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5 MHz CW TC with PLL tracking oscillator

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WaveRider

Tue Nov 28 2006, 11:58AM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

I have completed the audio modulator! It works well. The arc delivers a crystal-clear rendition of Tim O'Briens "Fiddler's Dream.."

Enjoy!

I will attempt to put together some "How I did it" documentation as soon as possible...

ragnar

Tue Nov 28 2006, 12:15PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

Beautiful work, WaveRider!

It looks and sounds a lot like my quadraphonic system. The cage I'm sure much reduces its susceptibility to nearby metal objects/sheet, interference with the driver circuitry, fingers, hands, hair etc! =D

I'm keen to hear about your modulator. This is what I'm hoping to pull off with my 13.56MHz coil -- a suitably 'fixed' drive frequency (i.e. +/- a few%) and a class-G/class-H modulator.

Suggestion, if you get the chance -- consider an electrode geometry that causes the discharge to fan out.

WaveRider

Tue Nov 28 2006, 02:36PM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

Hi BP,
Thanks for the kudos!

My modulator is simple... (Valve am transmitter enthusiasts would recognise it as a solid state version of "plate modulation") I will post the prototype schematic when I get the chance to make a presentable drawing, but in words: It's a push-pull DC-DC converter that utilises the PWM capability of the TL494 chip. It operates at 100kHz and uses a ferrite core transformar and ultra-fast rectifiers. Care must be taken to not use filter caps that are too large...in order to avoid excessive high-frequency audio roilloff. Like EVR's Plasmatronic, I modulate the "dead time" of the pulse waveform to modulate the output voltage...which is fed to the drain of the power amplifier stage of my PLL-based class-e TC. Even given the "point" nature of the arc, the sound is loud and clear. Modulation is only about 20% or so.... (It's being driven by my MP3 player... Measly output signal is in no danger of overmodulating the thing!

)

There are ways to improve the modulator, but I am pretty pleased with this prototype... The sound is better than I imagined it would be given the simplicity of the circuit!

...

Tue Nov 28 2006, 03:19PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Wow

That is really good!

Do you have any way to test the quiet and max modulation output? I would have imagined that with such a small amount spark it would be hard to get a decen't listening level...

ragnar

Tue Nov 28 2006, 09:55PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

The amplitude of sound you can get from a given 'volume' of plasma is often surprising. With my four ~4MHz coils I had to modulate quite deep to make it suitably audible amongst, e.g. people talking.

WaveRider's spark isn't all that 'small' either =)

...

Tue Nov 28 2006, 11:21PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I always imagined you needed the huge sparkage like on EVR's PlasmaSonic system. I tried making a small '494 based coil and even with 1" sparks (a lot of them) I couldn't get usable sound before it started to clip.

WaveRider

Thu Nov 30 2006, 02:25PM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

The spark is not huge, but it can be a few cms long. Remember, the coil is not operating at full power when being audio modulated. It runs somewhere near 60-75%. The sound is very clear and quite loud. The MP3 player clips before the coil does, so I need a better audio source to test the full range..

I have updated my PLL tesla coil page with the circuit that I used for the modulator. I don't think I made any errors in the schematic.. If you build this circuit, be sure ti use good low-loss capacitors in the 100V transformer circuit.

WaveRider

Fri Jan 19 2007, 03:08PM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

Hi all,
Sorry about the silence, but I have not been able to spend much time "projecting" lately. However, I have been taking little snapshots of free time to prepare a first draft of some detailed notes on my Class-E audio modulated coil. They can be found on my PLL Tesla page. Hope it is informative... I welcome any comments/questions on how to improve the presentation or correct errors.. Also, to those experienced people: I put in lots of warnings to discourage those who may attempt construction without understanding the dangers. Please do not be put off by my overcautiousness.

Cheers and enjoy..

ragnar

Fri Jan 19 2007, 03:46PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

hi Bill,
glad to know you're alright -- and cheers for the progress on your page. **reads carefully**

I've been talking to Steve about some of the quirks I encounter with high-frequency class-E setups, and I'm documenting them, along with how I got around them -- the problems (and the solutions) are definitely not obvious (at least not to me!) at first. I decided to grow a spine and stop being a pussy, and I made a proper flywheel circuit to couple the resonator the the drain/RFC. I also invested in some air-dielectric variable caps (which I've been meaning to do for a while for other projects); it's really a pain switching combinations of these 250pF 10kV ceramics in and out for effect.

Some of the issues I was confronted with:
-- If drive duty cycle is even just smidgin > 50%, the whole output circuit crashes with no explanation,
-- gatedrive from 9V-11V is fine... as I drive it to 12V-15V, less and less happens on the output until no power flows,
-- device's own output plus forgotten capacitance between the drain tab, e.g. silicone pad, and heatsink sometimes exceeds desirable shunt capacitance; no matter how little extra capacitance you add, it'll always be too much! As a ballpark figure, my particular silicone pads yield about 40pF capacitance between the drain tab and a heatsink.
-- triple/quadruple check that resonator is actually being driven at fundamental! Inspecting waveforms whilst driving at harmonics often gives the (wrong) impression that parts of the circuit are working
-- even a little stray inductance between the shunt capacitor and the switching device can ruin things at high frequencies; I've found it much better to connect the cap to the drain tab, rather than the drain pin or even an inch away on the PCB.

I'm not sure how much of this applies, but at 13.56MHz, I feels like I'm working with a totally different beast. Then again, that may just be the feeling of inexperience!

I've attached some pics, including
-- high frequency gate driver transformer techniques,
-- (what I pretend is a) good RF PCB layout,
-- more durable secondary wound around teflon PFA former (coin is approx one inch diameter),
-- my very ugly, very lossy "output board" (better than none at all!)
-- whole setup as seen in the dark (highlights are scope, power supply and breakout),
-- closeup of breakout (moire effects on camera are from CCD susceptibility to electric field?),
-- driver, output stage, resonator and groundplane all framed,
-- some not-too-bad drain-source voltage waveforms (but my 20MHz oscilloscope is probably lying),
-- EM-shielding cage which I will soon put to use (CD is for scale, Tiger is for effect)

Enjoy =)

...

Sat Jan 20 2007, 01:36AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

good stuff Matt!

One advantage for going with the 13.56mhz is that the resonator is nice and small

I can't imagine what that would look like with a couple kw being pumped into it...

I wonder what would happen if you bought some commercial piece of gear that runs at 13.56mhz (plasma etcher?) and added a resonator to it...

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