Modulation question

[DMOD]

If one were to design a Class B P-P modulator circuit for a plate modulated 1kW Class C final, and assuming an efficiency of 70% for the 800 Watt P-P modulator, the DC input power to the Class B P-P circuit would have to be 800 Watts/0.7  = 1,143 Watts.

To add to this, assume a Class C stage efficiency of 90% for the 1kW plate modulated final, the DC input power to this stage would have to be:

1,111 Watts.

A well designed (stiff) DC power supply for continuous 125% modulation would require a total DC input power of:

DCc + DCm = 1,111 Watts + 1,143 Watts = 2,254 Watts.

This does not include filament power or control power.

[steve_qix]

300 percent is 10x I think.

Maybe we need a chart!  Haha

Pos.      Mult
Mod      Factor
100% = 4x - Normal
150% = 6.25x - Reasonably common male voice natural asymmetry reproduced through a high quality system.  A minimum design/implementation standard IMHO.
200% = 9x - Artifical enhancement required to actually modulate at this level (not recommended); generally implies waveform modification (AKA, clipping) for normal program material.  Good headroom.  My personal standard for AM transmitter design (lots of headroom).
250% = 12.25x - Eh?
300% = 16x - Are you sure you're not using carrier suppression of some sort??  RX better have a sync detector!

[VE3AJM]

I'm certain for those running 150% etc. natural positive modulation that you're also reducing your carrier power so that you're not exceeding 1500w PEP output on AM when you're on the air. That would be 240 watts of carrier output.  

Al VE3AJM

[steve_qix]

Well fortunately, AM signals are made up of 3 distinct components - carrier, upper sideband and lower sideband.  A vacuum tube 1kW power input transmitter, putting out - say - 750 watts of carrier, 100% modulated will show, on a spectrum analyzer, the carrier power of 750w, and each sideband will show a peak (not average) power of 375 watts.

Ok, now we have a field day operation with 3 transmitters, each operating at - say - 1500 watts PEP, on the same band (ok, it's an example) - when viewed on a spectrum analyzer will show the 3 distinct signals of 1500w PEP, and most rational people would say this is a legal setup.  Are they actually running 1500W PEP?  If, somehow, we combined similar signals into a single feedpoint, and amplified them with a single linear amplifier, and fed this to an antenna system so that all of the signals were present at the same time on a single feedline, would one say that we now have 4500 watts PEP?

If I have a 750 watt carrier transmitter, and a 750 watt PEP double-sideband transmitter phase locked to the carrier generator, and I generate the identical signal as in the first example, how would this be measured?  Is it 750 watts?  How does this differ from the field day operation?  If we suppose that the 3 1500W (PEP) field day transmitters operating at the same time are legal, then by the same measure, I'm only running 750 watts!!

Anyway, you can see what I'm getting at...and the slope is very slippery 

[VE3AJM]

However you want to measure your PEP power output, instantaneous or average peak power on AM, is up to you of course. For me, the more AM stations out there with good signals, the better.

Whether your FCC is in agreement with your method of measurement and conclusions you've written about here, may be an issue for them if there were any enforcements of the rules with regard to AM power. Slippery slope and one foot on a banana peel deal

Al VE3AJM

[KM1H]

So, if Im using a CE-100V or any other rig that allows selection of DSB OR either sideband AM what is my PEP in both conditions assuming the carrier is the same at 25W? 100W on DSB and 50W with one SB......correct?

Since the receiver (most) only demodulates one SB it will sound the same if the listener has zerobeated the carrier and doesnt tune around.....correct again?

Now, if I reduce the carrier a bit but it remains constant, and I increase the mike gain, I can increase my modulation index to say 125% or whatever I want within reason and boost the perceived audio in the receiver as long as I dont exceed its detection threshold. Or I can boost the carrier and audio to 100W PEP and still stay in the rigs spec.    Am I still on the right track?

With slightly reduced carrier, the overall efficiency is better meaning less wasted heat and the slightly elevated single SB has less total power than both at 100% further reducing waste.  What is wrong with this scenario?

Since I dont have a Class AB or B modulator negative-positive cycle to worry about do I just have to clip the low level audio when exceeding 100%? The 100V has a clipper built in followed with a rather elaborate set of filters that limit the audio to 4KHz as well as substantially reducing higher distortion products.

Are there any other rigs with that flexibility? The 200V eliminated the switchable SB's on AM as part of their cost reduction. Other phasers from CE, Hallicrafters, Lakeshore, etc, can select a SB in SSB mode and then inject carrier which is a minor inconvenience while the 100V has a seperate carrier control for AM behind a front panel door.

