to a total of 800w PEP.  An SR of 4 means... what?  1.6kWPEP output.   I hit an SR of 4, and run 100w.  I would be considered to be 'breaking the law' were someone to show up at my antenna with an analyzer and check the output -at- the antenna.

This is not to say that there's a continuous sixteen hundred watts being kicked out, just only on the very extreme audio peaks, but this simply means that you have have ample enough audio power available in your modulator to -not- flat-top the audio imposed on the carrier.  90w and 1500w PEP output (thereabouts).  Hardly seems worth it, does it?  ;-)

Correct, Shane.. 750w of carrier is equal to 3000w PEP, at 100% modulation with a sine-wave.  And, with that figure, and considering that we're using a sine-wave, then the old story is true that in order to modulate a 1kW DC input carrier to 100% with a sine wave, you only need 500w of audio power.

We don't speak in sine-waves though.  Our speech patterns are complex wave-forms, and therefore we must consider how much power is required in our modulating stage to properly reproduce our voice characteristics without causing distortion or splatter, due to flat-topping or over-driving an audio stage.

If an audio stage is over driven at a low-level, the modulator is only going to do it's job, and amplify the distortion.
 

The average power over one cycle of a sine wave... hmmm.

Darn, I think that I have been expressing power wrong.  The idea of RMS was a way to determine that a given AC voltage was equivalent to a given DC voltage for the purpose of equal heating, equal brightness with light bulbs, equal temperature filaments, etc.  RMS, Root Mean Squared - you take the square root of the average of the square of the voltage as it varies over time, and then the RMS value is the square root of that average.  It is focussed on average power, but it is a voltage (or current) reading. Voltage, and current, are proportional to the square root of power, hence the 'root'.  So RMS is a way of expressing the voltage (or current) associated with an average power, and 100V RMS AC produces equal heat in a resistor as 100V DC.  When the square, average and square root process is applied to DC, then RMS = DC.  In square waves with no DC component, RMS = Vpeak to peak / 2.

RMS applies to volts and amps; average applies to watts.  Here the FCC wants to indicate the equivalent continuous power of a waveform that exists over a very short interval, and they mean not the instantaneous peak at the crest of the RF waveform (which would be 3000W for about zero time), but the average power over one full RF cycle, at the crest of modulation (which would be 1500W average over one RF cycle).  An RF sine wave with the characteristics of that single cycle - the same voltage, into the same load - would be a 1500W signal, and that is their PEP measurement standard.

I never realized that I was expressing power wrong.  The numbers were right - but I meant average, and I was saying RMS.

Are you saying that we can run 500w of carrier DC input to the final, modulate it to 100% with a sine-wave, for 1500w PEP out and then on asymmetrical peaks, let the positive peaks float to wherever they want?

PLEASE say you're saying that, Bob!  ;D

(I know you're not, but I could only hope)

The rules state no more than 100 percent modulation on am.

They don't talk about it being symmetrical or not, they just say no more than 100 percent modulation.

At least, what I was reading in reference to the above (4 watt / 12 watt blah blah).  I'm not sure about amateur regularions....  Is their an  actual text limit for mod percentage or index?

Shane

Part 19 has no "text limit" for modulation percentage or carrier level.

Show me where in Part 97 it says no more than 100% modulation on an Amateur Transmitter


Sorry, Shane... I'm pretty sure that's all we -do- talk about here, is Amateur stuff. 

 ;D

Er, Shane, no offense (many of us played with CB as kids or otherwise), but do you now operate AM phone on any HF amateur frequencies? Or are you an 11 meter op who has a Tech license? And if so, why not take the E-Z multiple choice General exam and join the fun in the AM windows on HF? Plenty of AM'ing activity on the W. Coast, and we can always use one more.

I think that a strict interpretation of the FCC rules here would apply to a classic AM signal this way: about 500W in, 100% modulation, about 375W carrier out, 1500W PEP.  Or about 250W in, 167W out, 200% positive mod, 1500W PEP out.

Nothing new, it's just that I forgot, or never realized, that power is not expressed in RMS, it is expressed as an average, or a peak, etc, but not as RMS.  AC voltage and current are expressed in RMS.

If you had a dual-isolated port antenna - and let's assume that it has the same pattern and gain of a half-wave dipole, or maybe even a full-wave CCD, and the pattern and phase are the same for both inputs - then you could put a 750 watt carrier into one port and 750W PEP double sideband suppressed carrier into the other port, and with the right phasing, you would produce a classic AM signal with the same signal as a 750W carrier and 3000W PEP. So, with this hypothetical dual-isolated-port antenna system, you could effectively run an extra 3dB of PEP on AM.  The real 1500 max combined watts of the DSB and the carrier would be measured, not the short 3,000 watt spike from the combined peak.  It's not magic, it's average power versus the short power peaks we get in AM.  

