Generate RF to drive coil

Discussion in 'The Projects Forum' started by tkchan, Jul 9, 2008.

  1. tkchan

    Thread Starter New Member

    Jul 9, 2008
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    Hi all. I'm at an internship, and my project is to modify a table-top plasma cleaner that currently uses a vacuum tube to drive a 23 micro-H coil. I basically need to think of ways to modify it to become steady state. Wanted to see if anyone had any ideas where I could generate the power with the specifications below to drive the RF coil:

    Frequency: 13.56 MHz, Sine wave
    RF Power: 1.5 KW

    Basically I need an RF generator on a circuit board that can fit inside a 11"H x 18"W x 9" box, which can use a 115VAC wall outlet.

    Anyone with some ideas I can start with? Something very similar to the PRF-1150 (http://www.directedenergy.com/pdf/rf-1150_data_sheet.pdf).

    Thanks for any ideas!

    - Trevor
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Why not simply modify that design, or better yet have them build one to your specifications?

    Unless you have a good bit of experience with high power RF, tuning filters using network analyzers, etc - I suggest that for your safety and reliability of the equipment, you have them build a module for your application according to your specifications.

    1.5KW is quite a bit of RF energy. You're going to have to keep the module within it's operating temperature specifications. A couple of computer fans won't cut the mustard.
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    Agree with the Sgt. Do you have a schematic you can post? It may be possible to modify the existing circuit to take a FET in place of the tube. The devices are somewhat similar in operation, and the FET's are lots more available.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    There's quite a bit of detail in the documentation provided by the link, including schematics, components and theory of operation. One would be hard-pressed to come up with a closer match without spending a lot of time searching and reading.
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    Sputter, sputter - you have to actually see the link in order to follow it...
     
  6. tkchan

    Thread Starter New Member

    Jul 9, 2008
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    I really dont' have alot of experience with high power RF or the like. Only 7 years experience as an avionics technician in the USAF and some ECE courses. With the USAF, it was mainly testing LRU's and swapping out SRU's. Not much digging down to the component level. SGTWOOKIE, are you a Marine?

    So basically this small company took me in as an intern to research ways to modify their plasma cleaner and the PRF-1150 (in the link) was a module a consultant suggested to them that may work. The module itself costs $950, and I think our units sell for $3,000; I don't think it's a viable option to purchase such an expensive component. I was asked to research if the PRF-1150 would work in the plasma cleaner and if there are other similar options out there. Also if we could modify the current module (as SGTWOOKIE suggested) to fit our plasma cleaner.

    I believe one problem we have is that since our load is a coil, pretty much an inductor, that when connected, the output of the PRF-1150 would be affected since the PRF-1150 may see the load as simply an additional inductor to the circuit. Does this make sense? Since "L2 functions as both the main Tank inductor and as part of the output "L-match" matching the tank impedance to the RF load" (pg 3), where L2 is located close to the output of the PRF-1150.

    What the comany wants is a fixed frequency (13.56 MHz) sine wave and output power that can be varied around 1.5 KW. I was looking into a crystal oscillator to serve as a clock, but then generating the rest seems to be above my head. They definitely do not want something like a VCO to generate the frequency as the frequency would be unstable and it's driven by an LC circuit.

    Thanks for all your responses!
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Thank you for your Service. :)
    I spent the better part of a decade working on fast movers in Marine Aviation; F-4J/S. Radar/Missile Fire Control Tech. Those were tough birds to keep working; the J's were all analog, the S's introduced digital. All of our birds had very high-time airframes. I then spent another 15 years working on the systems of a number of other fast movers. I've been out of the Corps for awhile, but the Corps hasn't left me yet ;)

    The buy/build decision is a tough one. If your company already has the equipment to machine things, and test equipment such as a network analyzer (they start at around $7,000) to align the filters, and enough of the units will be constructed to amortize the costs of labor, tooling, equipment, etc. will offset the cost of the other unit, then it may be viable. Tuning RF filters is not a particularly easy task. While at the outset it seems simple, there are a lot of subtle nuances you pick up with experience. It's not something that is easy to explain; volumes have been written on the subject.
    Yes, an inductive load would affect it. However, if you can make your inductive load an LC tank circuit that is resonant at the fundamental frequency, impact should be minimal. You may also realize substantial power savings.

    Exactly how stable do they want it?
    An XTAL oscillator will be a couple orders of magnitude more stable than an LC oscillator.
    A TCXO (Temperature Controlled Crystal Oscillator) will go another order of magnitude.
    There are also VCXO's, and myriad others including Rhodium-doped, which can be extremely stable.

    Here's one manufacturer of oscillators:
    http://www.mtronpti.com/
    There are a number of others.
     
  8. tkchan

    Thread Starter New Member

    Jul 9, 2008
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    Thank YOU for your service. Once a Marine, Always a Marine, right? It's sure an elite group, sometimes I wished I joined. I only worked on the C-5A Galaxy, big and slow birds. Took one to Spain for one of two two week annual tours there. It was quite a noisy and cold ride.

    We sure don't have a network analyzer, and tuning RF filters does not seem to be the most efficient direction for us. There are only two techs here that build everything, and they seem to be busy as it is already. I also found a "Reference Design Kit" (RDK) http://investor.microsemi.com/ReleaseDetail.cfm?ReleaseID=294925
    that now seems to be a more attractive option with the new details you've informed me about designing and building a high power RF generator. From some search on the web, it seems a RDK gives the consumer a reference design and allows us to modify it to our specs? If so, I think that may be the option I would suggest to my superiors...;). This RDK has most of what we need; it's a 1000W Class-E RF Generator operating at 13.56 MHz switching frequency. Only thing is we'd probably modify it for slightly higher power output.
     
    Last edited: Jul 10, 2008
  9. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    What the OP is describing is also known as a plasma asher, their are a lot of choices out there, the 13.5Mhz was selected by the FCC for industrial use. I have used quite a few makes/models (still do), there are off the shelf RF generators specifically designed for the application. The one I'm currently using is a March 500, a 500W model, and there is another March 1000 elsewhere in the shop, both use an external commercial RF generator. I used to work on a LFE model, that is very similar to what was described, a big glass jar surrounded by a coil, with an internal RF tube generator (they are out of that business now though). The vacuum seal is also a big deal, as well as the gas mixes (that has to be selectable), with the surface area we're dealing with there is well over 2 tons of pressure on the door (and the rest of the chamber). The March uses a motor connected to a large air capacitor to tune the impedance of the chamber for minimum power reflection.

    I wish you luck with it, and will be following this thread with professional interest.
     
  10. tkchan

    Thread Starter New Member

    Jul 9, 2008
    4
    0
    Thanks for the info Bill. I've found more info on plasma cleaners/ashers now. Many more devices came up from the search of plasma ashers and it seems many of these devices have a solid state RF power source, which is where we are in the process of moving. I guess one of the obstacles is we need the RF source to be contained in our fairly small table-top plasma cleaner/asher. Just spoke to superior, and a path we may take is ordering a ready built module such as the PRF-1150 or the DRF1200 and modifying it to our specs. Keep you posted!

    - Trevor
     
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