With 20-30W of AM carrier available driving a big amp is no problem. It can really generate serious RF from an Alpha 77SX (a pair of 8877's) Ive been repairing and testing with the TS-950SD on AM 

When driving a linear with only one SB on AM can the amps carrier output be increased a bit wiithout running into its distortion threshold? IOW since it will be driven at 50W PEP instead of 100W PEP with DSB can I diddle with carrier and audio controls to boost the PEP another 3dB?  I dont have the 100V ready to go yet so no scope or SA tests have been run.

Carl

[W2VW]

Why would anyone want to run only one sideband and carrier?

It takes away a giant advantage on the air.

Also cant be demodulated by simple means without plenty of unwanted distortion.

See KWS-1.

[KM1H]

Dont know as Ive not tried it. And Id of thought that a single SB with a 5-8kHz BW would be more desirable than DSB in the same space.

But some Googling answered some of my questions primarily in a 2006 post by our own K4KYV; thanks Don.

OTOH I wouda thunk that a simple plug in or wire in SS adapter for the 6H6/6AL5/1N34 would have been developed by now and corrected the high distortion.

Do all conventional AM detectors going back to the 20's suffer the same distortion with SSBAM? All I see mentioned is the half wave diode.

For weak AM that isnt FMing I find myself using the BFO or product detector anyway and leave it on when a strong station enters the conversation. Of course drift, FMing, and the inability of some to zerobeat doesnt help

Carl

[k4kyv]

Well fortunately, AM signals are made up of 3 distinct components - carrier, upper sideband and lower sideband.  A vacuum tube 1kW power input transmitter, putting out - say - 750 watts of carrier, 100% modulated will show, on a spectrum analyzer, the carrier power of 750w, and each sideband will show a peak (not average) power of 375 watts.

Ok, now we have a field day operation with 3 transmitters, each operating at - say - 1500 watts PEP, on the same band (ok, it's an example) - when viewed on a spectrum analyzer will show the 3 distinct signals of 1500w PEP, and most rational people would say this is a legal setup.  Are they actually running 1500W PEP?  If, somehow, we combined similar signals into a single feedpoint, and amplified them with a single linear amplifier, and fed this to an antenna system so that all of the signals were present at the same time on a single feedline, would one say that we now have 4500 watts PEP?

If I have a 750 watt carrier transmitter, and a 750 watt PEP double-sideband transmitter phase locked to the carrier generator, and I generate the identical signal as in the first example, how would this be measured?  Is it 750 watts?  How does this differ from the field day operation?  If we suppose that the 3 1500W (PEP) field day transmitters operating at the same time are legal, then by the same measure, I'm only running 750 watts!!

Anyway, you can see what I'm getting at...and the slope is very slippery 

Steve,

I'm glad to see you have come  round to my way of thinking on this issue.  Remember a thread about a year ago, when you took exception to my posting that was essentially an identical argument, and you posted a couple of diagrams that analysed an AM signal both in the time domain and frequency domain? The time domain, as would be viewed using the envelope pattern of a wide-band oscilloscope, clearly demonstrated the vectorial sum of USB, LSB and carrier, which indeed indicated the 4X power at 100% modulation.  But the frequency domain, as displayed on a spectrum analyser, showed just as clearly the carrier with distinct upper and lower sidebands each peaking at an amplitude far below that of the carrier.

And who says all the transmitters even have to be on one band. During field day, each transmitter operating on the same band would most likely still be modulated by different audio signals. But what about W1AW, when they simulcast on SSB on several bands at the same time?  If you had an oscilloscope connected to a non-resonant pick-up, or a simple untuned field strength meter, wouldn't the indicated peak amplitude of the sum be far above that of any one of the transmitters?

The 4X argument assumes a carrier that actually varies up and down in step with the audio doing the modulation. This theory was disproved sometime in the 1920s. As we have long known, there is, instead, a steady unvarying carrier,  with two mirror-image sidebands, emitted independently above and below the carrier frequency.

Regarding the audio power required to plate modulate a carrier, with a sine wave tone the audio power is 1/2 the DC input to the final stage. I have seen estimates that even a very good modulation transformer is only about 90% efficient, so the modulator tubes need to deliver slightly more than 50% of the DC input.  With voice modulation, the PEAK audio power required to modulate 100% is 50% DC input, so the average power with voice modulation will be more on the order of 25%.  The exact figure depends on  the characteristics of the voice doing the modulation.  Because of the high peak to average characteristic of the typical  human voice, the modulator tubes may be rated at substantially less plate dissipation than what would be required for sine wave tone modulation, but the DC plate voltages and modulation transformer turns ratio must still be such that the modulator is capable of delivering the required power albeit over a short duty cycle. Just don't whistle into the mic or run a sine wave tone generator for more than a half second or so.

