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50MHz crystal video transmitter

Discussion in 'Radio and Communications' started by alphactif, Feb 21, 2012.

  1. trash

    trash Member

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    The 5.5MHz component is a simple IFT. You can rip one out of the sound circuit of any TV. (it's a small metal can with a screw in the top).
    Make sure you know which one is the 5.5MHz one. It may also be a 4.5MHz, 5MHz or 6MHz IFT depending on which standard you're using.
    You can of course complete eliminate the sound circuit if you only want video or want to add sound later.

    2t = 2 turns. 2t 5mm = 2 turns 5mm diameter (air wound). Find a 5mm drill bit and wind some enamel copper wire on it.

    I have not specified the length of the stripline transformers in the amplier part of the circuit. These can be 1/2 turn of 12mm coil, center tapped for band 5 UHF. 1 turn center tapped for lower UHF.
     
  2. alphactif

    alphactif New Member

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    There is two 2t 5mm coil ??? how they are connected together ? have you a pics of this coils ?
     
  3. davenn

    davenn Active Member

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    ok the 2 sections you have arrowed and questioned
    The left one is a ferrite bead on the V= supply rail. This will probably consist of 3 -4 turns through a single hole ferrite bead. It is there to stop RF going out on the DC rail

    The coil to the right is made of 2 lots of 2 turns on a 5mm diameter plastic former with an adjustable ferrite slug. Most likely, one set of turns further up the former tfrom the other. This is interstage coupling between the oscillator to the left and the modulator and amplifier/multiplier stages to the right. It also provides buffering between the 2 stages.

    Dave
     
  4. dave

    Dave New Member

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  5. trash

    trash Member

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    Thanks Dave,
    yep the ferrite bead has "2t" 2 turns on it, but more doesn't hurt.

    The two coils are actually air coils. There's no former, they're just in close proximity to each on the circuit board.
    The coils are of course aligned to provide the best magnetic coupling. They are nothing but a simple high frequency transformer.
    The dotted lines are just to indicate that the two coils make up that transformer.

    If you want to wind them onto a plastic former that is ok, but there is no ferrite used.
    The circuit is designed for UHF, about ch35 (576MHz) but it will work give or take 100MHz.
     
  6. alphactif

    alphactif New Member

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    Ok, I found a modulator from an old analog sat receiver, it work between channel 30 to 42 (550 - 650MHz), so how can I build an amplifier stage with one or two transistor (common transistor not the RF transistor so difficult to found).
     
  7. davenn

    davenn Active Member

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    I gave you amplifing device choice way back in post #9

    Dave
     
  8. alphactif

    alphactif New Member

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    it's possible to use an UHF antenna booster to amplifiy the RF modulator output ?
    output modulator is approximatly -40 to - 60dbm !!!!:confused:
     

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