Asymmetrical modulation

AM Radio discussion. Directional arrays are FUN!
kcbooboo
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Re: Asymmetrical modulation

Post by kcbooboo » Thu Apr 03, 2014 5:08 am

The folding-over phenomenon that you see in modulators based on multiplier chips is strictly due to the way the math operates and the way we view the output on a scope. Many modern function generators come out with the same sort of result when AM is pushed past 100%. The only reason we don't see it on broadcast transmitters is that when you add the peaks and troughs of the modulation to the RF carrier, you can't go below zero RF output. The modulation would attempt to make the RF go below zero, and "fold over" like you see with the 1496 etc, but once the result goes to zero RF there's nothing left to fold over, although there could be some of the modulation energy trying to get through the tuned circuits. Perhaps that energy goes into sub-space.

There's a company in Greece that sells 30-60 watt pallet amplifiers that cover 0.54-2.0 MHz for under $100. One of them has about 30dB gain, so a milliwatt of input should produce a watt of output. One of these should provide plenty of input signal level for almost any mod monitor. I don't know how linear it is, or if it would affect the modulation indication. It would also be handy when checking out older phase monitors after repair that also need a fairly potent signal (2-10V) and an RF sig gen just doesn't cut it. Been there, done that.

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srcloutier
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Re: Asymmetrical modulation

Post by srcloutier » Thu Apr 03, 2014 6:17 am

Several comments....

Since part 15 is such a low power system, pulse width modulation would not gain you anything. You might save a few hundred milliwatts over analog, so why bother :D .

A properly designed analog series modulator is all that's necessary to be able to achieve modulation well in excess of 100% positive. The RF amplifier would need to be similarly designed, but this is not generally a problem with high level modulated designs (in other words, drain or collector modulation of a class C, D, E or F stage). Obviously, PWM is very advantageous when running high power due to the high efficiency of the modulator.

The most complicated part of a part 15 transmitter is the frequency source. The RF amplifier and modulator is relatively simple, and with a proper negative peak limiter incorporated into the modulator itself (this has certain advantages, one of which is that the N.P.L. is DC coupled to a DC coupled, series modulator), a design which will produce 200% positive modulation would be fairly straight-forward.

Whether you actually want to run it this way - well, that's another story, but at least the transmitter can have the capability of doing so and cleanly.

If you're interested, I could probably cough up a modulator/RF amplifier design that would do the job. You would need to provide a frequency source. I've built a number of part 15 transmitters. These all use a free running Hartley oscillator as the frequency source, but that's not the most stable oscillator design in the world, and a synthesizer or even a crystal would be much better for long-term operations where frequency drift might be a problem.

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Dale H. Cook
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Re: Asymmetrical modulation

Post by Dale H. Cook » Thu Apr 03, 2014 6:36 am

srcloutier wrote: ... pulse width modulation would not gain you anything.
That's not the point - I have no short-term interest in building a Part 15 AM station. I was simply curious about whether anyone has implemented a version of the Collins or Harris design at low power.
srcloutier wrote: ... that's not the most stable oscillator design in the world
If I were to build one I would only build it rock-bound or rock-locked as those are the transmitter oscillator designs that I've worked with for decades.
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srcloutier
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Re: Asymmetrical modulation

Post by srcloutier » Thu Apr 03, 2014 6:59 am

PWM is used in at least one modulated situation at low power that I definitely know of, and there are probably many others. In specific, I was asked to design a very efficient, 1.5mHz amplitude modulated 5 watt "transmitter" that drove ultrasonic transducers for a medical application. The device had to run off battery power, so efficiency was very important. I uses a PWM for that implementation and a class E RF amplifier. There's probably a lot of this sort of thing in medical devices.

On the amateur radio side, I have personally used PWM with transmitters running as low power as 100 watts. Below that, unless power efficiency is very important (such as with batteries or even mobile), the simplicity of analog is very attractive.

Once place where PWM is a *big* winner at any power level is in direct rectification of the AC line (no power transformer) implementations. The largest such modulator that I've personally built supplies modulated DC to a 450 watt class E RF amplifier, and up to 200% positive modulation (at 450 watts carrier). There are probably other similar implementations out there someplace.

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Deep Thought
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Re: Asymmetrical modulation

Post by Deep Thought » Thu Apr 03, 2014 8:38 am

srcloutier wrote:There are probably other similar implementations out there someplace.
BE's AM transmitters, for one (or three).
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

srcloutier
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Re: Asymmetrical modulation

Post by srcloutier » Thu Apr 03, 2014 10:05 am

Deep Thought wrote:
srcloutier wrote:There are probably other similar implementations out there someplace.
BE's AM transmitters, for one (or three).
That's neat - thanks! I didn't know they were using direct rectification of the line. Certainly cuts down on the weight and saves some cost!

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Re: Asymmetrical modulation

Post by PID_Stop » Thu Apr 03, 2014 11:51 am

Funny... a lot of Grass Valley switchers used MC1496 ICs in their video effects amplifiers to perform dissolves and linear keying. I bet we still have spares in our inventory.

Heck, we still have spares for our original RCA TRT-1B videotape machines... :shock:

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kcbooboo
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Re: Asymmetrical modulation

Post by kcbooboo » Thu Apr 03, 2014 1:43 pm

Here's a neat applet that shows the spectrum and scope waveforms for different levels of amplitude modulation. It runs Java.

http://www.home.agilent.com/upload/cmc_ ... plitudemod

Bob M.