The audio power level increases as the square of the percentage of modulation. At 141% positive peak modulation, the required audio power is the same as the DC input. At 200%, the audio power is twice the DC input power. The 1950s era ARRL handbooks have a section on asymmetrical modulation, and show a drawing of what 300% modulation would look like on a scope. That level would require 9 times the power for 100% modulation, or 4.5 times the DC input power. I doubt if anyone's natural voice asymmetry reaches 300%; now we are talking about DSB reduced carrier, like what W3PHL used to run.

Don k4kyv

[R. Fry SWL]

...The time domain, as would be viewed using the envelope pattern of a wide-band oscilloscope, clearly demonstrated the vectorial sum of USB, LSB and carrier, which indeed indicated the 4X power at 100% modulation.  But the frequency domain, as displayed on a spectrum analyser, showed just as clearly the carrier with distinct upper and lower sidebands each peaking at an amplitude far below that of the carrier.  ...

And such displays/descriptions have no conflicts with modulation theory.  They exist simultaneously when sampled by such respective instrumentation.

[W2VW]

And Part 97 does not seem to proscribe time domain measurement method.

[W2VW]

Maybe exalted carrier detection would work but those things were forgotten long ago.

Otherwise a simple detector isn't going to make reception much fun. Better off with that other phone mode.

[VE3AJM]

Have there been any known or recorded cases of any enforcements of this by the FCC on an AM station. If so, do we know which method was used/applied by the FCC? They may just use a Bird peak reading wattmeter for all we know. That wouldn't work all that well if the station was operating a link coupled output transmitter and OWL all the way to the antenna feedpoint.

Al VE3AJM

[W2VW]

Have there been any known or recorded cases of any enforcements of this by the FCC on an AM station. If so, do we know which method was used/applied by the FCC?

Al VE3AJM

Same number as DOC : )

[VE3AJM]

  OK FIINE!

I do recall that an Industry Canada inspector shut down AAM back about 10 years ago. 5kw output got their attention.

Al VE3AJM

[AB2EZ]

Of all of the points that have been made about PEP measurement.... I personally find this one the most interesting.

Steve posted:

"Ok, now we have a field day operation with 3 transmitters, each operating at - say - 1500 watts PEP, on the same band (ok, it's an example) - when viewed on a spectrum analyzer will show the 3 distinct signals of 1500w PEP, and most rational people would say this is a legal setup.  Are they actually running 1500W PEP?  If, somehow, we combined similar signals into a single feedpoint, and amplified them with a single linear amplifier, and fed this to an antenna system so that all of the signals were present at the same time on a single feedline, would one say that we now have 4500 watts PEP?"

In fact, if you measured the time-varying envelope power... which is the average, over 1 rf cycle, of the instantaneous power of the composite signal formed by the sum of the three signals that Steve described... then the peak value of the envelope power would be 9 x 1500 watts = 13500 watts. This result follows from the fact that since the 3 distinct signals are separated in frequency by only a very small percentage of the average of the three carrier frequencies... there will always be instants in time (that last many RF cycles) when the three signals add approximately in phase. During those instants in time when the three signals add approximately in phase, the combined RF voltage will be the sum of the three individual RF voltages (i.e. 3 x the individual RF voltages)... and the combined envelope power (whch is proportional to the square of the RF voltage) will be 9x the envelope power of one signal considered by itself.

However, there will also be instants in time (lasting many RF cycles) when (for example) two of the three signals are 180 degrees out of phase... and their RF voltages will cancel. At those instants of time, the envelope power of the composite signal will be equal to the peak envelope power of just one of the signals.

Therefore, at some instants of time, the envelope power of the combined signal would be 9x the envelope power of one signal. At other instants of time, the envelope power of the combined signal would be 1x the envelope power of one signal.

Averaged over a large enough number of RF cycles, the individual RF powers would add... but that is not the engineeriing definition of the envelope power of the composite signal.

This phenomenon is the same as what happens if two tuning forks at approximately the same frequency are vibrating simultanously in approximately the same location. You will hear, in each ear, the sum of the sound pressure waves from both tuning forks. You will also hear, in each ear, a "beat note" at a frequency equal to the difference of the two tuning fork frequencies. The beat note results from the pressure waves, falling on each of your ears, coming into and out of phase. The time to make one complete cycle of: in phase- out of phase- in phase is: 1/(the difference of the two frequencies).