Looking at the numbers briefly, it looks as though you would get the maximum effective PEP output with equal carrier and DSB PEP with this for some reason.  So if you increase or decrease the carrier, you can't make 3000 W effective PEP without exceeding 1500W PEP to the antenna.  However, you could run 333 watts carrier output and 1167 W PEP DSB for 187% positive modulation and about 2747 watts of effective PEP.  But, that hypothetical antenna...   I think it is possible, but I am not aware of any out there.

There's always the "resistor-in-the-water-with-thermometer" method to determin true power....


  ""  The last station was allowed to keep his final PA.  They took his "driver" and his two export style radios.   "

This is odd in that the PA is part of the "illegal" opperation and usually goes byby.... FromWhatIHear, the FCC asks for the  'illegal' equipment to be given up voluntarily.

 Without looking at the caselaw, the  power cords, extension cords, antenna, mic., feedline, anything to do with the "illegal" event can be collected. If the FCC has to come back with a search warrant and the Fed Marshals, a 'good' write up of the warrant can have the FCC toss the whole place.....  klc

It's pretty simple when you use the proper terms and definitions vice making them up. Bud posted the correct definition of PEP. Not sure why people don't know this one.


My point - there is no such thing as RMS power. RMS voltage, sure, but not power. Further, deriving power (average, peak or otherwise) from measured voltage on anything other than a pure sinusoids will mostly likely be inaccurate. A calorimetric based device is usually required for this and I've never seen a spectrum analyzer of this type. I would imagine a good sampling spec an could calculate RMS voltages of nonsinusoids.

It seems like we have been down this road before. 100% modulation is the technical limitation of the AM carrier. You have positive peaks which can go as high as 130% or more and then there are the negative peaks which cannot go beyond 100% or you have splatter and interference to other users. The carrier is cut off, the modulation transformer sees and open circuit, etc.
The FCC limited the positive peaks for the fools in the AM broadcast industry to end the loudness wars of the 60's and 70's. AM Bcast is limited to 125% pos peaks.
There are no limitations for us ( I think)
Fred

That  would be a gooder way to do it Steve. The FCC however has specs for analyzing the broadcashers where the SA is set to a pacific bandwidth, the span is set at 50 Khz centered on the carrier freq and the trace is set for peak-hold. The scans are taken over a 10 minute period. This leaves a trace that you can now see the important stuff like peak output, bandwidth and percent of mod. Ifn I remember tomorrow I'll do up a sample on 1380AM here in Lorain and post it. It really is neat to see.

I was chief engineer of a broadcast station, in the early '70s - during the "loudness wars".  As a note of interest, I had a Collins 21E, and I made modifications to the transmitter to allow it to modulate to around 140-150% positive.  The modifications included a high level 3 diode negative peak limiter...

Well, we were the loudest game in town (with GOOD audio, too - not too much compression), banging the old General Radio modulation monitor off the scale.

Now, I had studied some older materials when I got my First Class FCC license, and I actually DID NOT KNOW that the positive modulation was restricted  :o   So, one day we had a routine inspection from the FCC, and as part of this, went to the transmitter site, and I started talking about the modulation, among other things.  Oops - what do you mean that's not allowed  :-[   Anyway, no citation, but I did have to IMMEDIATELY adjust the modulation.  The FCC guy was pretty cool, and had some very interesting stories.

Ahhh, the old loudness wars  :D

Oh, I *HATE* over compressed or processed audio - always did, even during the loudness wars.  Dynamic range is a good thing!

Regards,

Steve

Ahh yes.... the trusty calorimeter ! The ultimate calorie counter. We built one at work to measure 50KW at 100Mhz once. Water loads are neat !!

Talking with an FCC field tech yesterday, on something job related, I axed him how they measure PEP during inspections (where applicable).

Answer;  Peak Power Meter (http://www.bird-electronic.com/products/product.aspx?id=52)

"We put this inline at the transmitter. If it reads more than it should you got a problem."

So my advise, although the discussion is quite enjoyable and tutoral (if that's a word);

Ifn ur real worried about it go out and buy urself a Peak reading wattmeter (Bird 4300 or Coaxial Dynamics 83000). Adjust ur carrier and modulation as you like but don't let the meter go over the 1500 watt little mark on the meter.
Here's how I keep an eye on my customary 1499 watts PEP !

When I ran my double loop, my Viking II which I ran at 200mA plate current and saw 80 watts carrier out, seemed to be putting 120 watts forward and 40 watts reflected.  I figure that the antenna reflected about 1/3 of the applied power, so the reflected power was circulating in the feedline until the 80 watts got radiated, and that took 120 watts forward, and the 40 watts reflected was again reflected at the transmitter and added to the 80 watts to make 120 watts.