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Shane
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Re: Asymmetrical modulation

Post by Shane » Sun Apr 06, 2014 3:41 pm

srcloutier wrote:
Deep Thought wrote:
srcloutier wrote:There are probably other similar implementations out there someplace.
BE's AM transmitters, for one (or three).
That's neat - thanks! I didn't know they were using direct rectification of the line. Certainly cuts down on the weight and saves some cost!
BE claims the AM1A meets audio specs down to 5 watts output. I can speak to outputs down to 50. When we go from 1000w to 54, if I am within range of AGC saturation I have to call the remote control to verify we actually did cut power because there is zero detectable difference in the audio.

When we were spec-ing 1kw rigs in 2003, other mfgs. recommended their rigs not be used below 100w.

Sweet rigs, the BEs, and nice to learn another piece of info about them in an unexpected forum for that to come up.
Mike Shane, CBRE
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Re: Asymmetrical modulation

Post by loveofradio » Fri Nov 08, 2019 3:25 pm

Dan here. I have enjoyed looking over the post on this topic. For some time now I have been looking at circuits that produce asymmetrical modulation using LTSpice software for designing and analyzing those circuits. I want to briefly comment that I believe in the future, if someone has not already done it that SDR software for two way communications might offer asymmetrical modulation as one of the modes.

Now as for my circuits, I have found several methods for producing asymmetrical modulation in hardware terms. I first began with using a circuit for a device called the Mauldulator submitted for me to model by the designer David Dowler an engineer, who designed the circuit based upon the concepts of his friend John Bartal both of which are amateur radio friends.

Since then I have realize a few other methods for producing asymmetrical modulation with much simpler circuits. However all of the circuits are what I would call elegant in design. In my involvement with these circuits I discovered that they produce high levels of harmonic distortion, so I set out to find methods of dealing with the total harmonic distortion, mostly for broadcast quality AM.

Crossover distortion along the zero axis between the upper and lower wave cycles is the reason for the harmonic distortion, however I discovered that if you can move the crossover distortion away from the zero axis and rotate its it around the wave cycle to another angle then it is easier to filter with filtering. This might sound complicated however it is not and the circuit is simple. I also had to come up with a different filter concepts so I began to look at Op Amp noise suppression filters which helped when used with low pass audio filters. The most useful circuit starts out with higher distortion than the Mauldulator circuit but is easier to filter and produces the lowest distortion after filtering, so that was unusual when I analyzed the end product at the end of the Op Amp circuit chain involved in the design of these modulation circuits.

I want to review all of my LTSpice models and offer them for use for analysis of the circuit concepts. I believe these are the only Spice models to date since I have not found any circuits on this subject in the public domain.

You can view a demonstration of the Mauldulator in the below video by John Bartal.

[youtube]https://www.youtube.com/watch?v=gZDCyDNBG5k[/youtube]

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Deep Thought
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Re: Asymmetrical modulation

Post by Deep Thought » Mon Nov 11, 2019 12:47 am

6+ year old zombie thread walks like zombie. ;)

I still stand by my (and many other's) assertion that all you add with that much asymmetrical modulation is distortion in the receiver. There is no way to avoid it and all that even order THD and intermodulation distortion just muddies up the audio, which is the last thing you want on AM.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

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Dale H. Cook
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Re: Asymmetrical modulation

Post by Dale H. Cook » Mon Nov 11, 2019 7:34 am

Deep Thought wrote:
Mon Nov 11, 2019 12:47 am
6+ year old zombie thread walks like zombie. ;)
I was going to make a similar comment, but Deep beat me to it.
Deep Thought wrote:
Mon Nov 11, 2019 12:47 am
I still stand by my (and many other's) assertion that all you add with that much asymmetrical modulation is distortion in the receiver. There is no way to avoid it and all that even order THD and intermodulation distortion just muddies up the audio, which is the last thing you want on AM.
I will second that. There is also a significant objection to the phase rotators or phase flippers used in some older AM processors (the Inovonics MAP-II and Texar Audio Prism come to mind). A minority of people have voices with significant asymmetry that is of the opposite polarity to the asymmetry of most voices and programming. If a member of that minority is on the air and is listening to an off-air signal on headphones, when they begin to speak the rotator or flipper will invert the audio, and hearing that in the headphones can be very disconcerting. I always defeated the rotators/flippers to prevent that.
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rfn
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Re: Asymmetrical modulation

Post by rfn » Mon Nov 11, 2019 10:00 am

And there is why I carry a phase-shifting XLR/XLR when doing work for stations using asymmetric modulation.

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Deep Thought
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Re: Asymmetrical modulation

Post by Deep Thought » Mon Nov 11, 2019 8:19 pm

rfn wrote:
Mon Nov 11, 2019 10:00 am
And there is why I carry a phase-shifting XLR/XLR when doing work for stations using asymmetric modulation.
All that does is invert the audio. It does not address the underlying issue.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

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Dale H. Cook
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Re: Asymmetrical modulation

Post by Dale H. Cook » Mon Nov 11, 2019 10:05 pm

I will second Deep. Anything that increases asymmetry increases distortion.
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