Assuming the two individual acoustic signals are appproximately equal in amplitude... then at instants in time when the two audio signals are in phase, the associated instantaneous acoustic power falling on each of your ears is 4 x the instantaneous acoustic power of either of the two audio signals. When they are out of phase, the instantaneous acoustic power falling on each of your ears is approximately zero. The time averaged [i.e. averaged over an interval of time that is equal to 1/(the beat frequency)] acoustic power falling on each of your ears is the sum of the individual time averaged acoustic powers.  

As an aside, spectrum analyzers generally do not display peak envelope power... They display the average power in a narrow band of frequencies... averaged over a period of time that is approximately 1/(the width of that narrow band of frequencies). For example, if the resolution bandwidth of the spectrum analyzer is 100Hz... then what is displayed is the average power in that 100 Hz band of frequencies... averaged over a time interval of at least 10 milliseconds.

I don't know how the FCC would measure compliance... but my guess is that they would interpret the rules by measuring the RF envelope of the signal with an oscilloscope... observing the peak positive modulation index (k)... and defining the the peak envelope power as: the unmodulated carrier power x (1+k) x (1+k). This interpretation may not be "fair" or "reasonable" in some people's opinions... but there are lots of legally enforceable rules that are not viewed as fair or reasonable by various people who may be subject to those rules.

Stu

[W1ATR]

300 percent is 10x I think.

Maybe we need a chart!  Haha

Pos.      Mult
Mod      Factor
100% = 4x - Normal
150% = 6.25x - Reasonably common male voice natural asymmetry reproduced through a high quality system.  A minimum design/implementation standard IMHO.
200% = 9x - Artifical enhancement required to actually modulate at this level (not recommended); generally implies waveform modification (AKA, clipping) for normal program material.  Good headroom.  My personal standard for AM transmitter design (lots of headroom).
250% = 12.25x - Eh?
300% = 16x - Are you sure you're not using carrier suppression of some sort??  RX better have a sync detector!

350% = 20x - They call me missa wurl wide audio drive!
400% = 25x - Creates tear in space/time and station gets crushed inside black hole. That's one way to clean the shack i guess. 

[steve_qix]

It sure is great to have Stu in here!  Thank you!

Stu gave more support relative to the "AM Problem".  AM is, in fact, 3 distinct signals - which, just like the field day operation - if everything were combined into a single electrical path, would, under some circumstances (in phase condition) add up to more than is effectively there.

So, just like the field day operation - I could choose to operate 3 transmitters - a 1500 watt carrier output transmitter, and 2  750 watt PEP single sideband transmitters - one per sideband.  The sideband transmitters are phase locked to the carrier.  I run 3 coax runs up to my combiner which is located at the top of the tower.  The combiner output is connected to the antenna.  Peak reading wattmeters inserted into any of the individual coax lines show a maximum power at 1500 watts (the carrier transmitter).  The others show 750w PEP, and like the field day operation - all is perfectly legal.

Which points to the obvious conclusion that measuring the 3 distinct pieces of an AM signal, combined on a single electrical path (a coax cable or other output) is only valid when measuring average power.  A PEP measurement of the combined signals is not valid.

[AB2EZ]

Steve

Minor comment (for clarification):

RF signals at different frequencies cannot be phase locked. They can, however, be forced to maintain a fixed frequency separation. If the frequency separation is fixed at B Hz, then they will move in and out of phase and back in phase again every (1/B) seconds.

When they are in phase, their sinousoidal voltages will add to produce (A+B) volts. When they are out of phase, their voltages will add to produce A-B volts... where A is the amplitude of one of the sine waves, and B is the amplitude of the other sine wave.

The composite envelope power (i.e. the instantaneous power averaged over 1 RF cycle) will peak at (A+B) x (A+B) / 2R... where R is the associated resistance between the two points across which the voltage is being measured. The minimum instantaneous envelope power will be (A-B) x (A-B) / 2R. The time averaged power in the composite signal will be [ A x A / 2R] + [ B x B / 2R]

Stu

[AB2EZ]

Don

I very respectfully disagree with the following posted comments:

"The 4X argument assumes a carrier that actually varies up and down in step with the audio doing the modulation. This theory was disproved sometime in the 1920s. As we have long known, there is, instead, a steady unvarying carrier,  with two mirror-image sidebands, emitted independently above and below the carrier frequency.

