Demodulating square wave from 2.22MHz carrier

Discussion in 'Wireless & RF Design' started by TBJ, Jun 15, 2012.

  1. TBJ

    Thread Starter New Member

    Apr 9, 2012
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    Hi all,

    I am currently designing a POV (persistence of vision) display with a rotating part and a stationary part. For this it is necessary to transmit both power and data wirelessly from the stationary part to the rotating part.

    I have built a resonant inductive wireless power system using a Royer oscillator. This operates at 2.22MHz and is working well. The receiver on the rotating part takes the RF from the LC tank circuit and feeds it into a bridge rectifier to produce about 30 volts DC, then into a smoothing cap and a voltage regulator to make a usable 5 volts DC at an amp or so.

    Now then, what I want to do is switch the transmitter circuit using a MOSFET in order to transmit data, in other words modulating the 2.22MHz carrier with a square wave, let's say at a purely arbitrary speed of 50khz. I can do that bit - it's on the receiver side that I'm struggling.

    How can I get this signal back out at the receiver end? Obviously I can't just take a signal from right after the bridge rectifier because there is a smoothing cap which would smooth out the signal, so what can I do?

    Hope this makes some sense and someone can help.

    Thanks!
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Can you post a schematic of the reciever / rectifier part?
    That way we can see where you can retrieve the data signal.

    Bertus
     
  3. TBJ

    Thread Starter New Member

    Apr 9, 2012
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    Hi Bertus,

    Here is a schematic. I literally had to draw this up on a scrap of paper as I am on my lunch break at work at the moment and don't have access to any of my schematic programs, so apologies for the quality... :)

    [​IMG]
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    You could try to isolate the DC and the modulated signal with a diode.
    See my try of the schematic below:

    [​IMG]

    You may have to adapt the 2k2 and 1nF in the schematic for the best demodulated signal.

    Bertus
     
  5. WBahn

    Moderator

    Mar 31, 2012
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    Out of curiosity, how much total power is your transmitter consuming? How much is it actually transmitting? Are the transmitting and receiving coils pretty tightly coupled?
     
  6. TBJ

    Thread Starter New Member

    Apr 9, 2012
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    Hi WBahn,

    I made a boob - just measured the max current I can pull from the receiver on the regulated side and I'm getting about half an amp, though that may also be limited by the diodes I am using in the bridge rectifier (they are 1n4148s so not rated for any real current)

    I don't have two ammeters so I can't measure the efficiency at the moment I'm afraid, but I will try to make that happen soon.

    Bertus: Thanks, that looks like it may work. I have set up an experiment where I am modulating the transmitter with a 4.8khz square wave (to start with), so tomorrow i will see what the signal looks like that I get out.

    Can I just ask - what is the function of the 2.2k resistor and the capacitor in your schematic? Is the capacitor acting to eliminate the high frequency (2.2mhz) component of the signal?

    Thanks for your help.
     
  7. bertus

    Administrator

    Apr 5, 2008
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  8. TBJ

    Thread Starter New Member

    Apr 9, 2012
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    Well, it worked!

    I am able to transmit data over the power coupling at 4.8kb/s using AVR's SoftwareSerial library. I had to play with the values of R and C a little (100pF and 2.2k at the mo, not really experimented with other values yet because that works just fine). I am revising my transmitter design slightly to use smaller, cheaper components, too - once that is done I will try transmitting data at a higher speed and see how fast I can go.

    For those who are interested, here is a video in which I demonstrate my experiment:

    http://www.youtube.com/watch?v=Gj-rLRbD5DI

    Thanks for the help :)
     
  9. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Good to hear you got it working.

    Did you think about the demodulated signal amplitude?
    That might be to high for the arduino.
    A resistor (of about 1k) and a zener diode of 4.7 Volts will protect the adruino.

    Bertus
     
  10. TBJ

    Thread Starter New Member

    Apr 9, 2012
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    0
    Hi Bertus,

    In that video I am using a voltage regulator (which is a bit of a hack) to bring the ~30 volts signal down to 5 volts logic level.

    Since then I have devised a way to do it with two small signal MOSFETs as a level converter.

    I did look at using a zener but found the current limiting resistor was wasting quite a bit of power.

    Cheers.
     
  11. SeeIon

    New Member

    Oct 7, 2012
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    I have a project that needs AM or AM/Square Wave modulation. Can you detail how you AM modulate a free-running royer oscillator? I was thinking about mixing/modulating the DC Power Supply with an audio power amp or high current square wave generator but I am hoping your method is something new that I have not seen. Thanks

    Please forgive the bump as I just joined the forum due to the content of this thread.

    SeeIon
     
  12. SeeIon

    New Member

    Oct 7, 2012
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    This is a schematic of the Royer Oscillator I plan to modulate. Thanks
    SeeIon
     
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