OK, but would the FCC accept, say, 2250 watts forward and 750 watts reflected?  Do they understand that this is 1500 watts output?

Some riceboxes transmit AM that approximates negative carrier control.  I believe it is due to the positive modulation peaks triggering the ALC. They sound like schzit!  With some plastic radios this phenomenon is unavoidable unless you feel like probing the innards of the SMT circuit boards, but why would anyone want to deliberately put a garbage signal out over the air?

And, regarding p.e.p., DILLIGAF?

(In case you're not sure what that means, click here (http://www.youtube.com/watch?v=WxmUKVrT0iI) for the definition!)

Here they are. Good reading !!

47CFR part 97 (http://www.access.gpo.gov/nara/cfr/waisidx_00/47cfr97_00.html)

... and for ballance, the broadcash specs....

Sec.  73.1570   Modulation levels: AM, FM, TV and Class A TV aural.

   (a) The percentage of modulation is to be maintained at as high a level as
   is consistent with good quality of transmission and good broadcast service,
   with maximum levels not to exceed the values specified in paragraph (b).
   Generally, the modulation should not be less than 85% on peaks of frequent
   recurrence, but where lower modulation levels may be required to avoid
   objectionable loudness or to maintain the dynamic range of the program
   material, the degree of modulation may be reduced to whatever level is
   necessary for this purpose, even though under such circumstances, the level
   may  be  substantially less than that which produces peaks of frequent
   recurrence at a level of 85%.

   (b) Maximum modulation levels must meet the following limitations:

   (1) AM stations. In no case shall the amplitude modulation of the carrier
   wave  exceed 100% on negative peaks of frequent recurrence, or 125% on
   positive peaks at any time.

   (i) AM stations transmitting stereophonic programs not exceed the AM maximum
   stereophonic transmission signal modulation specifications of stereophonic
   system in use.

   (ii) For AM stations transmitting telemetry signals for remote control or
   automatic transmission system operation, the amplitude of modulation of the
   carrier by the use of subaudible tones must not be higher than necessary to
   effect reliable and accurate data transmission and may not, in any case,
   exceed 6%.

   (2) FM stations. The total modulation must not exceed 100 percent on peaks
   of frequent reoccurrence referenced to 75 kHz deviation. However, stations
   providing  subsidiary  communications services using subcarriers under
   provisions of  Sec. 73.319 concurrently with the broadcasting of stereophonic or
   monophonic programs may increase the peak modulation deviation as follows:

   (i) The total peak modulation may be increased 0.5 percent for each 1.0
   percent subcarrier injection modulation.

   (ii) In no event may the modulation of the carrier exceed 110 percent (82.5
   kHz peak deviation).

   (3) TV and Class A TV stations. In no case shall the total modulation of the
   aural carrier exceed 100% on peaks of frequent recurrence, unless some other
   peak modulation level is specified in an instrument of authorization. For
   monophonic transmissions, 100% modulation is defined as +/−25 kHz.

   (c)  If  a  limiting  or compression amplifier is employed to maintain
   modulation levels, precaution must be taken so as not to substantially alter
   the dynamic characteristics of programs.

IIRC, there used to be a prohibition against modulation in excess of 100%, but it didn't say anything about negative or positive direction.  It was usually interpreted to mean excess of 100% in the negative direction.  That's how W3PHL got away with running 600 watts of carrier and several kilowatts of double sideband.  The FCC cited this case as one of their justifications for a p.e.p. rule.

But the prohibition of modulation in excess of 100%, along with the prohibition against running a modulated oscillator, were (inadvertently?) deleted when the rules were amended with the parts of Docket 20777 that were not denied by the Commission.  So actually, that docket ended up easing the rules with AM instead of eliminating the mode below 28 mHz, as Johnston had originally wanted.

But notice it refers to "angle modulated" signals.  That means phase and frequency modulation, where the phase angle varies from the reference carrier.  The purpose of that limitation is to keep the bandwidth of the FM or PM signal approximately within the same limits as conventional AM.

At one time, the definition of percentage of  modulation was defined as a function of the maximum vs minimum amplitude of the signal, compared to the amplitude of the unmodulated carrier.  I recall seeing the formula in a 1930's edition of RADIO.  I'll have to look it up.

As an example, if the unmodulated carrier amplitude is 1000 volts, and on the negative modulation peak the minimum amplitude is 500 volts instead of 0 volts, it would have been considered only 100% modulation if the peaks extended up to 2500 volts, since the maximum - minimum voltage would be 2000 volts, or twice the amplitude of the unmodulated carrier.