Regarding the audio power required to plate modulate a carrier, with a sine wave tone the audio power is 1/2 the DC input to the final stage. I have seen estimates that even a very good modulation transformer is only about 90% efficient, so the modulator tubes need to deliver slightly more than 50% of the DC input.  With voice modulation, the PEAK audio power required to modulate 100% is 50% DC input, so the average power with voice modulation will be more on the order of 25%.  The exact figure depends on  the characteristics of the voice doing the modulation. "

The instantaneous rf power in an RF signal is proportional to the square of the amplitude of the signal [e.g. instantaneous power = v(t) x v(t) / R)... where v(t) is the instantaneous voltage, and R is the resistance between the two points across which the voltage is being measured].
The envelope power of an RF signal is defined (in engineering terms) as the average, over one RF cycle, of the instantaneous power in the RF signal, and is equal to 0.5 x the peak of the instantaneous power in that same rf cycle.  If the positive peak amplitude modulation index is k, then the peak amplitude of the RF signal is (1+k) x the amplitude of the RF signal at carrier (i.e. in the absence of modulatiion). The peak envelope power of the modulated signal is (1+k) x (1+k) x the envelope power at carrier. If k=1 (i.e. 100%), then the peak envelope power is 4 x the envelope power at carrier.

We apparently disagree here (that's what make horse races)... but I will reiterate my result from above. The peak audio output power (on positive voice peaks) that the modulator must deliver (not including losses in the modulation transformer, etc) to produce a peak positive modulation index of k (e.g. k=1.25 => 125% positive peaks) = k x k x the DC input power of the modulated rf stage.

Since voice signals reach their positive peaks only a small percentage of the time, the average audio power that the modulator will have to deliver to its load will be much less than half of the peak audio power it must deliver.
 
Of note, modern audio amplifiers are rated in terms of the peak audio output power they can deliver to a specified load... even though they cannot deliver anywhere near that much peak audio power if the audio waveform is a sine wave. [With a sine wave of that level, the amplifier would be be producing too much average power... and it would overheat/shut itself off/destroy itself]

Classical audio amplifiers were generally rated in terms of the average audio output power they could deliver to a load when the waveform was a sine wave at 1kHz. When the audio waveform is a sine wave, the peak audio power delivered to the load is 2x the average audio power.

In any event, when designing a modulator, it is (in my opinion) better to think in terms of the peak audio output power (on positive voice modulation peaks) that the modulator must deliver.

Stu

[steve_qix]

Steve

Minor comment (for clarification):

RF signals at different frequencies cannot be phase locked. They can, however, be forced to maintain a fixed frequency separation. If the frequency separation is fixed at B Hz, then they will move in and out of phase and back in phase again every (1/B) seconds.

Yes, good clarification!  I should have said the sideband transmitters are derived from the carrier signal, but of course they are on their own frequencies, so cannot [by definition] be phase locked.

This is a good discussion, even if it is a deviation from the original thread 

[Steve - K4HX]

Excellent point. And this is where so many people go wrong in their frequency domain based "analysis" of PEP.

As an aside, spectrum analyzers generally do not display peak envelope power... They display the average power in a narrow band of frequencies... averaged over a period of time that is approximately 1/(the width of that narrow band of frequencies). For example, if the resolution bandwidth of the spectrum analyzer is 100Hz... then what is displayed is the average power in that 100 Hz band of frequencies... averaged over a time interval of at least 10 milliseconds.

I don't know how the FCC would measure compliance... but my guess is that they would interpret the rules by measuring the RF envelope of the signal with an oscilloscope... observing the peak positive modulation index (k)... and defining the the peak envelope power as: the unmodulated carrier power x (1+k) x (1+k). This interpretation may not be "fair" or "reasonable" in some people's opinions... but there are lots of legally enforceable rules that are not viewed as fair or reasonable by various people who may be subject to those rules.

Stu

[W4RFM]

and to think all I was looking for is how big a mod transformer to use. 

This has turned out to be a very good discussion.

[KA2DZT]

The composite envelope power will peak at (A+B)/R x (A+B)/R... where R is the associated resistance between the two points across which the voltage is being measured. The minimum instantaneous envelope power will be (A-B) x (A-B)/R. The time averaged power in the composite signal will be [AxA/R] + [BxB/R]

Stu

Stu,

Is that an error in the math,  (A+B)/R x (A+B)/R ??  I also see you corrected it later in the post.  Or maybe I'm wrong.

Fred

[W2VW]

there are lots of legally enforceable rules that are not viewed as fair or reasonable by various people who may be subject to those rules.

Stu

Grinning and thinking of how much fun it must have been for you to hear about that every morning even with the receive antenna disconnected.

Almost as much fun as I have with tourists at work AND at home in summer...