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Simple Power oscillator

Discussion in 'Electronic Design' started by [email protected], Sep 30, 2007.

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  1. Guest

    I'm trying to realize a simple RF power oscillator to experiment RF
    induction for educational goal. I found a lot of amateur radio
    transmitter schematics but my goal is something of more easy because
    I
    don't need of modulation, antenna matching and so on. My ideal scheme
    is an equivalent of "one tube Colpitts oscillator" but using a power
    mosfet ot transistor. My friend told me that there are "4 watts RFID
    reader working at 13.56MHz" scheme, but I found nothing in the web.
    Someone can help me?
    Thanks in advance,
    Antonio
     
  2. Tim Williams

    Tim Williams Guest

    Low RF, like 100-200kHz. MHz are for dielectric heating.

    Slap together a work coil, tank capacitor (look for polypropylene
    capacitors) and a power BJT. Stay away from MOSFETs, at least for now.
    Add bias resistor, bypass capacitor and feedback coil (making this a
    tuned-collector Hartley or so) and apply power.

    This is the sort of thing people would use 2N3055s for, but only in the 10s
    of kHz range. Try something faster, like fT > 5MHz.

    Tim
     
  3. Guest

    Internet is great! I've asked a question from Italy and in moment I
    had an useful answer from the Wisconsin!

    Thank you very much Tim for your fast reply.

    I've also given a fast glance at your very interesting web page. In
    particular I've seen your solid state Tesla coil.
    Probably this is the closest approach of the oscillator that I've in
    mind.
    In fact, when I spoke about "experiment on RF induction" I had in mind
    something like this: www.ecoupled.com
    So, any simplest solid state scheme to obtain some watts @ 1-10Mhz is
    the welcome.

    Best regards,
    Antonio
     
  4. Don Bowey

    Don Bowey Guest

    What you want to do is simple, but...........

    You need to go search FCC records to find the frequency allocation for
    dielectric heating. If you select a random frequency, you could find
    yourself in bad FCC trouble.

    I think there may be frequencies around 13 MHz in use for inductive heating
    (plywood mills for example), but not for RFID.

    What will be the load device for the MOPA (master oscillator power
    amplifier)? I always favored a Pi-net for random impedances, and a balun of
    some variety when the load was balanced.
     
  5. Guest

    Hi Don Bowey,
    I try to describe what I've in mind.
    Suppose a RF oscillator (e.g. Colpitts) which a LC resonator working
    at few MHz. The inductance L should be a simple loop of copper wire
    (one turn with diameter of about 0.7 feet). Placing near this loop
    another little LC resonating at the same frequency the RF energy will
    be efficiently transferred in this last one. If, for example, the
    power oscillator is about 5 watt, the transferred energy is able to
    light up a little bulb (for example).
    While I've experience with antennas, high Q LC etc. I've not a scheme
    for that type of oscillator. In my modest opinion it has to be as much
    as possible easy, just to begin some experiments.
    Thank you for your attention,
    Antonio
     
  6. I would strongly suggest you using a tube unless your father is a
    manufacturer of RF transistors. An uncontrolled transistor or FET power RF
    oscillator can go wild and blow itself in a fraction of a second. You need a
    carefully designed and very fast acting protection. A tube is much more
    forgiving. Also, since the gain of transistor is less compared to a tube,
    the mode of the generation will be strongly dependent upon the loading.

    Vladimir Vassilevsky
    DSP and Mixed Signal Consultant
    www.abvolt.com
     
  7. Guest

    I would strongly suggest you using a tube
    Thanks Vladimir, I will take in account your advice too.

    Antonio
     

  8. The last induction heater I was around used a pair of EIMAC 304TL
    triodes.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  9. Chris Jones

    Chris Jones Guest

    If you build a power oscillator, the frequency is likely to be unstable when
    the load impedance or power supply voltage changes. This means that you
    are likely to pollute an unnecessarily wide range of the RF spectrum and
    you might find that this causes you to receive complaints from nearby users
    of the RF spectrum, who may call in the authorities.

    I strongly recommend that you get a crystal oscillator and then feed the
    output of this into a power amplifier. The power amplifier could be built
    with transistors or valves (vacuum tubes).

    Chris
     
  10. Guest

    OK, thanks to Michael and Chris too.

    Now the design requirements are more clear.
    - crystal oscillator stage in an allowed frequency
    - buffer and PA stage

    Cheers
    Antonio
     
  11. Tam/WB2TT

    Tam/WB2TT Guest

    This is the right, and probably only legal way to do it. The maximum power
    that can be put through a crystal is on the order of 1 mw; so you need
    several stages to get appreciable power out.

    Several people have mentioned using vacuum tubes. My offhand guess is that
    the cost crossover point between transistors and tubes is around 200W
    output.

    Tam
     
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