A legal limit am transmitter for 75/160 meters from WO1U

[redhat]

I found this ages ago when researching what others were doing in the realm of class D on HF.  A google search for '75 meter am transmitter paper pdf' will get you the original from the AM Phone message board.

This has some neat practical information for those of you who want to experiment with high power levels and high level modulation.  His prototype will make 1500W PEP on 75 or 160 Meters (3.8 and 1.8 MHz respectively) at over 90% efficiency.  It also covers some of the pitfalls of this method.

This is not a call to action, but some practical info from someone who has done it.  With the drive system and fets in use, I doubt this would survive an excursion to 43 meters, but the same principles apply.

http://amfone.net/Amforum/index.php?action=dlattach;topic=30798.0;attach=31360

+-RH

[Stretchyman]

Great RF design with a WTF modulation scheme.

Why on earth design a decent RF generator only to modulate it with the most outdated of methods?

Yes saw this ages ago and commented on AMFONE (that's where they all hang out!)

Maybe the author has never heard of PWM?

There's people on the same site still advocating the use of HEISING modulation, FFS it makes me want to bang my head against the wall (actually I'd rather bang there's!)

Anyway people, please don't bother with mod trannies for any Voltage over 12V as there is a better way.....

Oh Yes.

 

[redhat]

Number one; your dealing with a crowd that values simplicity.  I agree, PWM is the way to go, but there is something to be said for simplicity.  My back says otherwise  

Number two; I was reminded of this when Antenna was mentioning all the trouble his friend had with modulating that kit rig, and some of the same problems I think this guy who wrote the paper discovered as well.

In this day and age, people are still building grenade clones, not because of their 'superior design and quality'...quite the opposite.  There is a lot to be said for something that can be made and repaired from commonly available parts, and that is where the desire to build these things comes from.  Its just another way to approach the problem.

*edit* upon further reflection, it could also be that the preference toward high level transformer modulation relates to nostalgia.  To a lot of the older guys, unless it 'feels' solid with lots of iron, like big cars, it just doesn't feel right.  Heavy iron may be reminiscent of old broadcast rigs.  Just a thought...

All the best,

+-RH

[Stretchyman]

Yes 'Nostalgia' !

Dont do that as I'm an Engineer at heart (albeit an 'Old' one!)

I rekon in 50 Years time folk will pick up PWM when the Earth's natural resources (i.e. IRON) run out!

Ah Well......

 

BTW, rebranding my PWM, with Audio BPF and 6:1 compressor, 50V @ 10A MAX, 1 inch square!

[WO1U]

I saw your comments on AMFone as well, glad to see you're not biased toward your PWM approach as being the only viable solution.... :-)  Your statement about "better" ways to modulate is interesting. Better in what way?  There are numerous ways to amplitude modulate a signal, all have advantages and disadvantages. I have built and used PWM, PDM, Outphasing, Class A series, and Heising type modulators. I am aware you think it is 'modern and best' to use PWM. Maybe so, maybe not....but the point of the article was not to make the best transmitter ever, as was clearly stated in the beginning.

If you didn't get your competitive back up about the modulator and read the intent of the article you would readily see, this was just an experimental approach to prove a couple of points:

1. Use of off-the-shelf hi-fi gear. Simple, cheap (like most of us hams! :-) )
2. Show that a simple, cheap off-the-shelf transformer can provide broadcast quality ( 3Hz to 30 kHz at -3dB frequency response ) using ONE CHEAP COMPONENT.

I had heard someone talking on the air who had the belief that tube AM is the only way to get good quality AM, and they emphatically made the statement "you can't modulate a solid state RF deck with a transformer" regardless. That narrow minded statement was partially what made me want to try it. I did. It worked. So what.

The bottom line is A) you're probably right PWM might be better by several measures (and of course YOURS would be so much better!)  B) You missed the point.  Criticism like this discourages sharing experimentation with wacky non-standard approaches. Yes Strandbeest isn't a very good car.

I too have a laundry list of critiques of that prototype. I later added a diode to ensure the modulation could not swing negative (D'OH!), an LED overmodulation indicator, and discovered much less audio power is needed (150W amplifier will do) and the transformer doesn't need to be nearly as big (300VA is sufficient).
The good that came out of that work though is a way to quickly, cheaply, slap together a quality transmitter in a weekend using a minimum of readily available parts. Also the H-Bridge RF Deck was another separate experiment, while it was successful it was unnecessarily complicated, and not easily scalable in frequency, e.g. not the best approach. Glad to hear you liked that though, thanks.

I hope this answers your WTF comment.

Regards,

Mike

Great RF design with a WTF modulation scheme.

Why on earth design a decent RF generator only to modulate it with the most outdated of methods?

Yes saw this ages ago and commented on AMFONE (that's where they all hang out!)

Maybe the author has never heard of PWM?

