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steve_qix
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« Reply #25 on: September 17, 2007, 08:01:35 AM »

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.


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

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

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« Reply #26 on: September 17, 2007, 08:52:24 AM »

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

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WD8BIL
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« Reply #27 on: September 17, 2007, 11:33:35 AM »

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  Cheesy
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w5omr
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« Reply #28 on: September 17, 2007, 03:08:26 PM »

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.

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

There's no doubt that the current -legal limit- output power is 1.5kW "output".  That's Power Output or Pout, 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.
 
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Bacon, WA3WDR
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« Reply #29 on: September 17, 2007, 04:41:08 PM »

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."
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.
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w5omr
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« Reply #30 on: September 17, 2007, 05:11:45 PM »

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.

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!  Grin

(I know you're not, but I could only hope)
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« Reply #31 on: September 17, 2007, 06:43:13 PM »

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.

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!  Grin

(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
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« Reply #32 on: September 17, 2007, 07:05:58 PM »


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.
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« Reply #33 on: September 17, 2007, 07:44:09 PM »

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

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

Quote
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?

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

 Grin
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W1GFH
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« Reply #34 on: September 17, 2007, 07:56:16 PM »


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


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.
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Bacon, WA3WDR
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« Reply #35 on: September 17, 2007, 08:17:05 PM »

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.
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« Reply #36 on: September 17, 2007, 08:43:35 PM »

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
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Steve - WB3HUZ
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« Reply #37 on: September 17, 2007, 09:10:14 PM »

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.

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

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.
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flintstone mop
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« Reply #38 on: September 17, 2007, 09:42:25 PM »

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)
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« Reply #39 on: September 17, 2007, 11:49:17 PM »

Quote
I would imagine a good sampling spec an could calculate RMS voltages of nonsinusoids.

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.
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steve_qix
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« Reply #40 on: September 18, 2007, 05:41:34 PM »

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  Shocked   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  Embarrassed   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  Cheesy

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

Regards,

Steve
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« Reply #41 on: September 19, 2007, 08:32:16 AM »

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

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

"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 !


* Monitoring Stuff.jpg (16.35 KB, 196x252 - viewed 641 times.)
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Bacon, WA3WDR
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« Reply #42 on: September 19, 2007, 10:47:22 PM »

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?
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« Reply #43 on: September 21, 2007, 12:20:44 AM »

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 for the definition!)
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« Reply #44 on: September 21, 2007, 08:30:13 AM »

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


Here they are. Good reading !!

47CFR part 97
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« Reply #45 on: September 21, 2007, 10:30:34 AM »

... 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.
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k4kyv
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« Reply #46 on: September 21, 2007, 12:37:33 PM »

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

Here they are. Good reading !!
47CFR part 97

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.
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« Reply #47 on: September 21, 2007, 02:16:21 PM »

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.
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