There's people on the same site still advocating the use of HEISING modulation, FFS it makes me want to bang my head against the wall (actually I'd rather bang there's!)

Anyway people, please don't bother with mod trannies for any Voltage over 12V as there is a better way.....

Oh Yes.

 

[Stretchyman]

Better as in more modern, cheaper, smaller more efficient, more linear and incredible simple as PWM and driver combined in a single package with virtually lossless FETs makes it for me! That and a KW modulator with aircored inductors will weigh very little in comparison.

Str.

[WO1U]

PWM is hardly a modern concept, been around since the 50s (I was there!) guaranteed not cheaper than a $30 transformer, it probably is smaller but you've not sent any specifications, it is not more efficient than a passive transformer, and probably not more efficient than the Crown XLS1000 + passive transformer, I doubt it is any more linear than the Crown but again you've not provided any specs.  Virtually lossless FETs? That's funny! No such thing exists. All FETs including Silicon Carbide and GaNFETs have on-resistance. 

What is the input and output voltage range of your modulator?
What range of modulation impedance can it drive?
What is the frequency response?
What is the Efficiency vs load? Is it duty cycle limited?
What is the linearity or IMD vs load?
What is the switching frequency leakage vs. current drawn?
What are the limitations of its operation? Have you done any critical analysis of your design?

If you have actually characterized these parameters please send along a link, I would like to see the data. I might be interested in buying one if A) you can tell me what it is in technical terms, and B) if I could get a sense that you were trustworthy. Being on a forum I am starting with neither :-)

Mike

Better as in more modern, cheaper, smaller more efficient, more linear and incredible simple as PWM and driver combined in a single package with virtually lossless FETs makes it for me! That and a KW modulator with aircored inductors will weigh very little in comparison.

Str.

[redhat]

I'm going to get some popcorn and watch this from a safe distance 

+-RH

[WO1U]

Hey Redhat, sorry to dissappoint.  I concede I have been trolled.

I am done here. 73

[redhat]

I'm sorry to see you go.  I am glad you made this project available to all of us, and I certainly learned a lot from it when I was starting out building my own stuff.

Please don't be a stranger, and I hope you won't let this experience taint your opinion of the rest of us.  We're all here to learn, some folks forget that.

+-RH

[Azimuth Coordinator]

Oh... did I just read Outphasing !!!    I thought I was the only one.   Redhat please pass the popcorn. 


tAC

[Pigmeat]

A lot of those guys on 75 like to hear that initial big "Chonk!" in their monitors that comes with high power and Heising modulation. A certain ham in Maine would likely throw himself off his collection of school buses of spare parts if he had to do it another way.

[Stretchyman]

Apologies for not answering sooner!!

Questions, questions, OK.....

PWM is hardly a modern concept, been around since the 50s (I was there!) guaranteed not cheaper than a $30 transformer, it probably is smaller.

Yes, much smaller about and inch square + the humongous filter tho'! that and you forgot the cost of the amp ($1000?)


What is the input and output voltage range of your modulator? ANY upto whatever the FETs will take so 12V-500V

What range of modulation impedance can it drive? ANY, filter designed to match.


What is the frequency response? DC to Infinity, limited by filter ONLY so usually use active filter on I/P to cut over 7KHz.

What is the Efficiency vs load? Is it duty cycle limited? 95%+ I2R as per, so with 90mR FETs (4 FETs = 22.5mR, u can do the sums..)

What is the linearity or IMD vs load? No idea...

What is the switching frequency leakage vs. current drawn? Depends on filter again, more poles for high power and less clock leak.
What are the limitations of its operation? Have you done any critical analysis of your design? None, er..No.

If you have actually characterized these parameters please send along a link, I would like to see the data. I might be interested in buying one if A) you can tell me what it is in technical terms, and B) if I could get a sense that you were trustworthy. Being on a forum I am starting with neither :-)

Looks like you're perfectly capable of building your own.
 
You're on AMFONE and there's a few designs on there, however I'd stick to using the UCC35702 as it's very linear, has ripple cancellation and a few other nice features, use this to drive any of the new half bridge FET drivers and some SiC FETs.
Whole design will fit in the palm of your hand, don't forget the chunky filter tho'!

Str

p.s. Please don't leave 

[JimIO]

With that circuit you need isolation because of the line operated power supply. An off the shelf transformer is a good choice. If I was going to build something like that to pirate with I would go with Bluetooth and Class A series.

[redhat]

Line isolation is not a big problem if you have transformer coupled output and drive signals.  Above the 1KW (5KW PEP) or so level, this makes more sense than stringing together more switchers, both from a weight and cost standpoint.  I see a lot of the high power Greek pirates doing this, and at the levels they are now pushing (12KW) the transformer required would be quite massive.  It all gets rather complicated with regard to metering, in this case you would need analog photocouplers with the associated circuitry to get voltage and current samples to the outside world, isolated from the mains.  Watch your signal grounds!

+-RH